ISSN:0366-9033    

DOI:10.1039/DF95212FX001

出版商:RSC    

年代:1952

数据来源: RSC

ISSN:0366-9033    

DOI:10.1039/DF95212BX003

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

RADIATION CHEMISTRY GENERAL INTRODUCTION BY F. S. DAINTON Chemistry Department, The University, Leeds 2 The absorption by matter of electro-magnetic radiation in the wavelength range 2000-7OOOA is generally a simple process. The destruction of the energy quantum occurs in a single act involving the quantum and the absorbing molecule only, and is governed by well-recognized laws. The absorption is selective, the primary products of the absorption process can often be identified unambiguously, and are found to be of similar reactivity, and their rate of formation and spatial distribution can usually be specified with some exactness and certainty. In these respects the primary photochemical act differs completely from the primary act in chemical reactions which are induced by the absorption of high energy quanta (say radiation A < 50 A) or by the slowing down of rapidly moving charged and uncharged particles of atomic and subatomic nature.The mechanism of energy transfer from the radiation or the particles is complex, selective and im- perfectly understood; it is not possible to make anything more than very approxi- mate and qualitative predictions as to the number, nature and initial and final distribution of the entities formed in the primary process. Moreover, in the most important reaction medium, namely water, reactions initiated by one of the par- ticles of the primary act may be reversed by one of the others. Despite the fact that at the turn of the century the development of radiation chemistry was com- parable with that of photochemistry, the present status of the former subject is similar to that of photochemistry 30 years ago.At the present time the most useful conclusions as to the primary act are still obtained by inference from the nature of the ultimate products. The last few years have seen a considerable strengthening of this nexus due primarily to a greater understanding of the chem- istry of free radicals and unstable ions, and it now seems that species of this kind must be intermediary between reactants and products. The main purpose of this Discussion, which is the first to be held on this subject by the Faraday Society, is therefore to appraise the present position, to attempt what synthesis is possible of the views of the physicists, chemists and biologists who, for varying reasons, have contributed ideas and methods to the subject, and to suggest future lines of development.In the first three papers, the authors summarize some of the current ideas concerning the physical processes involved in the formation of the primary products. The lacunae in our knowledge of the mechanism of energy loss by fast charged particles are emphasized by Professor Spiers. We are still ignorant of the Wvalues (i.e. energy required for creation of one ion pair) for liquids, and of the relation of the ionization potentials of the isolated molecules, which gain energy by inelastic collisions with the impingen t particle, to this quantity W and the empirical quantity, the mean excitation poten- tial l? which is employed in the Bethe theory in its origrnal and modified forms.Nevertheless we do know in qualitative terms how the mean ion density of com- monly-used radiations varies with the energy, mass and atomic number of the fast particle. The conclusions reached here are still the foundations on which are erected all hypotheses concerning the dependence of the reaction product in both A 9RADIATION CHEMISTRY GENERAL INTRODUCTION BY F. S. DAINTON Chemistry Department, The University, Leeds 2 The absorption by matter of electro-magnetic radiation in the wavelength range 2000-7OOOA is generally a simple process. The destruction of the energy quantum occurs in a single act involving the quantum and the absorbing molecule only, and is governed by well-recognized laws. The absorption is selective, the primary products of the absorption process can often be identified unambiguously, and are found to be of similar reactivity, and their rate of formation and spatial distribution can usually be specified with some exactness and certainty.In these respects the primary photochemical act differs completely from the primary act in chemical reactions which are induced by the absorption of high energy quanta (say radiation A < 50 A) or by the slowing down of rapidly moving charged and uncharged particles of atomic and subatomic nature. The mechanism of energy transfer from the radiation or the particles is complex, selective and im- perfectly understood; it is not possible to make anything more than very approxi- mate and qualitative predictions as to the number, nature and initial and final distribution of the entities formed in the primary process.Moreover, in the most important reaction medium, namely water, reactions initiated by one of the par- ticles of the primary act may be reversed by one of the others. Despite the fact that at the turn of the century the development of radiation chemistry was com- parable with that of photochemistry, the present status of the former subject is similar to that of photochemistry 30 years ago. At the present time the most useful conclusions as to the primary act are still obtained by inference from the nature of the ultimate products. The last few years have seen a considerable strengthening of this nexus due primarily to a greater understanding of the chem- istry of free radicals and unstable ions, and it now seems that species of this kind must be intermediary between reactants and products.The main purpose of this Discussion, which is the first to be held on this subject by the Faraday Society, is therefore to appraise the present position, to attempt what synthesis is possible of the views of the physicists, chemists and biologists who, for varying reasons, have contributed ideas and methods to the subject, and to suggest future lines of development. In the first three papers, the authors summarize some of the current ideas concerning the physical processes involved in the formation of the primary products. The lacunae in our knowledge of the mechanism of energy loss by fast charged particles are emphasized by Professor Spiers. We are still ignorant of the Wvalues (i.e.energy required for creation of one ion pair) for liquids, and of the relation of the ionization potentials of the isolated molecules, which gain energy by inelastic collisions with the impingen t particle, to this quantity W and the empirical quantity, the mean excitation poten- tial l? which is employed in the Bethe theory in its origrnal and modified forms. Nevertheless we do know in qualitative terms how the mean ion density of com- monly-used radiations varies with the energy, mass and atomic number of the fast particle. The conclusions reached here are still the foundations on which are erected all hypotheses concerning the dependence of the reaction product in both A 9RADIATION CHEMISTRY GENERAL INTRODUCTION BY F.S. DAINTON Chemistry Department, The University, Leeds 2 The absorption by matter of electro-magnetic radiation in the wavelength range 2000-7OOOA is generally a simple process. The destruction of the energy quantum occurs in a single act involving the quantum and the absorbing molecule only, and is governed by well-recognized laws. The absorption is selective, the primary products of the absorption process can often be identified unambiguously, and are found to be of similar reactivity, and their rate of formation and spatial distribution can usually be specified with some exactness and certainty. In these respects the primary photochemical act differs completely from the primary act in chemical reactions which are induced by the absorption of high energy quanta (say radiation A < 50 A) or by the slowing down of rapidly moving charged and uncharged particles of atomic and subatomic nature.The mechanism of energy transfer from the radiation or the particles is complex, selective and im- perfectly understood; it is not possible to make anything more than very approxi- mate and qualitative predictions as to the number, nature and initial and final distribution of the entities formed in the primary process. Moreover, in the most important reaction medium, namely water, reactions initiated by one of the par- ticles of the primary act may be reversed by one of the others. Despite the fact that at the turn of the century the development of radiation chemistry was com- parable with that of photochemistry, the present status of the former subject is similar to that of photochemistry 30 years ago.At the present time the most useful conclusions as to the primary act are still obtained by inference from the nature of the ultimate products. The last few years have seen a considerable strengthening of this nexus due primarily to a greater understanding of the chem- istry of free radicals and unstable ions, and it now seems that species of this kind must be intermediary between reactants and products. The main purpose of this Discussion, which is the first to be held on this subject by the Faraday Society, is therefore to appraise the present position, to attempt what synthesis is possible of the views of the physicists, chemists and biologists who, for varying reasons, have contributed ideas and methods to the subject, and to suggest future lines of development. In the first three papers, the authors summarize some of the current ideas concerning the physical processes involved in the formation of the primary products. The lacunae in our knowledge of the mechanism of energy loss by fast charged particles are emphasized by Professor Spiers.We are still ignorant of the Wvalues (i.e. energy required for creation of one ion pair) for liquids, and of the relation of the ionization potentials of the isolated molecules, which gain energy by inelastic collisions with the impingen t particle, to this quantity W and the empirical quantity, the mean excitation poten- tial l? which is employed in the Bethe theory in its origrnal and modified forms. Nevertheless we do know in qualitative terms how the mean ion density of com- monly-used radiations varies with the energy, mass and atomic number of the fast particle. The conclusions reached here are still the foundations on which are erected all hypotheses concerning the dependence of the reaction product in both A 9INDEX TO PHOTOGRAPH TAKEN AT GENERAL DISCUSSION ON AT LEEDS UNIVERSITY. 8TH-fOTH APRIL.1952 " RADIATION CHEMISTRY. " HELD Allen. Prof . A . 0 . . Allsopp. D r . C . B . . Alper. MissT . . . Amphlett. Dr. C . B . . Axford. Dr . D . W . E . . Bacq. Prof . 2 . M . . Barb. Dr . W . G . . . Barnard. D r . D . Bartindale. D r . G . W . R . Baskett. Dr . A . C . . Bates. Mr . T . H . Bawn. Prof . C . E . H . . Baxendale. Dr . J . H . . Behrens. Mr . H .. . Behrens. Mrs . . . Belton. Dr . J . W . . Bernas. Mme . A . . . Best. Mr . J . V . . . Blok. Mr . J . . . Boag. Mr . J . W . . . Bonhoeffer. Prof . K . F . Bradley. Mr . R . S . . Burns. Dr . W . G . . Burton. Prof . M . . Butler. Dr . J . A . V . . Capron. Prof . P . . . Chapiro. Mr . A . . . Coleby. Mr . B . . . Collinson. Dr . E . . . Conway. Dr . B . E . . Cook. Dr . E . W . . Coomber. Dr . D . I . . Cousin. Mr . C . . . Dainton. Prof . F . S . . Dale. Dr . W . M . Daniels. Mr . M . . . . 13 . 22 . 64 . 32 . loo . 2 . 24 . 102 . 135 . 83 . 31 . 9 . 33 . 118 . 125 . 1 . 3 . 37 . 123 . 90 . 8 . 26 . 75 . 10 . 35 . 98 . 88 . 127 . 19 . 106 . 68 . 114 . 30 . 16 . 58 . 142 Danon. Mr . J . . . Daudel. Dr . . . Daudel. Mme . . . Davies. Dr . J . V . . Davison.Mr . W . H . T . Dawson. Mr . T . L . . Day. Mr . M . J . . . Dewhurst. Dr . H . A . . Ebert. D r . M . . . Emeleus. Prof . H . J . . Fletcher. Mr . J . C . . Gaade. Dr . W . . . Garrison. Dr . W . M . . George. Dr . P . . . Gilbert. Dr . C . W . . Goodings. Mr . E . P . . Gray. Dr . L . H . . . Gueron. Prof . J . . . Haeffner. Mr . E . Haissinsky. Dr . M . . Hannon. Dr . R . S . . Harbottle. Dr . G . . Hardwick. Dr . T . J . . Hardy. Mr . H . R . . Hart. Dr . E . J . . . Hering. Dr . H . . . Ivin. Dr . K . J . . . Jackson. Mr . G . G . . Jacobowitz. Mr . G . . Johnson. Mr . G . R . A . Keene. Dr . J . P Kornitzer. Miss B . . . 52 . 59 . 60 . 72 . 56 . 78 . 143 . 92 . 121 . 7 . 79 . 101 . 14 . 23 . 73 . 139 . 89 . 132 . 141 . 5 . 96 . 49 . 45 . 80 . 4 . 1 1 1 . 20 .39 . 133 . 146 . 110 ; 122 Landler. Mr . Y . . . . 144 84 85 134 71 Lefort. Dr . M . . . . Luyckx. Prof . A . . . McDowell. C . A . . . McKinley.McKee. Mr . J . S .. M M M M M M M M M M M M M M .addock. D r . A..G . . .agat. D r . M . . . agee. Mr . J . . . .agee. Prof . J . L . . lartin. Mr . G . R . . [eerssche. Mr . M . van liller. Dr . N . . . lilling. Mr . B . . . linder..Dr . W . . . [ooney. Dr . R . W . . [oore. Dr . C . G . . loore. Prof . W . J . . [und. Prof . W . . . [unro. Mr . T . R . . Nicholls. Mr . A . M . . Regnaut. Mr . P . J . . Richards. Dr . E . W . T . Rigg. Mr . T . . . Rogers. Mr . G . T . . Rolin. D r . M . . . Rowbottom. Mr . J . . Rydberg. Mr . J . . . Saeland. Mr . E . . . Sangster. Mr . D . F . . Santen. Dr . J . H . van . Scholes.Dr . G . . . S 4 , M r . L . . . Shaw. Dr . P . F . D . . Roberts. Dr . R . . . Scott. Dr . J . R . . . . 65 . 17 . 81 . 87 . 18 . 129 . 47 . 116 . 27 . 107 . 103 . 86 . 6 . 109 . 53 . 138 . 115 . 128 . 46 . 104 . 130 . 91 . 124 . 42 . 95 . 69 . 126 . 38 . 55 . 97 Sheard. Mr . D . R . . Singh. Dr . K . . . Smith. Dr . Neville . Spiers. Prof . F . W . . Stein. Dr . G . . . Steinbrucher. Dr . . Sutin. Mr . N . . . Sutton. Mr . H . C . . Swallow. Dr . A . J . . Swarc. Mr . M . . . Szasz. Dr . G . . . Taplin. Dr . G . V . Taylor. Dr . E . H . Thomson. Dr . S . J . Tomlinson. Mr . M . Tompkins. Dr . F . C . . . . . . . 147 . 62 . 136 . 93 . 145 . 43 . 63 . 94 . 57 . 50 . 12 . 41 . 77 . 113 . 66 . 34 Trotman.Dickenson. Mr . A . F . 51 Tuck. Mr . D . G .. Valentine. Dr .L . Wahba. Dr . M . . Wake. Dr . W . C . * Wall. Dr . L . A . . Walton. Mr . G . N . Wardle. Mr . G . . Waters. Dr . W . A . Weiss. Dr . J . . Weston. Mr . M . . Whiteley. Dr . R .. Wiener. Dr . H . . Wild. Dr . W . . Wilkinson. Mr . J . Williams. Mr . T . F . Willis. Mr . E . H .. Wilson. Miss J . . Wright. Mr . J . . Wormwell. Dr . F . . . 117 . . 70 . . 61 . . 82 . * 99 . . 40 . . 112 . . 44 . . 1 1 . . 140 . . 74 . . 108 . . 15 . . 76 . . 119 . . 54 . . 21 . . 36 . . 2910 GENERAL INTRODUCTION quality and quantity on the particle mass and energy. Thereare plenty of examples of this providedin the papers of Allen, Hardwick, Bonet-Maury, Haissinsky and others. We still derive most of our knowledge concerning the properties of positive and negative ions (i) from experiments in gaseous systems (discharges, swarm experiments and mass spectrometry), and (ii) for the simplest ions from wave mechanics.Although several authors utter warnings concerning the " carry- over " of conclusions derived from such attcnuated systems into the liquid phase, there seem to me to be many such justifiable extrapolations. As two examples, I will cite the variation of the cross-section for the formation of a particular positive ion with the energy of the incident electron and the fact that dissociative electron capture processes such as AB + e -+A $- B- are resonance phenomena. This latter conclusion is of value because by photochemical means we can produce electrons of low energy in liquid systems adjacent to the molecules by which they can be captured, and thereby gain useful experimental information concerning the ultimate fate of the negative ion.By representing the process in terms of potential energy curves, we can predict relations in conformity with this notion which are susceptible to experimental verification, and also lead to interesting conclusions concerning certain thermal reactions which can take place. An example in point is the reduction of polyvalent cations by the hydroxyl radical in aqueous solution. Most radiation chemical experiments are carried out in polar media, and it is here that the cautious approach is most required, because the solvation energies and entropies are of such considerable magnitude and vary so greatly from ion to ion that processes considered to be of low probability in the gas phase may be of very high probability in polar media, and vice versa.Another warning is necessary here. The magnitudes of changes in thermodynamic functions accompanying solvation are derived from eqiiilibriurn measurements, whereas there is reason to believe that many charge transfer processes are so rapid that the reorientation of solvent dipoles which occurs is a slower process. This may make prediction of reaction probabilities, based on the consideration of the energetics of initial and final states, of little value. It is probably preferable at the present time to proceed empirically, studying various series of reactions so chosen as to reveal relationships between rates and some parameter such as a varying ionization potential or dis- sociation energy.Whilst it is not for me to predetermine the course of the Discussion, I would like to draw attention to some questions which seem to me to transcend others in importance. 1. DOSIMETRY AND ACTINOMETRY .-Before effective comparison between the results of workers in different laboratories can be made, we must have an agree- ment about methods of assessing energy absorption. If this is not easily possible, the case for a substandard by radiation actinometry is very strong. 1 feel sure I am right in expressing the gratitude of the meeting to Dr. Miller and Mr. Wilkinson for their efforts in this direction, and I hope it will be agreed that with X- and y-rays and using the dose rate range 1-103 r/min we will agree to use the aerated FeSO4 actinometer.Associated with this problem, but of less importance, is the desirability of expressing the total amount of chemical change per unit of energy absorbed, e.g. as the so-called G-value or radiation yield per 100 eV. Whilst a case can be argued for the retention of the use of ionic yield for gaseous reactions, there seems to be little advantage in this for liquid phase reactions as long as the chosen value of W for the liquid concerned has little more warrant than that it is the author's persona1 preference. 2. THE PRIMARY RADIATION YIELD.-Views have been advanced that a propor- tion of the energy absorbed is used in forming ultimate molecules and another fraction in forming radicals. No one doubts the latter but many will have some reservations about the former and about the validity of assigning immutable values G, and GR respectively to the yields.It has been said that the only authentic value of G, will be that corresponding to " precise agreement between independentF. S . DAINTON 11 reactions ”. But if solutes are chosen which are modified by one radical in such a way that this modification can be reversed by another, then G R obs will necessarily be a function of the solute chosen. And even if solutes are employed for which one radical does not reverse the effect of the other, the proportion of the total energy used to create radicals as judged by the chemical reactions they initiate may still depend on competition for the radicals of the solute with other reactions leading to radical destruction. The true value of G R may therefore be much larger than found by such methods, and, if this argument is valid, maximum values of G R obs are the more significant.We may then find a high efficiency of energy utilization. 3. TRACK DENSITY EFFECTS.-Earlier in this introduction 1 mentioned that track density considerations underlie most of our thinking in this field. In single-hit processes, differences between x-rays and X-rays have long been ascribed to the gross differences of mean track density. There are now indications that for a given type of radiation, differences in track density might arise from quite small changes of energy with consequential variations of the ionic yield. For example, with X-rays the mean track density appears to be substantially constant in range 30-200 kVp and evidence will be presented that chemical changes in this energy range may differ significantly from those induced by say, 1 MeV electrons where the mean track density is considerably less.If this should be established it is a point of considerable importance to the radiobiologist. initial distribution of the primary products, the majority of authors still deduce overall rate expressions based on the assumption of a uniform concentration of initial radicals. In some cases this treatment fails to account for the observed kinetic behaviour, but in others it is surprisingly successful. This fact poses some interesting problems. At very high dose rates we may expect complete track overlap and the initial distribution may in fact be almost uniform, but at low dose rates this may rarely be so.Chain reactions which are initiated by the radicals offer a promising field of investigation, the study of which may be expected to provide interesting information concerning track distribution. A simple example may make this point clear. If the mean distance from a radical in one track to a radical in an adjacent track is x, the probability that a chain of life r sec carried by reaction centres possessing a mean diRusion coefficient in the medium of I) will be terminated by mutual interaction with a reaction chain initiated in an adjacent track, will be high if 2Dr > x2. This is the reason, stressed elsewhere, why the kinetics of the radiolysis and photolysis of H202 are identical, and it is justifiable to apply “ homogeneous kinetics ” to the radiolysis, and why chain reactions in gases, e.g.the spin isomerization of hydrogen, can be treated in this way. Other systems are known which, when initiated photochemically (and therefore uniformly) may cause reaction to proceed at the same overall rate as when initiated radiochemically, and yet show entirely different reaction kinetics in the two sets of circumstances. and more will be said to-day about this subject. Views of great diversity have been expressed, but in all the theories which have been proposed, it is always conceded that the action of radiation on water is to produce inter alia some species capable of oxidation and other(s) of reduction of certain solutes, and that most of these species would have a transient existence in water even if the solute were not present.The basis for effective theorising on this subject would seem to be know- ledge of the following for each reaction of this kind : (i) a complete material balance under all conditions under which the reaction is carried out ; (ii) a thorough study of yields and equilibria as a function of dose rate and pH; (iii) measurement of the actual redox potential of the system under ail these conditions; (iv) the effect of additives which complex with the oxidized or reduced form of the solute. 6. NON-AQUEOUS SYSTEMS.-such a large proportion of current published work in radiation chemistry is concerned with water or aqueous solutions that it is often 4. UNIFORM OR NON-UNIFORM KINETICS.-DeSpite the known nOn-UnifOrm 5. OXIDATION AND REDUCTION REACTIONS IN WATER.-MUCh has been Written12 GENERAL INTRODUCTION overlooked that in some respects the investigation of non-polar liquids and vapours offers a better opportunity for elucidating the general principles of radiation- chemical changes. This difference was better realized 25 years ago by such pioneers as S. C . Lind and W. Mund. The outstanding advantage appears to be the virtual absence of secondary processes involving electron transfer ; and the papers by Burton and Gordon, and by Prevost-Bernas, Chapiro, Cousin, Landler and Magat are modern examples of the exploitation of this advantage, which one would like to see emulated. In each paper, very interesting general points are raised, such as the astonishing range of variation of radical yields (table 2 of Magat’s paper), and the energy transfer mechanism found in aromatic systems, which require discussion. 7. BIOLOGICAL SYSTEMS.-These are of particular interest in radiation chemistry, because arising from their study have come several new ideas, e.g. protection and sensitization, which may find application in simpler systems. Furthermore, many of us must always have at the back of our minds the very great importance of the effects of radiations on living cells. We cannot help but feel that had Dr. Douglas Lea still lived, Itis contributions to this discussion on all topics-but this one in particular-would have been both helpful and stimulating.

ISSN:0366-9033    

DOI:10.1039/DF9521200009

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

I. THE PRIMARY ACT RADIATION ABSORPTION AND ENERGY LOSS BY PRIMARY AND SECONDARY PARTICLES BY F. W. SPIERS Department of Medical Physics, The University, Leeds Received 14th February, 1952 The processes of absorption of ionizing radiation are reviewed and their relative importance is discussed in terms of quantum energy. Examples of the energies of secondary importance is discussed in terms of quantum energy. Examples of the energies of secondary electrons are given for commonly used radiations. The modes of energy loss by fast charged particles are described and the magnitude of the energy required to produce one ion-pair is discussed in reIation to particle typ:: and speed and the pro- perties of the absorbing medium. Some indication is given of the initial spatial dis- tribution of the ionization.The purpose of this article is to review the processes by which ionizing radi- ation transfers energy to matter and the manner in which secondary electrons dispose of the energy thus acquired. Knowledge of radiation absorption pro- cesses is now very complete and can be translated into forms directly applicable to the problems of radiation chemistry and radiobiology. The theory of the loss of energy by fast charged particles is also well founded and experimentally estab- lished-for example, by accurate a-ray range data. In some directions, however, knowledge is less complete. The stopping powers of various media for electrons are at present deduced from the more precise information on a-particles; the energy lost by an electron in producing one ion-pair in water is inferred from the value for a-rays in water vapour and the partition of this cnergy between ioniz- ation and excitation of the water molecules can only be very roughly estimated.Nevertheless a useful qualitative picture can be given of the initial distribution of ionization in a liquid, which accords with a considerable body of experimental findings on the chemical effects of radiation. A number of excellent reviews have already appeared and, among them, those of Gray 1 and Lea 2 contain most of what is known or can be reasonably surmised about this subject at the present time. INTERACTION OF RADIATION WITH MATTm-When electromagnetic radiation traverses matter it is attenuated by scattering processes and by transference of energy to atomic electrons.For X-rays and y-rays the photon energy may be transferred in whole or in part to an electron, with the result that the latter is set in motion with an energy greatly in excess of its binding energy in the atom or molecule. Unlike the photon, this pro-jected electron has a limited range in liquids and solids and usually dissipates all its energy in the medium in which it arises. For this reason the radiation energy transformed to kinetic energy of secondary electrons is referred to as the real or true energy absorption in the medium. Some of the scattered electromagnetic radiation will be subsequently absorbed in any extended medium and thereby contribute also to its real energy absorption. Factors such as multiple scatter and the extent and shape of the medium make it difficult to calculate the magnitude of this scatter contribution theoretically. The usual and most satisfactory procedure is to measure the 1314 ABSORPTION AND ENERGY LOSS total dose or energy absorption due to both primary and scattered radiation under conditions which accurately simulate those of the experimental irradiation.The energy absorption of a medium is determined by the sum T~ -t ua + K~ of the absorption coefficients which refer respectively to the photoelectric, Compton recoil and pair production processes. Since these depend in different ways on the energy of the radiation, it is of interest to indicate briefly their relative im- portance in the absorption coefficient of water. PHOTOELECTRIC ABSORPTION COEFFICIENT .r,.-This process is important for photon energies below 0-2 MeV and its magnitude varies approximately as $24, Values of T~ may be taken from compilations of experimental results 3, 4 or from empirical formulae.2> 5 The energy given to the electron is the photon energy less the binding energy of the electron which, for water, can never amount to more than 0.5 keV for an electron in the K shell of the oxygen atom.COMPTON RECOIL COEFFICIENT u,.-The magnitude of this coefficient, which gives the fraction of the photon energy imparted to the recoil electron, is cal- culated by the formulae given by KIein and Nishina.6 The mean energy of the recoil electron E is a& liv but the electron energies are continuous over a range given by h where 6A = - (1 - cos 4).The recoil electron energy will therefore vary from mc 0, when the photon is undeflected, to a maximum when SA = ~ for a photon scattered through c,b -- 180". PAIR PRODUCTION COEFFICIENT K,.-The absorption coefficient K for pair production is proportional to 2 2 and increases with increasing photon energy above a threshold energy at 2mc 2 (1.02 MeV). The energy available as kinetic energy of the created electron pair is therefore hv - 27nc2 which, in the most probable distribution, is divided equally between the positron and negatron. The real energy absorption is given by 2h mc Values of K have been calculated by Heitler 7 and values of K~~ for air and of real energy absorption of a number of elements relative to air, have been given by Mayneord.8 If absorption data for any medium and the radiation energy are accurately known then the energy absorption in ergs/g can be deduced from a measurement of the dose in rontgen," at least in the energy range up to 2 MeV.The energy absorption in ergs/g r for water and for a number of values of 2 is shown in fig. 1 over the range 0.01 to 1.0 MeV. At low energies the photoelectric coefficient predominates but above 0-2 MeV the absorption is due almost entirely to the Compton recoil process and depends only on the number of electrons per g. The energy absorption per r for water shows little variation throughout this range of photon energy, a fact which justifies the use of the rontgen as a unit of dose for therapeutic irradiation of the aqueous tissues of the body. Fig. 2 shows the absorption of radiations of high energies above 1 MeV, and indicates the region where pair production is effective.The energy absorption is given relative to water since in this region the rontgen becomes unsuitable as a unit of dose. The * The rontgen is defined as " the quantity of X or y radiation such that the associated corpuscular emission per 0.001293 g of air produces, in air, ions carrying one electro- static unit of quantity of electricity of either sign ". It is equivalent to an energy absorp- tion of 84 ergs in 1 g of air,F. W. SPIERS 15 absorption again becomes dependent on 2, the coefficient increasing with the photon energy. piled in table 1 for a number of photon energies in the range 10 keV and 1-25 MeV, where radiation absorption is due to the photoelectric and recoil processes.Radiations having average photon energies corresponding to the first four lines of table 1 are produced by X-ray tubes operating at approximately twice the listed DATA FOR WATER FOR SOME COMMONLY USED RADIATIONS.-Data are COm- FIG. 1. photon energies with suitably chosen filters. The fifth radiation listed is the mean y-ray energy of the C060 isotope. Proportions of the two types of secondary electron are given, together with the proportions of the dose to which they give rise. Large differences between photoelectron energies and the mean energies of the recoil electrons are evident and corresponding differences exist between their ranges. In these circumstances it becomes difficult to assign to the secondary electrons a mean energy which has much practical significance; still less can the electrons be regarded as having a mean range.TABLE DA DATA FOR WATER * photon energy 10 KeV 25 ? 3 50 ? 3 100 ? ? 1.25 MeV types of electron recoil photo Y ? 3 ) Y Y 9 ) :< numbers ”/, contribu- electron of elcctrons tion to dose energies kV ‘ $ ~ ~ ~ ~ ~ 4.2 95-8 41.0 59.0 84.0 16.0 97.4 2.6 100.0 0.1 99.9 2.9 97.1 30.0 70-0 84.0 16-0 100.0 0.18 9.5 1.1 1 24.5 4.05 49.5 13.7 99-5 600-0 0.006 2.3 0.06 12-1 0.52 42.0 4.35 140.0 2150.0 ion pairs per micron mean for 1160 148 629 74 279 57 112 72 25 - 10 10 _______- mean P and R 147 - 72 - 42 - 0.0 (also pair production < 0.2 %) * W taken as 28 eV per ion pair ; 9 range data from Lea 2 and Siri.10 MEAN ION DENSiTY .-If an electron has an initial energy E in keV and we follow Gray 9 in taking W as 28 eV per ion pair in water, a mean ion density can be calculated as - A = 1000 E/28 R = 35-7 E’/R ion pairs per micron, (3)16 ABSORPTION AND ENERGY LOSS where R is the range in microns of the electron of energy E.Ion densities calculated in this way are listed separately in table 1 for photoelectrons of single energy E p and recoil electrons of average energy ZR. The mean ion densities given in the last column of table 1 have been derived by taking the summated electron energies npEp + n R G and a total range equal to npRp + nRRR, where n p , Ep, R p and F ~ R , ER, RR refer to the numbers, energies and ranges of the photo- and recoil-electrons respectively. The variation of this mean ion density over a range of photon energies from 10 keV to 1.25 MeV is shown in curve J L of fig.3. The broken curve I in this figure is taken from calculations of mean ion densities by Gray l b who used the mean recoil and photoelectron energy and the range cor- responding to it as given by Lea.2 The curves show similar qualitative features but differ in absolute values by a factor of two in the region of photon energies between 30 and 70 keV. The two methods of calculating the mean ion density become identical where only one type of secondary electron is involved, but differ when significant numbers of both recoil and photo-electrons of widely different initial energy are produced. FIG. 2. The points B, C and D in fig. 3 are taken from some unpublished work which Prof. H.E. Johns has very kindly allowed me to use. Cormack and Johns 11 have made. detailed calculations of the energy distributions of secondary electrons from a number of commonly used radiations. By using the complete spectral distribution of the radiation, they have derived the numbers of electrons set in motion with energies within successive 5 keV intervals and from these they have calculated mean ion densities in a manner similar to that used by the writer. It will be seen that the mean ion densities obtained for complete spectral ranges by the detailed analyses of Cormack- and Johns are of the same order as the values given in curve IT in fig. 3 by the elementary calculation for photons of single energies. It is reasonable to take curve I1 in fig. 3 as giving the order of the mean ion density in relation to the average photon energy of a heterogeneous radiation.Tt is evident, however, that a mean ion density describes the ioniza- tion more adequately at both low and high photon energies than in the inter- mediate energy range. At low energies photoelectrons predominate arid at high energies the recoil electrons, although not homogeneous, have energies mainly in the region where specific ionization is a slowly varying function of electron energy. Tn the intermediate range of photon energies the interpretation of a radi- ation effect may need to be made in terms of a distribution of ion densities rather than a simple average.F . W . SPIERS 17 FIG. 3. FIG. 4.18 ABSORPTION A N D ENERGY LOSS Some graphical data given by Cormack and Johns illustrate in detail the energy distributions produced by heterogeneous X-rays.Fig. 4 reproduces parts of fig. 1 and 6 of these authors’ paper and shows the spectral distribution of dose for a lightly filtered 200 keV beam (B) and a heavily filtered 200 keV beam (C). Fig. 4 also gives the instantaneous distribution of electron energies at a point in water for these two radiations; the large differences in the character of the photon spectrum are present to a much smaller extent in the electron energy dis- tribution. Fig. 5 gives the distribution of ion densitjes among the secondary electrons for the radiations B and C together with the relative distribution for C060 y-rays (Cormack and Johns,ll parts of fig. 9 and 10). Fig. 5 also gives a FIG.5. representation derived from Cormack and Johns’ data of the dose distributed among the ion density values, which incidentally leads to the evaluation of a mean ion density. The results illustrate very clearly the small range of ion densities for the high energy C060 y-rays (the range is likewise small for Ra y-rays), and the difficulty of assessing the significance of a mean ion density in the intermediate X-ray range from 100 to 200 keV. Another way of arriving at a mean ion density would be to take a mean of the ion densities of the recoil and photoelectrons weighted according to their con- tributions to the dose (Gray, private communication). Values nearer to curve I in fig. 3 are then obtained and such a mean might have more significance for chemical effects.If an average is taken in this way, however, for the distribution shown for Cormack and Johns’ radiation C in fig. 5 , a mean ion density coincident with their value is obtained-i.e. one which is below that given by the various approximations considered.F . W. SPIERS 19 ENERGY LOSS BY CHARGED PARTICLES.-When a charged particle traverses matter it loses energy by inelastic “ collisions ” with atomic electrons in which the atoms may be excited or ionized. The electrical disturbance caused by the passage of the charged particle, and therefore the energy transferred to the atomic electron, depends on the particle’s charge and speed, on the binding energy of the orbital electron and on the closeness of the collision. In general, ejection of an atomic electron, i.e.ionization, will occur in close collisions near the track of the moving particle and excitation of orbital electrons will occur in glancing collisions at greater distances from the track. The loss of energy per unit length of the path of the particle can be written : where N is the number of atoms of atomic number 2 per cm3, z and 7) are the charge and velocity of the particle and e and rn are the charge and mass of the electron. B is the “ stopping number ” of the medium which can be calculated from the theory given by Bethe 12 based on quantum theory. A recent review and statement of this theory has been given by Bohr.13 The Bethe formula for the stopping number for electrons is R == 2 In --- - In (I - /32) - /3*, (23 where E i s the ‘ mean excitation potential ’ of the atom.The function B increases slowly with particle energy and hence dT/dx at first falls as the particle energy rises, owing to the 2.2 term in eqn. (4). This term becomes constant as w approaches the velocity of light and consequently the dT/dx curve passes through a broad minimum and then rises slowly for very high particle energies. The formula, which is derived for a simple hydrogen-like atom, can be applied to other electronic configurations if the mean excitation potential Ecan be deter- mined. This is usually done empirically by adjusting E to fit known energy- range data, Since experimental data of sufficient precision are difficult to obtain with electrons, values of have been deduced by fitting the theoretical formula for a-particles to a-particle range data.Mano 14 has in this way derived values of E which, in the absence of other data, are used in calculating B for electrons. From Mano’s values of 16 eV and 86 eV for H and 0 atoms, a logarithmic mean value of 69 eV is obtained for water, and this figure is used in Lea’s calculations of energy loss and range. Cormack and Johns 11 have used a value of 80 eV for ’E for water in some of their calculations but this makes only a very small difference in the value of dT/dx. The variation of energy loss of fast electrons in water is shown by the full curve A in fig. 6. The formula for B is not valid when the particle velocity approaches that of the orbital electrons of the atoms of the medium, and B is zero for velocities below the lowest velocity of the atomic elec- trons.The energy loss therefore follows the familiar Bragg type of curve showing a maximum and then a sharp fall to zero at low energies. Fermi 15 modified the Bethe formula to allow for polarization of the medium by the field of the moving particle which effectively reduces the value of B. Halpern and Hall 16 have elaborated this idea, replacing Fermi’s single frequency model of the absorbing atom by a multiple frequency model and deriving cor- rections which are illustrated in fig. 6. For water, Halpern and Hall’s correction begins to show at energies above 1 MeV, and when applied to the collision losses calculated on Bethe’s formula, gives the broken curve B in fig. 6, which only departs significantly from A at high energies. The insert in fig.6 relates to carbon, for which the corrections suggested by Halpern and Hall amount to a decrease in the stopping power of some 10 %, even at low electron energies.20 ABSORPTION AND ENERGY LOSS Halpern and Hall find thcir value of the stopping power of carbon for 10 MV /3 particles to be in good agreement with measured values at this energy. A careful comparison by Gray Ic of the ionization produced by Ra y-rays in a carbon- walled ionization chamber and in a gelatin-walled chamber did not reveal any differences of the magnitude suggested by Halpern and Hall’s theory for carbon as against water. Besides energy losses by collision, a charged particle can be accelerated in the field of the nucleus and lose energy by emitting radiation. This radiative process-the production of Bremsstrahlung-is insignificant for heavy particles but becomes important for electrons at energies greater than 2 MeV. The curve C in fig.6 indicates the incidence of radiative loss by electrons in water. The energy loss in this process increases roughly in proportion to the particle energy and is proportional to the square of the atomic number of the medium traversed. The expression for the stopping number B of light elements for a-particles can be simplified by omission of the relativity corrections necessary for P-particles. Since corrections for the Fermi effect depend on the particle’s speed and arise only for very high energies, the collision loss for cc-particles can be written dx 112 L.2 In (y), (6) d T 4re4z2NZ _. - - ___ where the symbols have the same meaning as in eqn.(4) and (5). Since the charge z is now twice that of the electron, the energy loss of the v,-particle is four times that of an electron of the same speed. The a-particle, however, has a mass some 7000 times greater than the electron and consequently much greater energy for the same speed. An cc-particle having the same energy as an electron has a much lower velocity and an energy dissipation of the order of 1000 times that of the electron. The formula (6) is not precise below 1 MeV because the cc-particle velocity is then comparable with the velocity of the orbital electrons in the absorbing atoms. A comprehensive review of the stopping powers of gases and liquids for a- particles has been given by Gray lL7 in which substantial evidence is presentedF .W. SPIERS 21 for the validity of the Bragg additive law of atomic stopping power. The stopping power of water, as measured by Michl17 and by Philipp,lS was greater, however, than the value for water vapour which itself followed the Bragg law. The only recent determination of the stopping power of water for a-particles, made by Appleyard,lg has confirmed the earlier experiments and gives a value about 12 % greater than that for water vapour. Gray 1" has pointed out that evidence of discrepancies for other liquids is not entirely convincing and until more extensive experimental data are available it is safest to assume the validity of the Bragg law in calculating the stopping powers of gaseous and liquid compounds. expended in creating one ion pair in air has been well established as 32.5 eV for electrons and 35.1 eV for cc-particles.1" The value of W for electrons is dependent on the electron energy only at energies below 10 keV.According to a formula given by Gerbes,zo W increases by 1 % as the energy decreases from 100 keV to 10 keV, and by amounts given in table 2 for energies less than 10 keV. TOTAL ENERGY EXPENDED IN THE FORMATION OF ONE ION PAIR.-The energy w electron energy in keV TABLE 2 10 5 3 2 1 % increment in W over value for fast electrons 1 3 5 8 17 Appleyard 21 has recently compared the mean energies expended per ion pair by a-particles traversing air and water vapour, and finds the value of Wfor water vapour to be only 88% of that for air. Applying the same ratio to ionization by electrons the value of W for electrons traversing water vapour would be 28 eV.This lower value would appear to be in closer accord with inferences from chemical experiments and has been used in calculating the specific ionizations given earlier in the paper. The mean energy expended per ion pair formed is also of the order of 30-35 eV For many gases in spite of great variation in the ionization potentials of the mole- cules concerned. Fano 22 has pointed out, however, certain complementary aspects of the partition of energy between ionization and excitation. In atoms having rather loosely attached electrons and low ionization potentials, the value of W is maintained by the considerably greater proportion of energy which then goes in exciting electrons to oscillate intensely in states of low excitation.If the electrons are stiffly bound and the ionization potential is high, excitation is less probable and less energy is expended in that process. Hence although W is nearly constant, the proportions of the energy appearing as ionization and excitation will vary with the type of atom or molecule irradiated. The value of W determines the number of ions formed by a given dose and this, for some substances at least, will be roughly constant. On Fano's view, however, the accompanying number of excitations may differ considerably from one substance to another. SPATIAL DISTRIBUTION OF THE IoNs.-The ion pairs created by the fast particle are not uniformly distributed along its track ; they occur discontinuously in groups or clusters of varying size.Lea 2 calculated the distribution of cluster size along electron tracks and also derived the contributions made by branch tracks, or delta rays, which arise both from electron and heavy particle tracks. Gray 9 has set out the presumed distribution of ions on the present available evidence and the salient features only of the distribution will be mentioned here. Less than half the clusters formed along an electron track contain one ion pair, 5 % of the clusters may contain as many as 16 ion pairs and the average cluster has 3 ion pairs. If a &ray track is produced by the electron, its characteristics will be similar to those of the primary particle and need not, in this case, receive special attention. The average cluster spacing can be derived simply from the mean ion demity as S =z 3,'E microns.The mean cluster spacings for the secondary electrons produced by the photon energies between 10 and 1000 keV, -22 ABSORPTION AND ENERGY LOSS are shown in fig. 7 ; they relate to the data used in table 1, and are derived from curve I1 in fig. 3. Since the positive ions probably lie within a few mp of the track, and the negative ions (produced by the ejected electrons) are some 15 mp from the track, it is evident that only at photon energies below 8 keV are the cluster separations of the same order as the cluster dimensions. At 1.5 keV the cluster spacing is the same as the mean separation of the positive ions and there is a more or less uniform distribution of positive ions along the track.The mean ion densities for a-rays in water are of the order of 3500 to 4500 ions/,u and the density of positive ions along the track is several times greater than that for a 1 keV electron. The conditions close to the track are such that dissociation of the positive ion H20f into H+ and OH leaves a very high concentra- tion of OH radicals, which can form H202, and a central region of positive charge which may well produce electric fields high enough to affect the spacing and movement of the negative ions surrounding it.23 With the exception of reactions sensitive to H202 this concentration of ionization is too great to be very efficient chemically and, in one case at least, the chemical action of radiation has been FIG. 7. accounted for wholly by effects outside the immediate vicinity of the cc-particle track.Thus Dale, Gray and Meredith24 showed that %rays, arising from the main track and carrying ionization out to distances of 1 to 2 p from it, could account for the observed x-ray inactivation of the enzyme carboxypeptidase. The proportions of the total ionization produced outside the a-ray column, re- garded as having a radius of 0.015 p, were given as below. initial or-particle energy TABLE 3 2 4 6 8 MeV proportion of dose beyond Y = 0.015 p 2.5 6 9 11 % The highest ion densities which can be produced are those along the tracks of atomic particles and fission fragments, the latter producing values as high as 130,000 ionslp. These ionizing particles do not lend themselves readily, however, to chemical experiments.Nevertheless, a very wide range of ion densities lies between the low ion densities associated with y- and hard X-rays and the high ion densities produced by ct-rays. Conditions in and near the track of the densely ionizing a-particle differ not merely in degree but in character from those associ- ated with fast electrons. Transition from the low to the high ion density condition might be expected when the cluster spacing along the track is of the same order as the separation of the positive ions in the cluster. = 4 mp This would occur atF . W . SPIERS 23 for electrons produced by 1.5 keV X-radiation. Effects which depend on the distribution of ions within the cluster would not be expected to vary much with mean ion density until 3 approaches values of the order of 15 mp.Little can be reported of the spatial distribution of excitations except that it will be unlikely to differ considerably from that of the ionizations. Although the excitations will be likely to occur farther from the track of the particle than the positive ions, the majority will still be within a few millimicrons of the centre of the track. The spacing of excitations along the track is probably very similar to the linear distribution of ions. If the In (Z,nv2/E) term in eqn. ( 5 ) and (6) is replaced by two separate terms in Ei and ze, for the mean ionization and mean excitation potentials respectively, the value & might be expected to be perhaps 10 times less than E . Since the terms are logarithmic, the excitation term might then be twice the ionization term at low particle energies.Over most of the 20 $0 6 0 80 FIG. 8. particle range, the difference in the separate log terms would not be great and consequently the effect on dT/dx, and therefore on &, would be small. APPENDIX.-h addition to the data given in table 1 and fig. 3, the method of Cormack and Johns 11 has been applied to the radiation spectrum of an X-ray tube operating at 100 keV. The mean photon energy of this beam is then derived as 53 keV and the numbers of recoil and photoelectrons generated in water are respectively 78 % and 22 %. The complete distributions of the secondary electrons are shown in fig. 8 where they are compared with the single recoil and photo- electron energies used to compile the data for the 50 keV photons in table 1. The mean ion densities, computed by methods I1 and I11 in fig. 3, are 61 and 105 ion pairs/micron and these lie respectively close to curve 11 and a little below curve I11 in fig, 6 .24 GASEOUS IONS AND THEIR REACTIONS 1 (a) Gray, Proc. Camb. Phil. SOC., 1944, 40, 72. (b) Gray, Brit. J. Rad., suppl. 1, 2 Lea, Actions of Radiations on Living Cells (Cambridge University Press, 1946). 3 Allen, Compton and Allison, X-ray in Theory and Experiment (Macmillan & Co., 4 Victoreen, J. Appl. Plzysics, 1943, 14, 95. 5 Walter, Fortsch. Geb. Roentgen, 1927, 35, 929. 6 Klein and Nishina, 2. Physik, 1929, 52, 853. 7 Heitler, Tlie Quantum Tlteory of Radiation (Oxford University Press, 1936). 8 Mayneord, Brit. J. Rad., suppl. 2, 1950, 138. 9 Gray, J. Chim. Phys., 1951, 48, 172. 1947, 7. (c) Gray, Brit. J . Rad., 1949, 22, 677. 1935). 10 Siri, Isotopic Tracers arid Nuclear Radiations, (McGraw-Hill Book Co., 1949). 11 Cormack and Johns, Brit. J . Racl. (in press). 12 Bethe, Handb. Physik, 1933, 24 (i), 273. 13 Bohr, Det. Kgl. Dattske Vidensk. Selskub, Math-fys. Meddel., 1948, 18, 8. 14 Mano, J . Physique Rad., 1934, 5, 628. 15 Fernii, Physic. Rev., 1940, 57, 485. 16 Halpern and Hall, Physic. Rev., 1948, 73, 477. 17 Michl, A k d Wiss. Wien., 1914, 123, 1965. 18 Philipp, 2. Physik., 1923, 17, 23. 19 Appleyard, Proc. Canib. Phil. SOC., 1950, 47, 443. 20 Gerbes, Ann. Physik, 1935, 23, 648. 21 Appleyard, Nature, 1949, 164, 838. 22 Fano, Physic. Rev., 1946, 52, 44. 23 Read, Brit. J . Rad., 1949, 22, 366 ; 1951, 24, 345. 24 Dale, Gray and Meredith, Phil. Trans. Roy. Sue. A , 1949, 242, 33.

ISSN:0366-9033    

DOI:10.1039/DF9521200013

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

24 GASEOUS IONS A N D THEIR REACTIONS GASEOUS IONS AND THEIR REACTIONS BY H. S . W. MASSEY Department of Physics, University College, London Received 25th Januury, 1952 The first part is concerned with the nature of gaseous ions. Information available from experiments on the formation of clusters by alkali metal ions is discussed. Clusters are formed not only with polar molecules but also rare gas atoms. The recent work on the molecular ions of the rare gases is also described. Some special features of negative atomic and molecular ions are summarized and attention is drawn to the importance of ions existing in metastable states. The second part is concerned with the rates of reactions in which ions are involved. These are classified under five headings : reactions leading to production of positive ions, formation of negative ions, change in the nature of the ions, detachment of electrons from negative ions and recombination. The different processes which can occur in these various categories are listed and some indication of the factors which determine their rates is given. The behaviour of ions in a condensed medium is very complicated and it is natural to turn towards gaseous ions in the hope that the phenomena concerned will be so much simpler that a start may be made in understanding them.This is only partly true for even the behaviour of ions in gases is often unexpectedly complicated as will be shown below. In this paper it will only be possible to give a very cursory account of certain selected aspects of gaseous ionics.The first part will be concerned with the nature of ions in gases and the second with the rates of reactions in which ions are concerned.H . S . W. MASSEY 25 ion, positive or negative, does not remain long in its atomic form if it is moving in a gas at ordinary pressures which has not been very specially purified. This is mainly because the charge on the ion leads to a considerable attraction between the ion and a polar molecule such as a water molecule, which is normally present in appreciable concentrations in an impure gas. The failure to realize the im- portance of cluster formation has vitiated many experiments designed to in- vestigate the properties of atomic ions. In the years just before the war some progress was made towards the establishment of quantitative methods of studying cluster formation by positive ions and post-war developments have shown that atomic ions may even combine readily with atoms, usually regarded as chemically inert, to form quite stable molecular ions.We shall begin by summarizing the information which has been obtained about the formation of complex positive ions. 1.1. Polar cluster formation by alkali metal ions.-One of the most effective ways of studying the nature of a gaseous ion is by measurement of its mobility in the gas concerned. A technique for carrying out such measurements under carefully controlled conditions of gas purity has been developed by Tyndall and Powell.1 By taking very special precautions they were able to measure the mobilities of the alkali metal ions in rare gases of such purity that no cluster formation occurred during the passage of the ions through the experimental chamber.Munson and Tyndall2 were then able to investigate the effect on the mobility of the addition of small but measurable quantities of water vapour to the rare gas. The mobility was found to be very much reduced by an amount which was independent of the concentration of water vapour down to very low concentrations (to partial pressures as low as 4 x 10-4 mm). Thus the mobilities of Li+, Na+, Kf, Rb+ and Csf ions in pure helium are 25-6, 24.2, 22.9, 21-4 and 19.6 cm?/V sec respectively whereas in helium containing a small amount of water the corresponding values are 11.70, 11.1 5, 11.85, 12.8, and 13.4 crn?/V sec. Similar results are obtained for the other rare gases. As the reduc- tion is independent of the water vapour concentration it follows that the size of the cluster is also independent of this concentration.It is possible by using Langevin’s theory 3 of the mobility of the ions to place an upper limit to the masses of the clustered ions and hence to the number of clustered molecules. This limit comes out to be about 6 for LiT, Na+ and K-+ ions. If an estimate can be made of the effective radius of a cluster the number of attached molecules can be determined more definitely. In this way it is found that the number of attached molecules is about 4 for Li+ and this is in agreement with predictions made by Bernal and Fowler 4 using their theory of the structure of liquid water. On the other hand it seems quite certain that Csf ions attach several molecules whereas according to.Berna1 and Fowler, they should not.A remarkable feature of the results is the rapidity of the association reaction. There is no evidence in these experiments of the presence of any ions intermediate between the unclustered and fully clustered varieties. The only reasonable ex- planation of this is that the rate determining process is the first attachment. Once this has occurred the full cluster builds up very rapidly. This is consistent with the requirement that the first attachment can only take place in a three-body collision. When the frequency of such collisions is examined it is seen, however, that even the first attachment takes place very much more rapidly than would be expected.Thus the estimated probability of an ion making a three-body collision while passing through the experimental chamber is only about 2 x 10-2 when there is 1 % water present. Nevertheless observable cluster formation is found in xenon when there is only 0.005 % of water present. The reason for the initial attachment taking place so much more rapidly than anticipated is still not clear and remains an important subject for further investigation. 1. THE NATURE OF GASEOUS IONS.-It has long been known that an atomic The reduced mobility must clearly be due to cluster formation.26 GASEOUS IONS AND THEIR REACTIONS 1.2. Association of alkali metal ions with inert gas atoms.-In 1939 Munson and Hoselitz,s who were studying the mobility of Lit ions in xenon, found evidence for the existence of ions with smaller mobility than the atomic ions which would not be eliminated even with the most careful purification of the xenon.This suggested that the heavier ion might be a molecular ion of the type LiXef. To investigate the matter further the mobility of Lif ions in helium at liquid hydrogen temperature was measured. In this case polar impurities must certainly have been frozen out and yet again it was found that an ion of smaller mobility was present. This could only have been LiHe+. From the proportion of clustered ions, their mobility and its variation with temperature it was possible to estimate the number of atoms attached in a cluster and also the dissociation energy. It was found that at least one and probably two atoms of Kr and Xe may attach to Li+ ions at room temperatures, the dissociation energy of the cluster being about 0.3 eV for Kr and 0.4 eV for Xe.At room temperature there is very little attachment of He atoms but at least one such atom is attached at liquid hydrogen temperatures, the dissociation energy being about 0.07 eV. This is rather smaller than that calculated by Meyerott 8 using quantum theory. 1.3. Molecular ions of the rare gases.-The existence of stable He2+ molecules has been known for some time and it was further known from the intensity of emission of helium band spectra that there must be a considerable concentration of such ions in the positive column of a glow discharge in helium. Despite this it was not considered at all likely that if positive ions were withdrawn from such a source they would be mainly He2+ and not He+.Thus Tyndall and Powell6 used a helium discharge as a source of the He+ ions for their experiments on the mobility of these ions in their helium. They obtained the value 20 cm2/V sec for this mobility. A little later Massey and Mohr 7 calculated the mobility using the quantum theory of collisions and allowing for the resonant charge transfer process He-+ + He 3 He + He+. (1.1) Their result, 11 cm2/V sec, was much smaller than the observed value. In 1946 Meyerott 8 suggested that the observed value might actually be that for Hezf and this has since been confirmed by Hornbeck’s recent measurements.9 He finds a mobility of 10.8 cm2/V sec for He+ and 19 cm2/V sec for He2+. The recent history of this matter is even more remarkable.In 1949 Biondi and Brown 10 applied a new microwave technique to investigate the rate of loss of electrons in a helium afterglow. At low pressures (< 20 mm Hg) the main loss is due to diffusion and from the measurements the mobility of the positive ions could be determined. It was found to be 12 cm2/V sec which we now know indicates that the ions were He+. At higher pressures (> 20 mm Hg) recombina- tion becomes the most important process of electron loss. Under these conditions Biondi and Brown measured the recombination coefficient for the electrons and found the large value 2 x 10-8 cm3/sec. To explain this large coefficient Bates 11 suggested that the recombination process is one of dissociative recombination to He2+, viz.(1 .2) He2-t + e -+ He’ + He” in which the energy released by the recombination is used in dissociating the mole- cule. This explanation has been confirmed by a remarkable experiment suggested by Holstein and carried out by Brown.12 The recombination coefficient in helium afterglow containing a small amount of argon was observed and found to be too small for measurement whereas in pure helium it was at least 104 times larger. Measurement of the mobility of the ions in the mixed helium-argon afterglow showed that they were A+ and the low recombination coefficient can be ascribedH. S . W. MASSEY 27 to the absence of molecular ions. is due to reactions of the kind The large proportion of ions present as A+ (1.3) where He’ denotes a metastable or other excited helium atom.It appears that under the low pressure conditions in Biondi and Brown’s first experiments the proportion of He2+ is small but it builds up rapidly as the pressure increases. This would be expected as the He2+ ions must be formed by some secondary process the probability of which will increase with the pressure. Bates 11 made an estimate of the rate of: production of He2f ions by three-body collisions : He‘ + A +He + A+ + e, He+ 4- He + He --f He2+ + He, and found it to be of the correct order of magnitude to explain Biondi and Brown’s results. The absence of molecular ions in the helium-argon mixture experiments suggests that HeA+ molecules are either unstable or very weakly bound. On the other hand studies of recombination and diffusion of ions in neon and argon 13 show that Ne2+ and A2+ ions are quite stable and play a vital role in determining the rate of recombination just as in helium.The much greater stability of the homonuclear molecular ions is understand- able in terms of the quantum mechanics. The resonance degeneracy due to the identity of the HeHe+ and He+He, for example, increases the strength of the bond in the molecule. 1.4. Analysis of ionic constitution in a plasma.-In view of the somewhat un- expected nature of the positive ions formed in simple gases it is important to have available a convenient technique for determining the ionic constitution of a plasma. Boyd 14 has developed a velocity analyzer for this purpose which operates on the same principle as the linear accelerator.It has the great advantage of being a very compact instrument so that it may be inserted in a discharge plasma without appreciably disturbing the discharge and can be used to explore the variation in ionic constitution at different points in the plasma. With this instrument Boyd 15 was able to show that He2+ ions are the most abundant species in a glow discharge in helium even at quite a low pressure (0-006 mm Hg, with 100 mA discharge current). A large range of possibilities await the use of this instrument for the analysis of discharge plasmas. 1.5. Negative ions.-l.5.1. Atomic ions.-Although all elements possess stable positive atomic ions this is by no means true for negative atomic ions. Thus no rare gases form negative ions nor does nitrogen. The most stable nega- tive atomic ions are those of the halogen atoms.Thus the electron affinity of fluorine is 4.13 eV which is actually greater than the ionization energy of caesium. The electron affinities of other important atoms are H (0.747 eV), 0 (2.2), C1 (3.1), Br (3.6) and l(3-2). Another important aspect of negative ions, as distinct from neutral atoms and positive ions, is that the number of stable excited states of negative ions is very limited. It seems that 0-- possesses one stable excited state with very small binding energy but H- does not.16 Hasted 17 has recently produced evidence derived from a study of the rates of detachment of electrons from negative ions on collision with rare gas atoms, that not only 0- but also F- and C1- possess one stable excited state of low binding energy. This supports the conclusions arrived at by Bates 18 for 0- and F- from an empirical study based on extrapolation of the properties of neutral atoms and positive ions.In the gaseous phase no doubly charged negative ions such as 0 2 - are stable. Thus the energy of 0-- is about 6.5 eV greater than that of 0-. It may seem re- markable that doubly charged negative ions are certainly stable in electrolytes. The reason for this is cluster formation with polar solvent molecules. These molecules enclose the ion in an electrical double layer. Owing to the presence of (1 -4)28 GASEOUS IONS AND THEIR REACTIONS this layer the energy required to remove an electron from the ion is greater than it would otherwise be. This means that the electron affinity is effectively increased and this increase is large enough to make clustered 0-- ions quite stable in an aqueous solution.It follows also that the electron affinity of an atom such as C1 is much greater in solution than in the gaseous phase. Negative ions naturally form clusters when present in a gas containing a polar impurity just as do positive ions. Just as in electrolytes the presence of the cluster tends to increase the stability of the ion. 1.5.2. Molecular negative ions. 19-The diatomic molecules of most of the elements which form stable negative atomic ions, also form stable negative ions. Thus Fz-, C12-, Br2-, 12- and 0 2 - are certainly stable. In this connection it is of interest to note that the alkali superoxides such as KO2 are polar compounds K+02-.The electron affinity of 0 2 has been deduced 20 as 0-7 eV from a study of the energy relations in cyclic processes involving these superoxides. Although H2- is stable as a molecule it is unstable towards the dissociation into H2 and an electron. H2 has a negative electron affinity. The resonance degeneracy in homonuclear diatomic negative ions is a stabiliz- ing factor as with positive ions but it is not sufficiently effective to render stable such ions as Hez-. Certain molecules such as OH which are isoelectronic with halogen atoms form quite stable negative ions. 1.6, Ions in metastable states.-It is important to remember that many ions possess low lying metastable excited states of quite long lifetime. As ions are likely to be formed in these excited states with quite high probability they may play an important part in determining the rates of reactions in which the ions are involved.In the same way, for negative ions which possess a stable excited state this state is metastable and excited ions may persist for times greater than that spent by the ion in an experi- mental apparatus. A different kind of ionic metastability has been observed by Hippel and Condon 21 in the course of mass spectrograph studies of the products of dissoci- ation of certain hydrocarbon molecules by electron impact. Certain ions such as C4H10+ and C3H5+ break up while passing through the spectrograph showing that they are only metastable towards dissociation. Effects of this sort would be expected for complex ions as it is necessary that the surplus energy should con- centrate on a particular degree of freedom before dissociation can occur.As the energy may be unduly dispersed among many degrees of freedom owing to the complexity of the system, the time before chance leads to a sufficient concentration in the dissociation mode may be quite long. Thus the electron affinity of OH is 2.1 eV. Some examples are given for positive ions in table I. TABLE 1 .-METASTABLE STATES OF CERTAIN POSITlVE IONS ion metastable excitation life-times states energy (eV) (set) - 0' 2D 3.3 *P 5.0 0.2 N+ IS 4.1 0.9 ID 1.9 300.0 2. REACTIONS INVOLVING IoNs.-We may classify the reactions which can occur in which ions are involved under the following headings : (a) reactions leading to production of positive ions, (b) reactions leading to formation of negative ions, ( c ) reactions leading to change in the nature of the ions, ( d ) reactions leading to detachment of electrons from negative ions, (e) recombination processes.H .S . W. MASSEY 29 Under these headings a great number of processes are included and it will be quite impossible to do much more than list the most important of them and give some guide as to the factors which determine their rates. In most instances it will be convenient to specify the rate in terms of an effective cross-section Q such that the chance of a collision leading to the reaction concerned in which one of the reactant atoms or molecules travels a small distance d through a gas containing N of the other reactant molecules CM-3 is Ned.2.1 Reactions leading to production of positive ions.-Typical reactions of this kind are : A + e --f A+ + e + c ( a l ) ionization by electron impact (02) polar dissociation (a3) photoionization (a4) ionization by positive ion impact (a5) ionization by neutral atom impact (a6) ionization by metastable or excited atoms (a7) associative ionization. AB f c --f A -t B- 4- D A + Lv --f A- 1- c A + B --f A+ -I- Bf t e A f B --f A+ 4- B -l- e A + B’ --f A ‘ + B 4- e A $- B’+AB+ + e Ionization by electron impact is an important process. The cross section rises from zero for electrons with energies EO just sufficient to produce ionization to a maximum of the order of the gas kinetic cross-section of the atom at an energy between 2 and 4Eo after which it falls off gradually. In general the velocity variation of the cross section is similar to that for ( a l ) but the maximum is of the order 10-3 to 10-4 times as large. It has been observed in 0 2 , NO, CO and Br2.22 The size and frequency variation of the photo-ionization cross-section varies from atom to atom in a rather irregular way.For many atoms such as 0 and A it may be of the order 10-17 cm2 for frequencies near the threshold 23 whereas for Na and K 24 it is several orders of magnitude smaller. Processes (a4) and (a5) both involve ionization arising from transfer of energy from relative translation of two systems of atomic dimensions to electronic excita- tion energy. In such cases the cross-section is very small if the relative velocity z’ of the colliding systems is small compared with the orbital velocity u of the electrons concerned in the transition. This is because, under these circumstances the collision takes place so gradually that it is effectively adiabatic.When ZJ > u the cross-section is very similar to that for ionization by electrons of the same velocity. It follows that, unless very energetic ions and atoms are present the contribution to the ionization arising from processes (a4) and (as) is negligible. Process (a6) is distinguished from (a4) and (a5) in that the energy necessary for ionization is not provided from that of relative translation but from the excitation energy of one of the colliding systems. The factors which exclude (a4) and (a5) as important sources of ionization do not apply and the cross-section for (a6) may be as large as gas kinetic or even greater.The process (1.3) is an important example. It is of course necessary that the excitation energy of B exceed the ionization energy of A. Otherwise some energy must be supplied from relative translation and the same limitation would apply as to (a4) and (a5). Process (a7) is a variant of (a6) in which the atomic products combine to form a molecular ion. If the process is exothermic the size of the cross-section does not depend markedly on the relative velocity v of the colliding systems but it is sensitive to the relative positions of the potential energy curves for the molecules AB’ and AB+. It can be quite large but it is difficult to predict whether in any particular case it will be large or small.An example is provided by the production of He2’ ions by the process inverse to (1.2). The inverse process to (a7) is one of the most important sources of electron recombination. The process (a2) is more important as a source of negative ions.30 GASEOUS IONS A N D T H E I R REAC'TIONS 2.2. Reactions leading to formation of negative ions.-Typical reactions of this kind are : A -1- c -+ A- -j- hv AB -t ~9 --f A -1- B' AB + e -+ A+ + B- -1 c (h 1 ) radiative attachment (h2) d issocia tive at tachnien t (03) polar dissociation A -i- B -f- c --f A- -j- B A .!. c + c -+A- -t e } (65) three-body attachment. These processes are distinguished by the way in which the energy released on attachment is disposed of. The cross-section for this process is always very small, of order 10- 22 cm*, and is rarely important except under conditions of very low pressurc, (h3) has already been considered as it is identical with (a2).The cross-section for (62) behaves quite differently. The process is essentially a resonance one in which the electron energy must be nearly equal to the energy difference between the ground level of AB and that which corresponds to a " vertical " transition to a potential energy curve of AB-. The cross-section has a sharp maximum of order 10-19 to 10-20 cm2 at a particular electron energy and is negligible for energies greater or smaller than this by a few electron volts. It has been observed with 0 2 , CO, NO, 12 and Br2 and other molecules.2~ In (64) the surplus energy is first transferred to excite vibration of the AB- molecule.The vibrationally excited molecule will dissociate by the reverse pro- cess to that which leads to its formation unless the excitation energy is further transferred either to a second molecule C on collision or is radiated. The possi- bility of a process of this kind depends on a coincidental intersection of a potential energy curve of AB- with that of AB at a point near the normal separation of the atoms A, B in the neutral molecule. It seems likely that production of 0 2 - ions from thermal electrons in 0 2 occurs in this way 26 but some doubt has recently been thrown on the validity of these results.27 In (65) the surplus energy is taken away by a third body, eithcr a neutral atom or an electron. These processes occur at rates greater than that of radiation attach- ment only if the pressure exceeds one atmosphere or the electron concentration is greater than 1018 cm3.2.3. Reactions leading to changes in tkc nature (1-f the ions.-Ty pica1 reactions In (61) it is emitted as radiation. In (b2) and (63) the surpliis energy is used in dissociating a molecule. charge transfer dissociation by electron i rnpac t dissociation by molecular impact three-body association cluster formation. The charge transfer process ( ~ 1 ) is similar to (a4) and (a5) in that it involves exchange of energy between relative translation and electronic excitation. The process is nearly adiabatic when the velocity of relative motion is much less than aAE/h, where AE is the magnitude of the energy transferred from or to relative translation, h is Planck's constant and a is a length of the order of gas kinetic diameters.The cross-section rises to a flat maximum at a velocity comparableH . S . W. MASSEY 31 with aAE/h and then falls off rather more rapidly than for ionization collisions such as (a4) and (05). In general the most probable charge transfer processes, when the ion energies do not exceed a few eV, will be those for which AE is smallest. It is important in reckoning AE for a particular process to remember that the initial ions A+ as well as the final atom A and ion B+ may be in excited states (see 1.6). As an indication of orders of magnitude the cross-section for charge transfer between two atoms whose masses are of order 10 times that of a hydrogen atom will be negligible if the energy of relative motion is much less than 500AE2 eV, where AEis measured in eV.Dissociation by electron impact (c2) is a process closely similar to ionization by electron impact ( a l ) . In each case part of the kinetic energy of the electron is transferred to the atom or molecule to excite it to an upper state. If this state of AB+ is an unstable one dissociation follows. The cross-section will behave in much the same way as for the process (01). Dissociation by atom impact (c3) will be restricted in the same way as processes (a4), (a5) and (cl) if it proceeds via electronic excitation. It is possible, however, to cause dissociation by transfer of energy to molecular vibration. Whether this will give rise to much larger cross-sections is still very uncertain.Three-body association as in (c4) is exactly similar to the corresponding process with neutral atoms and molecules which plays such an important role in chemical reactions in the gas phase. For information about such processes the reader must refer to some such book as The Theory of Rate Processes by Eyring and Kimball. Once a sufficiently complex ion has been built up further association may take place without direct intervention of a third body. This is because the activated complex which is formed has a long lifetime and may be stabilized before it dissoci- ates even if the collision frequency is very small. 2.4. Reactions leading to detachnient of electrons from negative ions.-Typical reactions of this kind are : A- + hv+A f e A- f B’ --f A f B + e A- 4- e -+ A + e + e A- f B -+ A + e -k B A + B + A B + e (dS) associative detachment.(dl) photodetachment (d2) detachment by excited atoms (d3) detachment by electron impact (d4) detachment by atom impact Since these processes involve the removal of a bound electron most of them are analogous to processes which lead to positive ion production. Thus (dl), (d2), (d3) and (d4) are respectively analogous to (a3), b6), (01) and (d), and the remarks made in $ 2.1 about the latter are applicable to the corresponding negative ion reactions. It is important to remember, however, that electron affini- ties of atoms are usually much smaller than ionization energies so that a process such as ( d l ) will occur with quanta of much lower frequency than will (a3). Also the cross-section for (d4) will become negligible only for energies of relative motion much smaller than for (a5).No analogue of (dS) appears in 0 2.1. This is because the dissociation energy of a molecule AB is often comparable with or larger than the electron affinity of A or B while that of a molecule ABf is usually much smaller than the ionization energy of A or B. The rate of a process such as (dS) is likely to depend on the relative position of the potential energy curves of the AB- and AB molecules and at present there is little information about even the order of magnitude of the cross-section for any particular case.32 GASEOUS IONS AND THEIR REACTIONS 2.5. Recombination reactions.-Typical reactions of this kind are : A+ + e -+A + hv ABf + e -+A + B A+ + e + B -+A + B (el) radiative eIectron recombination (e2) dissociative recombination (e3) three-body electron recombination (4 mutual neutralization A-+B+->AB+liv (e4) radiative ionic recombination A- + B+ -+ A' + B" A- + B+ + C + A + B + C 1 (e6) three-body ionic recombination.+ A B + C I It is usual to give the rate of recombination in terms of the recombination co- efficient a rather than a cross-section Q r . If 1; is the mean relative velocity of the colliding ions IX = P e r . The first three reactions involve recombination between electrons and ions and as they involve electron capture they have analogues among the processes leading to negative ion formation. Thus (el), (e2) and (el) are respectively analogous to (bl), (b2) and (b5), but the presence of the Coulomb force between the interacting ions and electrons leads to some modification of the considerations applying to the negative ion formation reactions. The coefficient for radiative recombination is of order 10-12 cm3 sec-1 for electrons and ions at ordinary temperatures.Although the great number of states into which the electron may be captured raises the cross-section above that for radiative attachment by a factor of order 100, radiative recombination is still a very slow process.28 The importance of dissociative recombination in deter- mining the rate of loss of electrons in afterglows has already been referred to in S 1.3. Observed recombination coefficients in these cases show that the dissoci- ative recombination proceeds between lo4 and 105 times faster than radiative recombination.Three-body electronic recombination is only important at very high pressures owing to the difficulty of transferring energy from an electron to an atom or molecule. To give a coefficient comparable with that for dissociative recom- bination the pressure would need to be greater than 1000 atm. The three reactions (e4), (e5) and (e6) all involve recombination of positive ions with negative ions instead of electrons. (e4) may be dismissed as unim- portant in almost all circumstances. Mutual neutralization (e5) has been discussed theoretically by Bates and Massey29 who show that the coefficient for ions of thermal energy probably varies widely with the states of excitation of the resultant neutral atoms or mole- cules, but may be quite large, of order 10-8 cm3 sec-1.Three-body ionic recombination (e6) is much more important than for the corresponding process with electrons. It has been discussed theoretically in a classical paper by Thomson.30 The coefficient for simple ions at ordinary tem- peratures increases at a rate which is initially proportional to the pressure but saturates at a pressure of about 1000 mm Hg at a value of order 10-6 mm. At greater pressures the coefficient decreases inversely at the pressure. This decrease is not allowed for in Thomson's theory but was explained by Langevin.31 Experi- ments by Sayers32 and by Marshall, Luhr and Gardner33 have confirmed the validity of the theory. 3. CONCLUDING REMARKS.-AlthOUgh a good start has been made towards understanding the factors which determine the rates of reactions involving ions and estimates of varying accuracy may be made about the rates in particular cases, much still remains to be done before reliable values can be given for all the reactions which may be important in any particular set of phenomenaH .S . W. MASSEY 33 General reference : Massey and Burhop, Electronic and Ioiric Impact Phenomena (Oxford, 1952). 1 Tyndall and Powell, see Tyndall, Mobility ofPositive IOJZS in Gases (Cambridge, 1938). 2 Munson and Tyndall, Proc. Roy. SOC. A , 1939, 172,28. 3 Langevin, Ann. Chem. Phys., 1903, 28, 289. 4 Bernal and Fowler, J . Chem. Physics, 1933, 1, 515. 5 Munson and Hoselitz, Proc. Roy. SOC. A , 1939, 172, 43. 6 Tyndall and Powell, Proc. Roy. SOC. A , 1931, 134, 125. 7 Massey and Mohr, Proc. Roy. SOC. A, 1934, 144, 188. 8 Meyerott, Physic. Rev., 1946, 70, 671. 5, Hornbeck, Physic. Rev., 1951, 84, 615. 10 Biondi and Brown, Physic. Rev., 1949, 75, 1700. 11 Bates, Physic. Rev., 1950, 77, 718 and 78, 492. 12 Biondi, Physic. Rev., 1951, 83, 1078. 13 Biondi and Brown, Physic. Rev., 1949, 76, 1697 ; Redfield and Holt, Physic. Rev., 14 Boyd, Nature, 1950, 165, 142. 15 Boyd, Proc. Physic. SOC. A , 1950, 63, 543. 16 Massey, Negative Ions (Cambridge, 1950), 2nd ed., p. 58. 17 Hasted, Proc. Roy. SOC. A (in course of publication). 18 Bates, Proc. Roy. Irish Acad., 1947, 51, 151. 19 Massey, Negative Ions (cambridge, 1950), 2nd ed., chap. 2. 20 Kazarnovski, Cornpt. rend. U.R.S.S., 1948, 59, 67 ; Uri and Evans, Trans. Faraday 21 Hippel and Condon, Physic. Rev., 1945, 68, 54; Hippel, Physic. Rev., 1947,71, 594. 22 Massey, Negative Ions (Cambridge, 1950), 2nd ed., p. 46. 23 Seaton, Proc. Roy. Sac. A , 1951, 208,408. 24 Ditchburn and Jutsum, Nature, 1950, 165, 723 ; Ditchburn, Tunstead and Yates. 25 Massey, Negative Ions (Cambridge, 1950), 2nd edn., p. 58. 26 Bloch and Bradbury, Physic. Rev., 1935,48, 689. 27 Biondi, Physic. Rev., 1951, 84, 1072. 28 Bates, Buckingharn, Massey and Unwin, Proc. Roy. SOC. A , 1939, 170, 322. 29 Bates and Massey, Phil. Trans., 1943, 239, 269. 30 Thomson, Phil. Mag., 1924, 47, 337. 31 Langevin, Ann. Chem. Phys., 1903, 28, 289, 433. 32 Sayers, Proc. Roy. SOC. A , 1938, 169, 83. 33 Marshall, Luhr and Gardner, Physic. Rev., 1938, 33, 75. 1951, 82, 874; Hornbeck, Physic. Rev., 1951, 84, 615. SOC., 1949, 45, 217. Proc. Roy. SOC. A , 1943, 181, 386.

ISSN:0366-9033    

DOI:10.1039/DF9521200024

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

H . S . W. MASSEY 33 CHARGE NEUTRALIZATION BY REACTION BETWEEN POSITIVE AND NEGATIVE IONS BY JOHN L. MAGEE Department of Chemistry, University of Notre Dame, Notre Dame, Indiana, U S A . Received, 5th February, 1952 The mechanism of reaction between positive and negative ions is examined for application to polyatomic ions of interest in radiation chemistry. It is found that polyatomic ions are expected to have larger neutralization cross-sections and a greater diversity of products than corresponding atomic cases. Metathetical processes of the type are not expected to occur. A+ + B- -> C 4- D Estimates of the cross-sections for neutralization of Of by 0- and by 0 2 - are made. 1 . INTRODUCTION.-charge neutralization by reaction between positive and It has negative ions occurs in any irradiated system which forms negative ions.2CHARGE NEUTRALIZATION 34 been recognized that the reaction may be of considerable importance in radiation chemistry293 although little attention seems to have been given it in this con- nection.The mechanism of the reaction of atomic ions has been discussed.4.5 However, reactions in which at least one of the ions is diatomic or polyatomic are of most interest for our present purpose. Ion neutralizations which leave both reactants in their lowest states are usually very exothermic. On the basis of energetics alone, ion neutralization could go by many paths, leading to a variety of products, including various excited states of atoms, radicals and molecules. Products can form via dissociation, rearrange- ment or chemical reaction of the initial ion pair.The actual products in any given case are determined by competition between various processes and depend upon the potential energy surface of the individual system. We have not solved the pertinent reaction-rate problem, but have merely attempted to sketch its general features. In view of recurrent evidence for specific “ hot atom ” and “ hot radical ” effects,6 it is of particular interest to know the kinetic energy with which the pro- ducts are formed as well as their electronic and vibrational states. 2. GENERAL FEATURES OF MECHANISM.-The initial state of the system A+ + B- will usually correspond to a very highly excited electronic state of the system7 A + B. After charge transfer, the system will almost invariably be in a lower electronic state.Hence, the process will usually be “ non-adiabatic ”.8*9 The various configurations of the system at which the state A+ + B- has potential energy equal to that of other states correspond to conditions for possible transition from the ionic state to the various neutral states. For large separations of A and B the wave function for the system corresponds closely to a product of wave functions for the two isolated components and a relative motion term. If A and B are both neutral, they will not interact very strongly until the separation RAB is very small, of the order of kinetic theory molecular diameters. The charged state A+ + B- is very different in this respect, since it has a long range coulombic interaction. The potential energy for these states contains the coulombic energy term - e2/RAB for values of RAB appreciably larger than kinetic theory diameters.Thus the state A+ + B- has the property that its potential energy may cross that of many neutral states during a collision; at any intersection a transfer can be made to the neutral state. The problem of the transition probability for a system with two crossing potential curves has been examined.lo.11312 The system of most interest here is more complicated than the simple case usually treated for two reasons. (1) Ions in general will be poly- atomic and thus the system will have many degrees of freedom in addition to RAB. (2) During a given collision there will usually be many crossing points so that the various transitions cannot be treated as independent events.We con- sider the influence of each of these factors. Transition is improbable to states which have distant crossing points (RAB large) because the interaction between initial and final states is too weak. The system cannot transfer to a state which has a crossing point too close. There is strong interaction and so transition occurs both on approach and separation, with no resultant reaction. The situation is illustrated by the schematic potential curves of fig. 1. In fig. 1, at the distant crossing point a, interaction with the terminal neutral state A’ + B’ is negligible. At c the interaction is so strong that there is no possibility for transfer to the terminal state A”’ + B”‘. At the crossing point b the interaction is of such a magnitude that a transition is probable.Since any crossing point is passed twice in a given collision (if at all) the total reaction prob- ability must be 2p(l - p ) where p is the probability for making the transition and 1 - p is the probability for failure of reverse transition on separation. The maximum value of 2 p ( l - p ) is one half and gives the maximum value of the probability for neutralization in a collision in which only one intersection is passed. There is a most favoured set of final states for any neutralization.J O H N L . MAGEE 35 In 5 5 it is shown that for several intersections the neutralization probability can approach unity under favourable conditions. For polyatomic ions such a simple one-dimensional curve cannot represent the behaviour except under special conditions.It may represent the system with fair accuracy for large separations. It must, however, break down badly for small separations. For example, in systems which approach as closely as c very definite possibility for a chemical process exists. A+ + B- --f C + D C + X + Y etc. The importance of such chemical contribution depends, of course, upon its ability to compete with all the neutralization processes possibly occurring at large separa- tions. The summed cross-section of the latter can be quite great. The number of crossing points for a given system, their positions and inter- actions determine the cross-section for neutralization and the product distribution. FIG. 1 .-Schematic potential curve for A+ + B- and representative neutral states of A 4- B.There may be no crossing point at all: e.g. for Cs+ + F- the ionic state is the lowest state for all separations. In another case, Brf + Br-, there are only two possible final states ; one has a crossing at about 50ao and is of the type a of fig. 1 and the other is almost certainly of type c. Such cases are probably of no interest in radiation chemistry. Polyatomic ions are generally expected to have a greater density of crossing points. These will tend to have smaller individual transition probabilities than corresponding atomic cases, but this condition actually tends to make the net neutralization cross-section larger. The explanation is that a large transition probability made up of a sum of small transition probabilities is favourable because the back-reaction becomes extremely improbable.crossing points of the various excited states Aj + Bk with A+ + B- are easily calculated. At any such point, R j k , the potential energy of the state for singly charged ions is approximately Many cases will have practically a continuum of final states. 3. DETERMINATION OF CROSSING PoINTS.-The approximate positions Of distant e L Rjk’ E(A+, B-1 - - while the neutral state has the approximate potential energy E(Aj, Bk).36 CHARGE NEUTRALIZATION The E terms are the values of the energy of the entities in the parenthesis for infinite separation. (3.1) If the system changes from one state to the other, it means that simultaneously the two transitions The existence of a crossing point means that E(A'-, B-) - E(Aj, Bk) =- e2iRjk.R- + Bk -1 c ; AE = &(B), (3.2) A ' 4- c + A ; AE.7 G(A), (3.3) take place very much as in the isolated entities. Ek(B) and E'(A) denote the corresponding energy changes. We can also write E,(A) -t Ek(B) e2/Rjk. (3.4) The positions of the available states can be known from spectroscopic data for A and B which include the ionization potential of A, along with the electron affinity of B. All values of Ej(A) and &(B) which yield positive values for Rjk give possible final states. Thus, for cases where either A or B or both are poly- atomic, each possible electronic state will have a series of crossing points corre- sponding to the various possible vibrational and rotational states of the products. The value of EI,(B) depends on the electron affinity of B defined for the par- ticular vibration state involved and the vibrational state of the product.It is actually possible to have Ek(B) negative. Those transitions occur most which involve least extensive electron rearrangement. The most important end state may frequently be the ground electronic state of B in various possible vibrational states. Since A+ is in an ionized state, there is a Rydberg series of states into which the electron can be captured. The smallest possible value of Ej(A) is, therefore, very close to zero. However, the relation (3.4) restricts the accessibility of these states. Only those states are accessible which have an ionization potential greater than the electron affinity of B (otherwise the value of Rjk turns out to be negative and without physical significance).This restriction usually decreases the density of crossing points from the near continuum resulting from the Rydberg series to somcthing smaller. If the ion B has a large electron affinity, all highly excited states of A may be unavailable. We have noted that the negative ion B- may have vibrational excitation so that its " electron affinity " is actually negative. The 0 2 ion furnishes an example. According to the Block-Rradbury theory13 of electron capture, in 0 2 the end state is a vibrational state of 0 2 - which is 0-12 to 0.18 eV higher in energy than the original 0 2 molecule. The relative importance of negative ions in such states must be small since they will lose their extra electron rather quickly unless collisional deactivation is effected.The reaction 0' + o--Z oj f ok (3.5) has been considered in some detail.49 5 Since the products are atoms, the number of available final states can be known with certainty. These states are listed in table 1 along with values of ,!$(A) + &(B) (in electron volts) and Rjk. The electron affinity is uncertain to & 0.2 eV; the resulting uncertainty in Rjk is in- dicated. The first five states certainly have interactions with the coulombic state of type c (fig. 1) and cannot possibly contribute to the neutralization. The end states listed as 6, 7 and 8 have crossing points of type b and determine the neutralization cross-section while 9, 10 and 11 are certainly of type a. The reaction Of + 0 2 - - oj 3- ( 0 2 ) k (3.6) can have a variety of initial states since 0 2 - can be in several different vibrational states.14 Let us consider 0 2 - in its lowest vibrational state which is - 1 eV lower in energy than the lowest state of 0 2 (plus, of course, a free electron of zero energy).JOHN L.MAGEE 37 In table 2 we have listed only crossing points to final states which involve the first 11 vibrational levels of the lowest electronic state of 0 2 and which occur in the TABLE 1 .-CROSSING POINTS FOR REACTION Of + 0- product designation 1. 3 p 2- 3p 2. 3P + 1D 3. 1D ~ ID 4- 3 P i 1s 5. ID 7 1s 6. I S - I S 7. 3P 1- (3S)5S 9. 3P + (3p)5P 10. 3 P + (3p)3U 11. 11) -t (3S)SS 8. 3P -+ (3~)3S range of R;k .ti E(ev) Rjk(ao) 11.35 1’ 0.2 - - I - 9.39 7.43 7-1 8 5.22 - - - 1 - - 3.0 I 9-0 8.5-9.7 2.25 12.0 11‘1-13.2 1.87 14.5 13.1-162 0.65 42.0 32-109 0.40 68.0 45-1 36 0.29 94.0 56-300 TABLE SOME CROSSING POINTS FOR THE REACTION Of f 0 2 - - ~ ( 0 2 ) is the vibrational quantuni number of 0 2 (3S)3S 2 10.1 (3S)3S 9 18-9 (3S)5S 4 10.1 (3PI3P 1 19.3 (3S)3S 5 10.8 (3S)3S 10 21.3 (3S)3S 3 11.0 (3P)jP 3 21.6 (333s 4 11.7 (3PI3P 2 22.5 (3S)5S 6 11.7 (3P)jP 4 24.5 state of 0 V ( O 2 ) Rjli state of 0 d 0 9 ) R;k (3S35S 7 12.6 (3PI3P 3 26.9 (3S)3S 5 12.7 (3PYP 5 29.2 (3S)5S 8 13.7 (3P>3p 4 31.6 (3P)jP 0 14.6 (3PYP 6 35.2 (3S)SS 9 15.0 (4S)3S 0 40.0 (3S)3S 6 13-9 (4S)5S 0 35.3 (3S)3S 10 15.0 (3PI3P 5 40.0 (3S)3S 7 15.2 (3PYP 7 46-8 (3S)3S S 16-8 (3Pj3P 0 16.9 (3P>3P 2 18.6 (3PYP 1 16.4 (4S)5S 1 47.7 range 1Oao < R;k < 50ao.There are 33 of these.15 An uncertainty in the electron affinity of 0 2 enters this calculation but this has not been indicated; the affinity is taken as 1 eV.Several other known electronic states 16917 of 0 2 furnish final states in the same region, as we see from fig. 2. One of these, the 3&- state which has a dissociation limit at 7 eV above the ground state furnishes a continuum of final states for R 2 2 0 4 ; a second state, the 3Zu+ which has a dissociation limit at 5 eV furnishes a continuum for R Z 8 ~ . There is a considerable increase in the complexity of a system over the atomic case when only one ion is diatomic. The most important change is the great increase in the number of accessible final states. It is probably a safe conclusion that for polyatomic systems the density of states is always high.However, cases must be examined individually. For Of + 0 2 - we have the possibility of a direct transition to a dissociated state of 0 2 , which is one mechanism for the formation of atoms in such a reaction. The transition to a stable excited state followed later by an internal conversion process may actually be more probable, but this question must be examined. For the system 0 2 f + 0 2 - it is certain that the densities of crossing states is higher than for O+ + 02-, but the highly excited electronic states of 0 2 are not known well enough to justify a calculation.38 CHARGE NEUTRALIZATION 4. TRANSITION moBABILITY.18-We are concerned with a system A+ + B- which suffers a collision such that its classical hyperbolic orbit crosses N neutral states of the system, where Nmay be a very large number.First we would like to know the total probability that the system emerges from the collision in an uncharged state. The orbit crosses each state twice (except for the special case of a single contact at the turning point for radial motion, which we shall ignore). Let us call the apriuri probability that a transition is made at the nth intersection, FIG. 2.-Some potential curves for the system 02-. FIG. 3.--Position of 2N intersections of a system which has ,V neutral states. Pn ; 1 - Pn will be the a pviori probability that a transition is not made. We must count the total number of ways in which success can be obtained in passing the 2N intersections. Fig. 3 shows a classical orbit which has approximately zero total energy. Intersection of the circles of radii R, with this orbit give the 2N crossing points of the N neutral states.Let us assume that the neutralization probability is known for a system which differs from the one in consideration only in having the first state removed. We call this neutralization probability P ~ - 1 ( 2 , 3 . . ., N). We shall always mean by PN-k the total probability of neutralization for the completeJOHN L. MAGEE 39 shell of states k + 1, k -1- 2, . . ., N, which involve the innermost 2(N - k ) crossing points. The probability we want can be derived from PN-1 by the simple consideration which follows. At the first crossing the probability for making the transition is po ; in systems in which transition occurs at R1 no further intersection is passed until return to RI on separation.Here there is the probability p1(1 - p1) for emerging neutral Systems which do not make the first transition have the probability (1 - p l ) P ~ - . for becoming neutralized in the 2 ( N - 1) inner intersections. The fraction (1 - pl)(l - P N J ) will emerge ionic as they approach the R1 intersection on separation. Here pl(1 - p1)(1 - P N J ) will finally lose their charges. The value of PN (i.e. the net probability of neutral emergence) is obtained by adding up the three classes of such emergence. P N = pl(1 - P1) + (1 - P1)pN-1 PI(1 - PI)(] - PN-l), (4.1) It is a very simple matter to develop from this recurrence relation the general ex- pression for PN since it is known that This can be written in a more compact form : (4.4) (4.5) where the symbol 17 denotes a continued product.If we take all the pn the same, we can sum the series in (4.5) and write (1 - p2N) = 2p(1 - (1 - (1 - p)2”). (4.6) A N pN = A p2j-2 = 1 - (1 - p)2 j = 1 As For large N and small p we have the limit p~ w 1 - e-2Np. (4.7) the product 2Np gets large, PN approaches unity. For N moderately large and p > i, eqn. (4.6) approaches As p + 1, we see that practically all neutralization occurs at the first transition point. The second term on the right represents the small contribution to the total prob- ability of the remaining neutral states. Thus a large number of states of the type c (fig. 1) still have a negligible neutralization probability. Returning to eqn. (4.1) we see that success indicated by the first and third terms on the right leads to the first neutral state and the second term gives the total of transitions into the other N - 1 states.If we collect these terms, we can write The product distribution is, of course, also of great interest. PiV = Al(1 ipN-1) $. p1PN-I. (4.9)40 CHARGE NEUTRALIZATION The second term on the right of this equality is subject to the same analysis, and by the process of iteration we obtain N pN = PK, (4.10) K= 1 where (4.1 1) and ,on represents the fraction of transitions to the nth neutral state. (4.5) we have From eqn. In $ 5 cross-section estimates have been made. as yet been studied for a specific case. collisions of ions with positive energy (&vm2) can be written The product distribution has not 5.Low PRESSURE CROSS-SECTION.-The cross-section for charge transfer in u(uW) == 2n P(b)bdb, JOm (5.1) where b is the distance between the asymptote of the classical hyperbolic orbit of the pair and a parallel line through the origin of co-ordinates. P(b) is the prob- ability for successful transition on this particular orbit as obtained by the methods of the previous section. It is more convenient to express this integral in terms of bo, the radius of closest approach. The expression for the angular momentum (5.2) L = p a b = pobo relates b and bo. Since TO is also a function of bo, we must use and the cross-section can be written as (5.4) In order to determine the appropriate value of uc0 to use in (5.4) for a thermal distribution, an average must be made.The thermal reaction rate is given in terms of a bimolecular rate constant k' which has been averaged over thermal velocity distribution. Since the value of P(b0) does not depend upon vcO and make the average, is so much larger than rW, we make the assumption that (5.7) Division of (5.7) by the average thermal velocity (SkT/p~)g gives the thermaI cross-section (5.8) 03 a(th) = 27i J 0 P(bo)hodbo+~~m(hli)dho.JOHN L . MAGEE 41 This cross-section can be approximated by the second term, which is much larger than the first, and the cross-sections of table 3 were obtained with the approxima- tion. Use of the formulas of the previous section for calculation of P(b0) requires an explicit expression for p,. Since the problem presents quantum-mechanical difficulties beyond the scope of this paper, we shall take the expression of Zener,lo P,, == 1 - exp i_ 1 (5.10) where Ha’ is the matrix element of the perturbation Hamiltonian between the ionic state and the nth neutral state, v, is the velocity along the trajectory at the nth crossing point and S, is the absolute value of d(Eo - E,)/dx at the crossing point.Eo, En are the potential energies of the charged and neutral states respectively. As examples we shall estimate the cross sections for the reactions Of + 0- + 0’ + 0” (5.1 1) o+ + 0 2 - -+ 0‘ -t 0 2 ” (5.12) We need only the matrix elements H,’ in order to (5.13) The basis for this formula and probable values of the parameters a, and q, are discussed in $8. It is shown that tc, depends upon the asymptotic form of the one-electron wave function and for the cases (5.11) and (5.12) varies between about 0.3 and 0.6.The factor qn is the overlap integral for all parts of the wave function other than that of the jumping electron. In the diatomic case, therefore, qn contains a vibrational overlap. The integral in (5.9) was computed numerically. Results are reported in table 3. which were discussed in $ 3. make the calculation, and we take for all of them H,’ = 4 x 10-3 xn (~~R,)3e-~“~nq~. TABLE 3.-cROSS-SECTION SUMMARY ; VALUES ARE GIVEN IN 1Ov13 CM2 j: P(bo)dbo, in ao, is indicated for each case in parenthesis Y. reaction 42 042 0.50 1.00 o++ 0- 10.0 - 4.2 (4.5) - 1.0 6.0 (6.5) 8.5 (9.2) 0.44 (0.48) 0.1 - 2.5 (2.7) - O+ 1- 0 2 - 1.0 32.0 (35.0) - - 0-1 27-0 (29.0) 11.0 (12.0) - 0.01 7.4 (8.0) - - For the case of O+ + 0-, calculation was made as if only states 6, 7 and 8 (table 1) existed.If q n ~ 1 we have a thermal cross-section of almost 10-12 cm.2. Variation of the parameter qh was made to determine the sensitivity of the calculation to the absolute value of the matrix element. This variation is of considerable interest in view of the uncertainty in the wave functions used in the calculation of H”’. The most favourable value of q, occurs because of the changing of the intersections in the direction of type c (fig. 1) for large 4, to type a for small qn. There is a rather high sensitivity of CJ to increase of cc, since the range of the interaction is reduced thereby. In the calculation for Of + 02- only the levels listed in table 2 were considered. The most probable value of ar is somewhere between 0.25 and 0.50.The factor q, is, of course, smaller for this case because of the vibrational overlap integral. The most probable value of is about 0.5. B42 CHARGE NEUTRALIZATION It is expected that qn2 should lie between 0.01 and 0.1. We have taken the same value of qn for all final states, although each one should have its own value. For a transition between two electronic states from a givcn initial vibrational state, = 1. Fig. 2 suggests17 that about 10 vibrational states are accessible, and so some of the qn2 must be greater than 0.1 and some of them less. There is, of course, in this case also some uncertainty in the magnitude of the matrix element which can also be put into q 2 and so we have taken from 1 to 0.01.The calculated cross-sections for Of + 0 2 - are larger than for Of -1 0-. It is to be expected that a system which has a large number of accessible final states should have larger cross-sections since they are not as greatly influenced by such variable factors as the actual positions of crossings, magnitudes of inleraction, etc. It should also be noted that many final states for Of + 0 2 - have been ignored in this treatment. 6. EFFECT OF pmssum.-The theory of Thomson 19 describes satisfactorily the pressure dependence of ion recombination below an atmosphere total pressure. In Thomson’s theory all ion pairs which suffer a collision such that their relative energy drops below zero will react without separating again.However, we must note that the latter condition is met for values of b w 1000ao. Very few orbits so described have values of bo such that charge transfer can be expected to take place during the first transit, for 5 5 shows that bo must be about 30ao or less for approximately unit transfer probability. On the capture collision most ions will be trapped into orbits which have such extremely small charge-transfer probability that they will have to change into more favourable orbits before discharging. The mechanism for changing orbits is, of course, collision with neutral molecules which take up small amounts of energy and increase the binding of the ions. The qualitative effect of this pressure-dependent mechanism on the reaction products is clear. Orbits which have such a small transition probability per cycle that they would contribute a negligible amount to the zero-pressure cross-section make the dominant contribution at high pressure.There are two reasons for this change: the statistical favouring of these orbits will keep them relatively more highly populated; the velocity in the negative energy (is., bound) orbits is lower than for the free case, and (see eqn. (5.10)) the probability for transition at any crossing-point is increased. In the pressure dependent region, the states of higher product excitation will be favoured. At the same time, the kinetic energy of the products will be smaller, both because the products have more electronic excitation and the transfer has been made from bound orbits. -The considerations above have strongly suggested that the charge transfer which effects the neutralization in molecular ion recombination take splace at a rather large distance.In this initial act, therefore, the reaction partners remain intact, i.e. keep their constituent atoms and configurations. Thus a simple metathetical reaction of the type 7. ROLE OF POSITIVE-NEGATIVE ION NEUTRALIZATION IN RADIATTON CHEMISTRY. A+ + B- +- C + D is not to be expected. Dissociation may, of course, follow immediately, but the probability for chemical rearrangements which involve both A+ and B- would seem small. Another conclusion is that rather highly excited states are favoured as end states. This means that large amounts of kinetic energy will generally not be given to the products-one electron volt would probably be an unusually large amount for the pressure dependent mechanism.Along with the high energy states one expects molecular rearrangements and dissociations (predissocjation). Radi- cals should be very common reaction products, as has been assumed. If comparison is made between the neutralization of a given molecular positive ion by an electron and by reaction with a negative ion there would seemJOHN L . MAGEE 43 to be two differences. (i) The available excited states of the ion have lower energy for the ion reaction, because the electron is already bound to the extent of the electron affinity and furthermore the coulombic potential energy is effectively degraded in the pressure-dependent mechanism. (ii) The negative ion can share the excitation energy and furnish dissociation products to the subsequent reactions.Since it is known that negative ions form in irradiated systems, particularly those containing oxygen, it is clear that this neutralization step must be included in the reaction mechanism. The neutralization products will greatly influence the subsequent reactions, and they must be determined for each case separately. 8. APPENDIX: EVALUATION OF MATRIX ELEmms.-We shall use a one electron ap- proximation for the matrix element Hjk’. Since interatomic separations are so large at the crossing points of most interest it is clearly the asymptotic form of the wave function which determines the matrix element. The one-electron wave function obeys the wave equation (8.1) where T + V(u) is the Hamiltonian operator and I is the ionization potential.V(r), the potential energy of one electron, in the field of the nucleus plus the other electrons. goes to zero exponentially with the distance for negative ions. The asymptotic form of he radial part of the wave function for 0- is, therefore, (T + W>>$ = - 44 where CI is related to the ionization potential as cc = 4T.I. (8.3) The value of the wave function is undetermined by this procedure, but the exponential The wave function for 0- used in the calculation of dependence should be accurate. he matrix element is (8.2) normalized to unity. The oxygen atom has a potential V(r) which is coulombic at large distances, and so the radial wave function behaves asymptotically as In this case we have taken the nodeless wave function, normalized for this state.uo(r) N une - OCr. (8.5) As pointed out for the 0- case above, the absolute value is not well determined but the Y dependence should be reliable. The 0 case is different, however, in that as the excita- tion increases, the wave function approaches the hydrogen wave functiops, especially for large angular momenta. For such cases the wave function can be known almost exactly. In the initial state the electron is on the negative ion B and we take for the perturbation operator (8.7) Integration with wave functions (8.4) and (8.6) with n = 3 gives H‘(R) = - 4 x 10-3 cce --DIR[(aR)3 - 5 ( d ) - 15 - - (8.8) ccR 30 1 * The same value of CI has been used in both wave functions. The lead term of (8.5) was used (5.13).The ionization potential of 0- is 2.2 eV, which gives 0-4 for a. The final states of most interest correspond to 3s excitations (see table 1, states 8 and 9) with ionization potentials approximately 4 eV, or cx 0-55. It would seem, therefore, that M = 0.5 is the most reasonable value to take for this case.44 GENERAL DISCUSSION The Sam': integral was used in the case of O+- + 0 2 - for the electronic interaction. In this case the ionization potential is approximately one eV and so a a 0.25. The sam': final states of oxygen are important (table 2). The value of cc should be somewhat lowzr, bztwzen 0.25 and 0.50. The matrix element in the diatomic case also has a vibrational overlap integral q. The schematic potential curves 17 of fig. 2 suggest that in the 0 2 - --f 0 2 + e transition there are possibly 10 final states accessible. Thus, since Eq,2 = 1, an average value of q n 2 of 0.1 would seem reasonable. 1 This work is a contribution from the Radiation Project of the University of Notre, Dame, supported in part by the Atomic Energy Commission under contract AT(11-1)-38. 2 Magee and Burton, J . Amer. Chem. Soc., 1951, 73, 523. 3 Burton, J. Physic. Chem., 1947, 51, 611. 4 Bates and Massey, Phil. Trans. A , 1943, 239, 269. 5 Massey, Negative Zons, (Cambridge, 2nd edn., 1950), p. 93. 6 Hamill, Williams, Schwarz and Voiland, forthcoming publication ; Prigogine, J . Physic. Chem., 1951, 55, 765 ; Hamill and Schuler, J. Amer. Chem. Soc. 1951, 73, 3466 ; Schultz and Taylor, J . Chem. Physics, 1950, 18, 198. 7 Here and throughout the paper A and B designate arbitrary entities : molecules, radicals or atoms. 8 Glasstone, Laidler and Eyring, Theory of Rate Processes (McGraw-Hill, New York, 1941). 9 Laidler and Schuler, Chem. Rev., 195 1, 48, 153. 10 Zener, Proc. Roy. Soc. A , 1932, 137, 696. 11 Landau, 2. Physik. Sowjet, 1932, 2,46. 12 Stueckelberg, Helv. physic. Acta, 1932, 5, 370. 13 Bloch and Bradbury, Physic Rev., 1935, 48, 689. 14 There is some uncertainty as to the positions of the low electronic states of 0 2 - . In this paper we tacitly assume that 0 2 - has the potential curves given by Massey in fig. 6(a), ref. (5), p. 31. 15 Energies of the states of atomic oxygen were taken from Bacher and Goudschmidt, Atomic Energy States (McGraw-Hill, New York), 1932. 16 Herzberg, Spectra of Diatomic Molecules (Van Nostrand, New York, 2nd edn.), p. 446. 17The relative position of the curves for 0 2 - and 0 2 is not known very accurately. Fig. 2 must be considered as schematic. 18 This section was prepared in collaboration with Mr. W. C. Hourt. 19Thomson, Phil. Mag., 1924, 47, 337.

ISSN:0366-9033    

DOI:10.1039/DF9521200033

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

44 GENERAL DISCUSSION GENERAL DISCUSSION Prof. F. S. Dainton (Leeds University) said: In the features of the primary act which are discussed in the first three papers very little has been mentioned con- cerning the possibility of light emission by liquid and gaseous systems subjected to ionizing radiation apart from the fact that radiative recombination and radia- tive electron capture processes are intrinsically unlikely. An exhaustive experi- mental study of this subject would be worthwhile for two reasons. Firstly because a close study of the spectrum of the light might enable the emitter to be identified and thus provide unambiguous evidence for the formation of excited species and for the recognition of their chemical nature. Clearly with the limited source strengths usually available such experiments are much more likely to be successful in gases than in liquids, and some years ago Dr.Burcham and I made observations on the spectra of the luminescent 600 keV proton beams in air and dry nitrogen, which served to identify excited nitrogen molecules in both gases and to show that nitric oxide was rapidly formed in the former. We also attempted to carry out a similar experiment with water vapour, which was for various technical reasonsGENERAL DISCUSSION 45 unsuccessful, but which we hope to repeat with a better reaction cell. In 1949, Dr. Collinson and I attempted to take the spectrum of the feeble luminescence generated when a 400 mc radium source was immersed in water contained in a spherical quartz flask. This luminescence was just visible to the dark adapted eye, but on further examination proved to be merely the Cerenkov radiation.Repeti- tion of the experiment with an aqueous ferrous sulphate solution in place of the water demonstrated the capacity of the system for self absorption of that part of the Cerenkov spectrum lying at wavelengths less than 2900A. Since the overall chemical change induced in ferrous sulphate solutions by absorption of light of these wavelengths may be represented by Fe2f f H20 --f Fe3+ i- OH- + 4H2 which is superficially the same as the radiation induced reaction, this experiment raised in our minds the question as to whether any significant fraction of the yield of radiation reactions is due to secondary photochemical processes. This is the second reason for studying the light emission.For the vast majority, the photo- chemical fraction of the yield stimulated by the Cerenkov radiation is zero or very small. Recently, however, it has been suggested by Richards that in a system in which Cerenkov radiation is non-existent, namely Po c( particles an ultra-violet emission is produced which is strongly self absorbed and in so doing induces a photochemical change of similar nature to the radiochemical change. Since the existing publications by Dr. Richards are brief can he tell us whether he has any evidence which indicates (i) what proportion of the total chemical change is due to this effect, (3) what are the magnitudes of the quanta emitted? Dr. E. W. T. Richards (Glasgow University and A.E.R.E. Harwell) said: In any satisfactory theory of the effects of alpha particles irradiation of solutions it is necessary to postulate some mechanism by which the energy dissipated by the particle is spread throughout the bulk of the liquid.This is especially the case where experiments have been carried out using dilute solutions. From the evidence of physical experiments on the ionization produced in liquids by alpha-particles, it would appear that 99 % of the ions recombine in a time less than 10-7 see. It is therefore necessary to consider some process other than simple ionization to explain the chemical changes which occur. Various theories which make use of free radicals have been put forward, but the method of production and their spacial distribution do not appear to have been fully elucidated. If the radicals are produced directly from the original ions, then it is difficult to see why they should have a longer life time than that given for the ions in Jaffe’s original column theory, in which the electrostatic forces between the ions were not considered.If this is the case, then chemical effects which have a time constant long compared with this cannot be explained by these theories. It has been shown that when a solid is bombarded with alpha-particles quanta are emitted.1 These quanta are strongly absorbed by the parent material and therefore the percentage number which escape is small. If such an effect was present in liquids it would be a mechanism whereby radicals could be produced having a lesser concentration than that of the original ionization.It should be noted that whilst the volume over which the radicals would be produced is large compared with that of the ion columns it would still be very small compared with the bulk of the liquid. Experiments have been performed to test the validity of this theory and positive results obtained.2 Some evidence of the presence of the quanta have also been obtained using purely physical methods and it would appear that the wavelength involved is between 1500-17OOA. The quanta are strongly absorbed by water vapour but transmitted to some extent by crystalline quartz. However, the possibility that the radiations concerned are K or L X-rays cannot be entirely ruled out. 1 Richards and Cole, Nature, 1951, 167, 286. 2 Dee and Richards, Nature, 1951, 168, 736.46 GENERAL DISCUSSION Dr.N. Miller (Edinburgh University) said: The experiments reported by Dee and Richards 3 were of considerable interest to us in Edinburgh in view of the fact that we had spent much time in the study of the chemical action of alpha- particles on ferrous sulphate solutions. We set out at once to confirm their obser- vations, but unfortunately found not only that we could not repeat their chemical experiments, but that sensitive physical experiments, designed to detect directly any ernission of ultra-violet light from water under alpha-bombardment, also gave negative results. In our attempts to repeat their chemical experiments, we used an apparatus resembling that reported in their publication as closely as possible, with the excep- tion that our 20 mc polonium source was covered with a mica sheet of about 1.5 mg/cm2 mass thickness to avoid the health hazards associated with the use of a bare polonium source of this strength. The method of covering the source will be explained in detail in another publication from this laboratory.The quartz plates used were (i) a fused quartz microscope cover slide, 1/2 mm in thickness, which showed over 90 % transmission down to 2000& (ii) slides made by cutting and polishing crystalline quartz, which showed similar excellent properties of near ultra-violet transmission. Maintaining a thin film of water on these slides was found to be exceedingly difficult: in fact we are somewhat sceptical as to whether Dee and Richards themselves satisfactorily solved this problem for the long period (11 days) of their irradiations.It proved a relatively simple matter, however, to maintain, at least for 24-hr. periods, uniform films of dilute sulphuric acid which could be shown by weighing to have a mass thickness less than the range of the alpha-particles. Carrying out many such irradiations for 24-hr periods, and using analytical techniques (spectrophotometry, cf. our paper above) much more sensitive than those employed by Dee and Richards, we were unable to detect any significant oxidation of ferrous ions. From the results of our work on the direct oxidation of such solutions by alpha-particles, we were able to say that any oxidation of this type must have been less than 1/300th of that observed under direct alpha-bombardment.(Reference is made here to conditions under which as many alpha-particles entered an “ infinitely deep ” solution as entered the thin layer of acidified water, the geometrical factor in the latter case being un- corrected for.) In a direct search for any such ultra-violet light emission, Mr. L. 0. Brown of our laboratory next carried out the following physical experiments : (i) The apparatus used in the first type of physical experiment resembled closely that employed in the chemical experiments just described, save that underneath the quartz, in place of the solution, was a thin coating of Apiezon grease, followed in succession by a Perspex light guide and the photocathode of an E.M.I. Type VX 5013 photomultiplier. Much weaker sources (ca.100 pc) were adequate for these physical experiments. These sources were not mica-covered, and the alpha- particles from them were collimated by placing the Pt discs on which the polonium was plated at the ends of brass tubes, 2 mm diam. and 10 mm long. All of the operations were carried out within a light-tight box, which was kept inside a dark room and only opened under red light, to avoid “ memory ” effects in the tube. The purpose of the Apiezon grease was to convert by fluorescence any ultra- violet light emitted to light of a wavelength which could pass through the envelope of the photomtuliplier. Bolton and Williams 4 have recently shown, using a similar type of photomultiplier, that light down to 972A can readily be detected in this way. The experiments were started with a film of water, thick enough to stop the alpha-particles completely, on the top of the quartz, and counting was carried out continuously as the water evaporated.After careful cleaning of the quartz, it could be shown’that such a water-film evaporated evenly and went through 3 Dee and Richards, Nature, 1951, 168, 736. 4 Bolton and Williams, Natirrc, 1952, 169, 316.GENERAL DISCUSSION 47 a phase in which it showed interference colours. There could, therefore, be no doubt that for some time during the evaporation the film was thin enough to allow the alpha-particles to penetrate. At the start of the experiments, with the thick water-film, a little light was detected, but it was established in a separate experi- ment that this was due to the passage of the alpha-particles through the air above the slide, a well-known phenomenon.5 At the end of the experiments, when the alpha-particles were being stopped in the quartz, about 5 times as much light was emitted, the increase being due to scintillations produced in the quartz itself.The interesting part of the experiment, of course, was the investigation of the intermediate region, when the water-film was thin enough to allow the alpha- particles to penetrate. If Dee and Richards had been right, the counting-rate should have gone through a very large maximum in this region. We observed no such behaviour. As the water evaporated, the counting rate merely rose smoothly to the value for quartz alone. (ii) In the second type of experiment the alpha-particles encountered only liquid water and water vapour in the whole of their paths.Here the polonium source, with its brass collimating tube, was inset into the bottom of a Perspex light guide. The light guide was suspended above the water, and the base of it, which was covered with a thin layer of Apiezon grease, was separated from the water surface by a gap varying between 1 and 2.5 cm, depending on the water level. The photomultiplier, suspended upside down above the system, looked into it from the upper end of the light guide, i.e. from behind the source. The whole of the apparatus could be evacuated before the experiment began, and de-aerated water introduced from an adjacent vessel. Some experimental difficulties had to be overcome here, which will be described in detail in a later publication, but no light emission was detected which could be ascribed to the water or water vapour whatever the distance between the bottom of the light guide and the water surface.The very small light emission observed was noted also when the end of the alpha- particle collimating tube was stopped up so that no alpha-particles escaped, and was probably to be attributed to scintillations caused within the Perspex light guide by y-rays from the polonium. Rough calculations based on a knowledge of the strength of the source and of the intensity of the y-emission 6 from Po210 showed that the counting-rate was of the right order of magnitude to be accounted for in this way. These experiments have now convinced us (i) that the chemical effects observed by Dee and Richards, whatever their cause, could not have been due to U.V.light emission from the water-film, and (ii) that no appreciable emission of light of wave- length greater than 1800 A, the upper absorption limit of water, takes place when water is bombarded with alpha-particles. With regard to possible light emission below 1800 A, one cannot, of course, make such categorical statements, as present knowledge of the transmission of liquid water and water vapour in the vacuum ultra-violet is very limited. We estimate, however, that any appreciable emission of such light from the alpha-particle tracks would have been detected in the second type of experiment described above unless the linear absorption coefficient of liquid water for the light were greater than 105 cm-1, a very high figure.If the absorption coefficient were indeed appreciably greater than this, the quanta would be absorbed so close to the ion column in liquid water as barely to affect the radical density. Quite apart from these considerations, Mr. Wilkinson and I find that we can explain the results of our studies on the oxidation of ferrous sulphate solutions by alpha-particles adequately without postulating any such effects. This evidence mill be dealt with in the publication which Mr. Wilkinson and I are now preparing on our chemical work with alpha-particles. 5 Greinacher, 2. Physik, 1928, 47, 344 ; Kara-Michailova, Sitzber. Akad. Win. Wien (HA), 1934,143,15 ; Audubert and Lormeau, Compt. rend., 1949,228,318 ; Lormeau, Compt. rend., 1950, 230, 956.6 Grace, Allen, West arid Halban, Pmr. P/zysic. Sor. A , 1951, 64, 493.48 GENERAL DISCUSSION In conclusion I would like to point out that in spite of the criticism to which their views are susceptible, Professor Dee and Dr. Richards have rendered a valu- able service to radiation chemistry by attracting attention to the possibility of effects such as they postulate. EfTects of this kind may quite possibly be important in other non-aqueous systems, Dr. M. Magat (Paris) said: Prof. Audubert asked me to report a few experi- ments performed by two of his co-workers, Miss Lautout and Dr. BUSSO, which concern the findings of Prof. Dee, Dr. Richards and Dr. Cole.73 I n the experiments of Lautout and Busso photocounters, sensitive in the range of 2000- 3200 8, were used ; these recorded about 1 kick per 105 photons, which represents a sensitivity about 107 times larger than that of Prof.Dee and Dr. Richards. Using a polonium source of 23 mc, they observed a weak light emission of a quartz plate bombarded by cc particles which was about of the same order of magnitude as the light emission of air, previously detected and investigated by Audubert and Mrs. Lousteau.9 Tn this last case the quantum yield was of the order of 1 light quantum/ 107 ion pairs, that is about lo7 times smaller than the effect reported by Dee and his group, and much smaller also than the emission produced by irradiating quartz with X- and y-rays. The emission from the quartz plate could be separated from that of air, by working either in vacuo or by covering the quartz plate with thin layers of A1 or Au.Tn this last case, according to Richards and Cole, the light intensity ought to increase due to the emission of the metal film or at least remain constant. The experimental results obtained were absolutely to the contrary- the light emission decreased in the classical way when the thickness of the absorbing layer increased, i.e. in the ratio of energy dissipated in quartz, when compared to that obtained in vacuum. No method could be found for spreading stable, 40p thick layers of water on a clean quartz surface, such as were used by Dee and his group. Instead the light emission of a thick layer of water when bombarded by CI particles was investi- gated. Should an emission, representing about 10-6 of that observed by Dee and his co-workers exist in the range of 2000-3000 8, (as it should to explain the observa- tions on chloroacetic acid) it would have been easily detected, particularly since water does not absorb light in this region.The result was, however, entirely negative, the count being identical with the background. The light emission of liquid water irradiated by cc rays is at least 10 times weaker than that of air. Dr. J. Weiss (Dzuhanz University, Newcastle) said: T think that even jf one assumes a very rapid recombination of the primarily formed ions, as suggested by Dr. Richards, it is extremely unlikely that in solutions this will lead to an appreci- able light emission, the reason being that the life time of the excited state for emission in the ultra-violet is of the order of 10-8 to 10-9 sec, and it is therefore rather more likely that, in general, the excitation energy is dissipated by the per- turbing influence of the surrounding molecules, leading to loss of energy in some other form, e.g.by dissociation or predissociation processes. I should be very glad to have Professor Massey’s comments on this point. Dr. E. W. T. Richards (Glusgow University and A.E.R.E., Hurwell) said: It i s generally accepted that the ionization produced in a liquid by the passage of an alpha-particle may be expressed by the “ column theory ” of ionization proposed by Jaffe. Kramers has pointed out that in Jaffe’s original theory no account is taken of the electrostatic forces which must exist in such columns and that under normal conditions these forces have a greater effect than the normal diffusion processes.However, according to both theories there should be a difference in the field strength against ion current relationship depending on the direction of 7 Dee, Richards and Cole, Nature, 1951, 167, 286. 8 Dee, Richards and Cole, Nature, 1951, 168, 736. 9 Audubert and Lousteau, Compt. rend., 1950, 230, 956: Conipt. rend., 1950, 230, 1771 ; 1952,234, 330.GENERAL DISCUSSION 49 the applied field relative to tracks of the alpha-particles. Experiments were designed to test these theories. The ion current which could be collected from a liquid when a field was applied at right-angles to the paths of the alpha-particles was measured and compared with the case when the field was applied parallel to the particle tracks.Within experimental error there was no difference in the two cases, both with respect to the magnitude of the current and to the variation of the current with the field strength. Experiments were then undertaken to investigate the variation in the percentage ion current which could be collected as a function of the original ion density. This was carried out by inserting absorbers between the alpha-source and the liquid under irradiation. The relationship obtained was entirely different than that which is obtained from a gas. However, the results were in agreement with the current which would be theoretically predicted if all the current were due to the delta rays which had an energy of greater than 1 kV. It would appear that there is little or no contribution from the primary ion columns to the ion current which can be collected from liquids.Dr. E. Collinson (Leeds University) said : The Halpern-Hall theory predicts a decrease of stopping power in the condensed state, yet the stopping power of liquid water is about 1 3 % higher than that of water vapour. Are there any reasons in the light of our present knowledge why this should be so? Bearing in mind this anomalous behaviour is it not particularly dangerous to assume that W will have the same value of 28 eV for both the liquid and the vapour? Applying the Bethe formula to calculate numerical values of rates of energy loss, etc., Lea 10 rejects the value of 45 eV given by Mano for the average excitation poten- tial of water, and obtains a value of 69 eV which depends on the validity of the Bragg additive law. In fact it now seems certain that liquid water provides an important exception to this law 11 and the value of 45 eV, though based on an incorrect derivation, apparently gives calculated values nearer to those found experimentally. It would be of interest, therefore, to know how Cormack and Johns derived their figures of E = 80 eV. Prof. F. W. Spiers (Leeds University) said: The value of W = 28 eV for electrons in water has been derived, as Dr. Collinson remarks, from experiments in water vapour. This procedure, although unsatisfactory, would appear to be better than the simple assumption that the value of Wfor water is the same as that for air. Studies of effects depending on ion density are not yet sufficiently critical to distinguish between postulated values of W. Changes in W cannot be inferred from changes in stopping power on condensation, since the proportions of energy lost in ionization and excitation are not necessarily the same in the liquid and vapou r phases. The value of E = 80 eV for water used by Cormack and Johns is that given by Halpern and Hall and is derived by these authors from the generalized geometric mean of the atomic frequencies of their mutliple-frequency model. Its derivation is similar, therefore, to that used by Lea applying the Bragg additive law to a simple molecular model. 10 Lea, Actions of Radiations on Living Cells (Cambridge University Press, 1946). 11 Appleyard, Puoc. Camb. Phil. SOC., 1950, 47, 443.

ISSN:0366-9033    

DOI:10.1039/DF9521200044

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

11. ACTINOMETRY AND RADIOLYSIS OF PURE LIQUIDS ACTINOMETRY OF IONIZING RADIATION BY N. MILLER AND J. WILKINSON Department of Natural Philosophy, The University, Edinburgh 8 Received 30th January, 1952 A number of systems which have recently been proposed for the chemical dosimetry of ionizing radiation are compared and criticized, the object being to review the present position and assist others who may be considering the use of these methods. The common acceptance of the term ionizing radiation as applied to particles and quanta of high energy, together with the wide use of the roentgen, a unit based on ionization, and allied units derived from the roentgen, in assessing the energy delivered to matter under bombardment with high energy radiation, is a reflection of the faith which is placed in ionization methods at the present time.Just as ionization is only one consequence of the absorption of this type of radi- ation in matter, however, other typical effects being the excitation of molecules to higher energy levels, evolution of heat, etc., it may well prove that the measure- ment of the energy absorbed from beams of radiation using physical principles other than ionization may under certain circumstances be preferable. The term dosimetry as applied to energy measurements of this type is an in- heritance from the field of radiotherapy, and reflects the extensive development of this subject in medicine, where until very recently ionization methods were used almost without exception. Much effort has been devoted in the past to attempted direct correlations between chemical changes and ionization, but it is now becoming generally recognized that in many cases such correlations may be misleading. In the early days of radiotherapy, chemical methods of dosimetry were for a time in vogue.1 The methods proposed were somewhat crude, depending largely on colour changes induced in solids which had mean atomic numbers very different from that of tissue.Since the radiation used in those days was usually soft X- radiation, where the absorption was largely by the photoelectric effect and thus sensitive to differences in atomic number, the methods fell into disfavour. Recently, however, interest in chemical procedures has revived, as it has been realized that chemical methods may have advantages over ionization methods in systems where the intensity of the primary X- or y-radiation is very inhomogeneous, as, for instance, in close proximity to a radioactive source, or under conditions where the absorption of the primary radiation within the medium is appreciable.These are often the conditions prevailing in studies in radiation chemistry. Chemical methods may also under certain circumstances be more rapid and convenient. While the field has been reviewed twice within the last year,2 the purpose of this article is somewhat different ; it is to compare the various systems which have been proposed in the light of the experience gained during the few years in which they have been in use, and to assist others who may be con- templating using these methods. A few general remarks may first be made.The extension of the term actino- metry, already well established in photochemistry, to cover the measurement of a flux of high-energy radiation through its chemical effects, now seems appropriate. 50N . MILLER AND J . WILKINSON 51 The term dosimetry may then be reserved for the more common case where what is measured is not the flux of radiation but the dose, i.e. the energy which has been abstracted from it within the irradiated medium. Similarly the terms intensity and dose rate may be distinguished, the former referring to the energy flux of. the radiation incident on the medium, e.g. in eV cm-2 sec-1, and the latter to the rate of energy absorption per unit mass of medium, e.g. in eV/g see. With particulate radiation, the energy flux becomes in most cases an arbitrary concept, and the dose is the physically significant quantity.It may be noted here that the conversion of a dose of quantum radiation quoted in roentgens into absolute units (eV/g) requires care, as rather intricate physical principles are involved, which cannot be enlarged upon in this article ; while similar considerations are involved in the computation of an energy flux from dose rate data, and vice versa.3 The methods at present in use may be broadly divided as they employ aqueous and other systems, and then subdivided according to the types of radiation which have been used. 1. AQUEOUS SYSTEMS The characteristics to be sought for in a reaction in an aqueous medium for use in dosimetry were laid down by one of the authors in an earlier paper.4 In brief, the amount of chemical change brought about by unit dose should be in- dependent of (i) the concentration of the reactant and of the product, (ii) the dose rate, (iii) any other conditions which are likely to change during the exposure, such as the pH, content of dissolved gases, etc., and (iv) the quality of the radiation.In addition, (v) the analytical procedure should be as simple as possible, (vi) the ordinary analytical grade reagents should be usable without further purification, and (vii) the solutions should be usable in their normal condition of equilibrium with the atmosphere. Though no system has yet been found which satisfies all these requirements, some systems fulfil some of them over quite wide regions.The requirement (iv) has been found most difficult to satisfy in practice: no system has yet been discovered which has a similar yield, defined as the chemical change per unit of energy absorbed in the solution, with X- and y-radiation as with heavy particle radiation. Several systems can be satisfactorily used over a wide region of X- and y-ray energy and also with direct electron irradiation from dissolved p-emitters. As far as analytical procedures are concerned, spectro- photometric or colorimetric methods have been found to be the most convenient in practice. Tables 1 and 2 give a summary of pertinent data on certain systems which have been shown to have possible applications in the field of X-, y- and p- ray dosimetry. The reactions concerned are: (i) the oxidation of ferrous ions in air-saturated 0.8 N sulphuric acid solutions,5 (ii) the reduction of ceric ions in similar soh- tions,6 and (iii) the hydroxylation of benzene in air-saturated water.7~ 8 For the conditions under which these methods are recommended readers are referred to the original publications.The second column of table 1, headed G, gives the yield in each case, as observed by various investigators and expressed in molecules per 100 eV absorbed in solution, These results have for the most part been ob- tained since one of the authors drew attention to the stringent requirements for accuracy in the measurement of the G value of a reaction intended as an actino- meter.4 The third and fourth columns of this table refer respectively to the type of radiation which was used in each particular investigation, and to the dose rate at which the studies were carried out, where this is given.Table 2 gives a summary of spectrophotometric data appropriate to these systems. In the sixth column of this table approximate values are given for the molar extinction coefficients of the chemical species opposite to that measured (see second column), at the same wavelength and in the same solution as that used in the normal analysis. These corrections are small in the usual case where the initial solution contains the reactant only. The final column gives the product52 A C T I N O M E T R Y OF I O N I Z I N G R A D I A T I O N TABLE 1 .-RECENT DATA CONCERNING VARIOUS AQUEOUS SYSTEMS PROPOSED FOR THE system G molecules/ 100 eV DOSIMETRY OF type of radiation oxidation of 19.9 t Y (Ra) Fez+ ions in 20.4 f 0.3t 'r' (coho) air-saturated 15.5 f 0.3 y (C060) 0.8 N sulph- uric acid 21.1 I: 1.3t X (1.2 MV) 20.3 X (250 kV peak) 19.7 X (200 kV peak) 21.1 & 0.5 B P32) 20.2 f 0.9 B (S35) 16 f 1 P (H3) 15.4 P (H3> reduction of 3.24t Y (Ra) ceric ions in 3.24 f 0.03t Y (Co60) air-saturated 3.63 X (200 kV peak) uric acid -6 X (55 kV peak) 0.S N Sulph- 5.2 X (-14 kV) 3.24 i 0.1 B (S35) hydroxylation 2.3 X (200 kV peak) of benzene in water air-saturated 2.1 B (%) X-, y-, AND ,&RADIATION dose rate, prays) : energy observers rjmin (X- or mode OF r eauiv/min* measurement (B-rals) 93.7 98-104 1500-1 5,000 442 N 3000 342 16.2 10-500 - 42-240 -30 24-29) - 3000 590 - 15.6 - 3000 30-45 ionization Miller 4 Hardwick 13 ionization and Ghormley and calorimetry Hochanadel 14 ionization Miller 15 Todd and Whitcher 16 Rigg, Stein and Weiss 17 4n counting and Hardwick 18 calorimetry 4n counting and Hardwick, 18 ionization 4n counting and Hardwick 19 ionization gas density Hart 20 measurement and ionization comparison with Hardwick 19 Fez+ oxidation Hardwick 6 ionkation Milling, Stein and .Weiss 21 Clark and Coe 22 Haissinsky, Lefort and Le Bail 23 4n counting and Hardwick 18 ionization ionization Stein and Weiss 8 counting rela- Day 21 tive to standard sample * One roentgen equivalent is here defined as that quantity of /?-radiation which liberates 93 ergs per g of the aqueous medium ; cf. Lea, Actions of Radiations on Living Cells (Cambridge University Press, Cambridge, 1st ed., 1946), p.8; Mayneord, loc. cit.3, p. 145. These values are calculated using the figure of 32.5 eV for the quantity W, the average energy expended by a fast electron per ionization in air, and they are also corrected for the electron density of 0-8 N sulphuric acid. I n a recent publication 13 Hardwick has calculated his G values using a figure of 31.9 eV for the quan- tity W appropriate to the recoil electrons produced by the Y-rays of C060. This value, quoted by Wang,9 is in fact an instantaneous value computed by Gerbes 10 from the experimental data of Eisl11 and Pigge,l2 for electrons having exactly the energy 500 keV, and is not strictly applicable to the present case. Gerbes' integral value of about 32.1 eV for electrons having this initial energy is perhaps a more accurate one than the figure of 32.5 eV normally used, which is derived directly from Eisl's work with cathode rays of 10-60 keV, but the authors think that until a further thorough experimental study of this problem is made it is better to standardize on the commonly accepted value of 32.5 eV to avoid confusion.TABLE 2.-SPECTROPHOTOMETRIC DATA ON THE SYSTEMS DESCRIBED IN TABLE 1 species opposite analyzed species Fe*+ Fe3+ Fe3+ Fe2+ Fe3+ Fez+ ( 3 4 ' Ce3i- phenol benzene , I t mode of analysis a-phenanthroline complex in acetate buffer at pH 4-5 thiocyanate complex in large excess of CNS direct, in 0.8 N H2S04, a t 20" C. direct, in 0.8 N HzSOJ Folin reagent, solu- tion neutralized by Na2CO3 direct, in 0.1 N NaOH direct, in neutral solution molar wavelength of extinction absorption peak, Tg:$zt mjL analyzed, E l 330 28 330 6 5.580 28 5,800 6 760 15 ~ 1 0 .0 0 0 15 287 15 -2,645 15 270 15, 24, 30,46 -1,450 15 -1,495 46 molar extinction coefficient of opposite species in G(EI - E Z ) same solution at same wavelength, E2 -37 ' 5 3.2x 10s < I 27 4 . 3 ~ 104 < 1 2 9 1.9x104 - 2 . 0 ~ 104N . MILLER A N D J . WILKINSON 53 of the G value for the radiation-induced reaction and the difference between the molar extinction coefficients of the measured and the opposite species, a measure of the sensitivity of the system as a dosimeter. In this connection it should be realized that such figures as these are only strictly comparable when the product is analyzed rather than the reactant.This is so because a certain minimum concentration of the reactant is usually necessary in order that the subsequent reaction may proceed with a yield independent of reactant concentration. A small change in reactant concentration cannot obviously be measured with the same accuracy as the appearance of a small quantity of product. Also, in those cases where the irradiated solution is not examined directly, it is customary to dilute it by the addition of solutions of reagents. Finally it should be noted that other systems exist which cannot be studied spectrophotometrically : thus, aqueous formic acid solutions apparently show very reproducible behaviour,32 but the reaction is usually followed by measuring gas evolution, a more laborious procedure. Before giving detailed consideration of the various systems, it must be said at once that the degree of agreement between different teams of investigators on the absolute G values of these reactions has in recent years proved rather disappoint- ing, as is evident from table 1.This lack of agreement is at the moment the chief obstacle in the way of the wider use of radiation actinometry. Only with ferrous sulphate solutions at dose rates less than about 1000 r/min are the majority of workers in accord, and at high dose rates variations of up to 25 % are recorded between different teams even in this system. In the authors’ opinion these dis- crepancies are in most cases not due to errors in measurement, but represent real differences in the behaviour of the solutions concerned due to factors which have not yet been satisfactorily elucidated.The fact that the mechanisms of these processes are not as yet fully understood and that empirical procedures have to be adopted is at the root of the trouble. FERROUS SULPHATE SOLUTIONS-X- AND Y-RAYs.-In view of the discrepancies just referred to in the measured values of G for this system using 7-rays at high dose rates, the authors have recently carried out a series of tests examining variables which might have been suspected of causing irreproducibility in this system. These tests have had to be carried out at low dose rates, as the authors have not recently had available any intense sources of X- or y-rays, but they serve to show that the G values of about 20 quoted in table 1 are well founded in the low dose-rate region.The variables concerned were (i) purity of the water used, (ii) differing brands of reagents, (iii) differing modes of cleaning of the glassware, and (iv) differing glass-liquid surface areas during irradiation. Any oxidation induced by the emission of ultra-violet light during irradiation, either by Cerenkov radi- ation or any other process, was found to be negligible. EXPERIMENTAL A N D RESULTS The experimental system used consisted of two cylindrical blocks of teak, the vertical axes of which were drilled out to take two sources of C060, the first of about 800 mc and the second of about 1800 mc activity. Distributed equidistantly around the central hole in each case were eight vertical holes drilled to take Pyrex test-tubes of about 12 mm diameter used as irradiation vessels. Both sources and tubes fitted snugly into their holes : holes and tubes were numbered, the sources inserted in a reproducible fashion, and the tubes filled with a known volume of liquid in each case.The geometry was thus as nearly constant as possible. Small differences in the oxidation rates observed in the various tubes became known during a set of preliminary experiments and were corrected for. Each relative oxidation rate quoted in table 3 represents the average of at least eight separate irradiations : in most cases the tubes were removed in pairs at four different times. When a tube was removed, a dummy tube filled with water to the same level was inserted in its place to keep the geometry constant. The oxidation rates observed in the tubes surrounding the 800 mc and 1800 mc sources corresponded to dose rates of about 6.6 and about 12.4 r/min respectively, using a G value of 20.4 13 for ferric ion production.54 ACTINOMETRY OF IONIZING RADIATION The independence of the observed yield on the initial ferrous ion concentration is now so well established in this system that details of this type will not be given for each individual case, but all solutions were initially between 2.5 and 25 x 10-4 M in ferrous ion, and total y-ray doses varied between 5,000 and 40,000 r.The temperature during the irradiations was about 17" C . The analytical method now preferred by the authors for most actinometric applica- tions of the ferrous sulphate system is the direct spectrophotometric estimation of ferric ion in 0-8 N sulphuric acid solution by its own absorption at 304 mp, first suggested by Hardwick.13 This in most cases permits a measurement to be made directly on the ir- radiated solution. (i) Purity of water.-In preliminary experiments carried out some years ago in Montreal one of the authors (N.M.) found that the yield observed was unaffected by further distillation of the water used provided it had already been distilled twice and once from alkaline permanganate solution. Distillation through silica tubing at 900" C in an oxygen atmosphere, as recommended by Fricke, Hart and Smith 33 was also carried out, without significantly affecting the results. These experiments were repeated in Edinburgh, where similar behaviour was noted.As a routine procedure in all aqueous solution work now carried out in Edinburgh, tap-water is distilled once in a commercial still and then again from alkaline permanganate, the vapour passing through silica tubing at about 800" C. (ii) Purity of reagents.-It had been suggested to one of the authors 34 that different G values might be observed using different brands of reagents. Traces of organic materials were known to affect the observed yield, but it was considered unlikely that appreciable quantities were present either in the ferrous salt or the sulphuric acid normally used, as the yield was observed to be quite independent either of the initial ferrous ion con- centration as long as this was greater than 2 x 10-4 M, or of the acid concentration in the region 0.2 to 1-5 N.Nevertheless, this point was examined in some detail. Two different brands of ferrous ammonium sulphate and one of ferrous sulphate were used, and also two different brands of sulphuric acid, in various combinations. The results, presented in table 3, show that yields within 2 % of each other were observed in all the combinations studied. In addition, the yield in one typical case was unaffected by the addition of 1-4 x 10-3 M chloride ion to the solution as sodium chloride. Chloride ion has been shown by Baxendale et a1.35 to suppress side-reactions involving organic impurities during the Fez+ + H202 reaction, and has also been shown by Dewhurst 36 to suppress the effect of deliberately added organic impurities during the irradiation of ferrous sulphate solutions.TABLE 3.-RELATIVE OXIDATION RATES OF FERROUS IONS BY cob0 y-RAYS differing sources of reagents: relative oxidation rates : 1.01 1 1.00 1 "01 dose rate :::: 1 6.6 r/rnin. > 9 ? Y A, ,, A + 1.4 x 1 0 - 3 ~ ci- + 1.0 x 10-2 M C1- 0.95 diflering modes of cleaning of irradiation vessels : chromic-sulphuric acids at 20" C 1 : 1 sulphuric-nitric acids at 100" C 0.99 I 12-4 r/min, effects of packing of irradiation vessels: packed with Pyrex balls 1.035 (so4)2, 6 H20 ,, ,, ,, capillary tubing 1.06 B, H2S04 B 1.00 1 dose rate, 1 Fe(NH4)2 I , ,, quartz capillary tubing 1-04 J in all cases. FeS04,7 H20 Fe(NH&(SO&, 6 H20 A)Morson's Ltd., Ponders End, Middx. Fe(NH&(S04)2, 6 H20 B : Analar, Hopkin & Williams Ltd. H2SO4 A : pure, nitrogen-free, Houlder & Son, Southall, Middx.H2S04 B : analytical grade, Berk & Co., London. A analytical grade ;N. MILLER AND J. WILKINSON 55 (iii) Cleaning of surfaces.-Glass tubes cleaned with chromic-sulphuric acid mixture gave G values within 1 % of those cleaned by steeping in a 1 : 1 sulphuric acid-nitric acid mixture at 100” C, both sets being washed out subsequently with water purified as described above (see table 3). Steaming of the tubes subsequently was also carried out, without affecting the results. The use of polystyrene irradiation cells was shown a number of years ago by one of the authors (N.M.), and also more recently by Hardwick,l3 to be justifiable, as solutions irradiated in such cells gave yields essentially identical with those obtained in glass cells of similar geometry.Cold chromic-sulphuric acid mixture was found to be satisfactory for cleaning polystyrene cells. Perspex (Lucite) cells could also be used : here the most satisfactory cleaning agent found by the authors was 0.5 M ceric sulphate in 1 N sulphuric acid. (iv) Surface e$ects.-The atmosphere of secondary electrons within a water-filled Pyrex tube irradiated with the y-rays from Coho is not markedly affected by packing the vessel with Pyrex balls, as most of the absorption of the radiation is by Compton scattering processes. It is therefore legitimate to test the ferrous sulphate system for any surface effects by this means. The results quoted in table 3 show that any such effects were found by the authors to be small, and easily attributable to the slight increase in the electron flux expected to be brought about by the presence of the glass.(v) Possible effects due to ultra-violet light.-When aqueous media are irradiated with C060 y-rays, the secondary electrons may be expected to give off Cerenkov radiation, and part of this radiation will be in the ultra-violet. Since ultra-violet light can readily be shown to be capable of oxidizing aerated ferrous sulphate solutions, it is as well to con- sider whether any appreciable oxidation of ferrous ions can be brought about by this route. Calculations, based on the quantitative estimates of Cerenkov radiation induced by 1.9 MeV electrons made by Collins and Reiling,37 show that any oxidation of this type should be much less than 1 % of that due directly to the passage of the electrons.That any such effects are in fact negligible is clear from the observation that the yield is independent of the initial ferrous ion concentration, and also to a large extent of that of ferric ion, despite the great differences in the transparency of the solution to ultra-violet light resulting from changes in these concentrations. Experiments such as those described in the previous section, in which the irradiation vessels were packed with glass balls, thereby greatly affecting the absorption of ultra-violet quanta within the solution, also confirm this. Finally, the authors carried out a set of experiments in which the vessels were packed longitudinally with Pyrex capillary tubing and then with quartz capillary tubing of identical dimensions, the yield being essentially unchanged in the two cases (see table 3).Any effects of this sort can therefore be ruled out. (vi) Yield with 1.2 MV X-rays.-As a result of the above tests and the good agreement obtained between the four sets of independent investigations reported in table 1, the authors now feel confident that the ferrous sulphate system may be used for the dosi- metry of y-rays or hard X-rays up to a dose-rate of at least 1000r/min. To provide a further check on the absolute yield of this reaction, one of the authors (N. M.) carried out a set of experiments using X-rays from the 2 MeV Van de Graaff generator in the Chemistry Department at A.E.R.E., Harwell. A system very similar to that described in an earlier publication 4 for use with 220 kV peak X-rays was used.A Perspex ioniza- tion chamber and irradiation cell of identical geometry were constructed, the collecting volume of the chamber and the internal volume of the irradiation cell being both flat cylinders 3.2 mm high and 1-5 cm in radius. The ionization current from the chamber was measured exactly as described in the earlier publication, the Victoreen VW4lB electro- meter valve circuit used being calibrated as a function of grid voltage using voltages sup- plied from an external circuit and measured on a substandard voltmeter. As an addi- tional precaution, the chamber was calibrated at about 25 cm distance from the target of the Van de Graaff generator against a Baldwin Ionex standard thimble chamber which had been adjusted after manufacture to conform to a N.P.L. standard chamber.At this distance from the target the dose rate was shown by movement of the Baldwin instrument to vary only a few per cent over all positions within the collecting volume of the chamber. When the Baldwin instrument was placed in a position corresponding to the centre of the collecting volume of the chamber, the dose rate observed agreed with that calculated from the known volume of the chamber within 1 %. The doses, all of which were in the region of 5000 r, were corrected for fluctuations in the tube current and voltage of the machine by using the records from the recording instruments which measured these quantities.56 ACTINOMETRY OF IONIZING RADIATION During these experiments the Van de Graaff generator was run at 1.2 MeV and 200 PA.The gold target used was t in. in thickness, and the resulting primary X-radiation was shown by experiment to be absorbed negligibly within a 3.2 mm sheet of Perspex. The precision of these studies was of course lower than that of Hardwick’s work accurately establishing the G value of this reaction at low dose rates,l3 but the results provided further confirmation of a G value in the region of 20 (see table 1). At higher dose rates than this all observers agree that the G values eventually become lower, but they do not agree as to the point at which the value begins to fall or as to the extent of the fall. Thus, Hardwick,29 using 2 MV peak X-rays, reports a yield remaining unchanged up to 4200 r/min, and then falling.Rigg, Stein and Weiss,l7 who used a vessel irradiated with 200 kV peak X-rays such that the average dose rate within the solutions was about 3000 r/min, quote a value of G =- 19.7, in gaoa agreement with the author’s figures and those of Hardwick, but Ghormley and Hochanadel,l4 on the other hand, using a C060 y-ray source in the form of a hollow cylinder surrounding their irradi- ation vessel, report a value of G - 15.5 remaining unchanged in the region 1500-15,000 r/min. This work of Ghormley and Hochanadel was carried out both by ionization and calorimetric methods, the agreement between these types of energy measurement being, incidentally, much closer than that between the earlier ionization measurements of the y-ray energy emitted by radium due to Gray 38 and the calorimetric measurements of the same quantity by Zlotowski.39 In 1935 Fricke and Hart 40 showed that when aerated ferrous sulphate solu- tions were irradiated with X-rays at low dose-rates about one equivalent of oxygen was used up for each ferrous ion oxidized.This observation was later confirmed by one of the authors.4 The symptoms of the fall in G value at high dose rates may be those of effective oxygen depletion in the solutions, as at high dose rates much of the oxygen originally in solution may be converted into the form of intermediates such as H202 and HO2, existing at high steady-state concentrations. If these observed discrepancies do represent real differences in the behaviour of the system at high dose rates, they must presumably be attributed to particles of suspended matter or traces of impurities on surfaces or in solution, conditions which are known to have extremely important effects in systems where the con- centration of H202 is appreciable.41 FERROUS SULPHATE-P-PARTICLES.-The confidence felt by the authors in the applicability of the ferrous sulphate system to electron dosimetry in the low-dose rate-region is increased by the recent results of Hardwick, Hawkings and Bayly 18 on the actinometry of P32 and S35 P-particles in solutions containing these isotopes.The results, agreeing well with the results just described in the case of X- and y-rays, are also presented in table 1. Here the sources used were standardized by counting in 4~ geometry or by ionization measurements, and also by calori- metry.With the low energy P-particles from tritium, both Hardwick and Hart agree that the value of G falls below 20, Hardwick obtaining G - 16 19 and Hart G = 15-4.20 FERROUS SULPHATE-HEAVY PARTIcLEs.-It has been realized for many years that reactions induced in aqueous solutions by heavy particles such as a-particles and protons may be expected to proceed by a different mechanism from similar reactions induced by electron or y-radiation. In most cases where direct com- parisons have been made the heavy particle radiation has been found to be less efficient,42 and the suggestion has been made that the primary columns of the particle tracks are less efficient as compared with the delta-rays, in promoting chemical change.43 Dee and Richards 44 have also called attention to the possible importance of quantum radiation arising as a result of the rapid recombination of the ions in the primary columns.The authors have been carrying out for some time an experimental programme on the oxidation of ferrous sulphate solutions by external sources of polonium a-particles. The details of the work will have to be reserved for a subsequent publication, but it may be said here that the ferrous sulphate system does appear to offer promise, even with a-particles, in that, under a given set of conditions of irradiation, the observed yield remains independent of initial ferrous ionN . MILLER AND J. WfLKfNSON 57 concentration over quite wide limits. This conclusion could not be reached from the earlier work of Nurnberger 45 on the action of cc-particles from radon on ferrous sulphate solutions, as the initial concentrations of ferrous ion used in Nurnberger’s work were considerably higher than those used by the authors, and only in fact extended down to the upper limit of the region of concentration independence.The G values observed by the authors are, however, only one-half or less of those quoted above in connection with light particle radiation, and the possible variation of the G value with ion density, as along the track of an individual particle, has yet to be examined. thorough study of the reduction of ceric ion in 0.8 N sulphuric acid solution by the y-rays from C060 and found that, under his experimental conditions, this reaction, although having a lower G value than that of ferrous ion oxidation, has the advantages of being independent of initial ceric ion concentration down to the lowest limits which can be studied by spectrophotometry, of oxygen con- centration or indeed of the presence of oxygen, and of dose rate up to about 36,000 r/min.Similar yields (G - 3.2) were also found by Hardwick and his co-workers for ceric reduction by S35 P-particles as by y-rays from C060 or radium (cf. table 1). This system has, however, since been found to suffer from two important disadvantages in addition to its low yield : (i) the requirements for the cleanliness of all surfaces in contact with the irradiated solution are extremely rigid, and (ii) the yield is found to increase markedly as the energy of the irradiat- ing electrons is decreased.Ceric sulphate solutions cannot, for instance, be irradi- ated in plastic vessels, as the results are quite erratic; while traces of organic materials on glass surfaces, which have no effect on the rate of ferrous oxidation, have been found by the authors to render the behaviour of this system irrepro- ducible. The rapid rise in yield with decrease in electron energy, which will be examined by Hardwick elsewhere in this Discussion, means in practice that the system is unsatisfactory for use with conventional X-ray equipment, where the emitted X-ray spectrum has an appreciable low-energy “ tail ”. These two difficulties probably account for the considerable divergence in the X-ray yield values for this system reported in the literature (cf. table 1). Although the ceric sulphate system may, in fact, well prove superior to the ferrous sulphate system when high energy y-rays or ,&particles are used at high dose rates, for most dosimetric applications at low dose-rates it is less satisfactory.Day and Stein in 1949 as being one which showed a linear relationship of chemical change with dose, and also little change of yield with initial reactant concentra- tion or pH. Its advantage lies in the ease of preparation and stability of the solutions, but the concentration independence observed is less rigorous than that of the ferrous sulphate system, variations of the G value up to 20 p/o with initial benzene concentration being recorded in one of the original publications,g while the yield is only about one-tenth of that for ferrous oxidation.The somewhat cumbersome analytical procedure originally proposed 7 (Folin-Ciocalteu reagent) can be avoided, as was shown by Carr,30 if the phenol produced is estimated directly by its own absorption at 270mp. Under these conditions, however, the ex- tinction coefficient becomes much lower than that of the colour produced by the Folin reagent, while the accuracy of the method is limited, as in the irradiated benzene solutions there is no absorption peak due to the phenol, but only a slight inflection on a steeply-sloping curve.15 The situation can be improved somewhat by carrying out the analysis in alkaline solution, as was suggested by Sworski.46 The phenate ion then measured, e.g. in 0.1 N NaOH, absorbs at a higher wave- length, where interference by benzene is reduced, and the extinction coefficient is raised; but a difficulty with both methods is that components other than benzene or phenol are produced on irradiation which affect the absorption in this region.ls.46 i n the alternative method suggested by Day and Stein, involving the use of sodium CERIC SULPHATE-x- AND y-RAYS, P-PARTrCLEs.-HardWiCk 6 has made a BENZENE-X- AND y-RAYS, P-PARTICLES.-ThiS system was recommended by58 ACTINOMETRY OF IONIZING RADIATION benzoate solutions, the product consists of a mixture of isomeric hydroxy-benzoate ions.47 These produce different depths of colour with the Folin reagent and absorb in different regions of the spectrum.The mixture can be analyzed by somewhat arbitrary procedures,7 but the assumption that the relative proportions of the ions remain identical under widely-changing conditions of irradiation needs further examination.With cc-particle radiation the reactio,? becomes com- plex, as hydroxylation may occur more than once in the same benzene ring.48 It remains to be seen whether the benzene system is free from the irrepro- ducibility impeding the wider use of the ferrous sulphate system at high dose rates. If this proves to be the case, it may be worth while to carry out the rather large research programme needed to make the method a precise one. 2. OTHER SYSTEMS For the comparison of relative depth doses the gels 2, 49 and plastics 50 con- taining dyes developed by Day and his co-workers and by Proctor and Goldblith offer considerable promise. Owing to their chemical complexity it is unlikely that materials of this type obtained from different sources will show identical behaviowr, so that they will probably prove less satisfactory as actinometric standards than reactions of the type just discussed, but for the studies of relative depth dose for which these methods are primarily suited this is not a serious objection. The dose rate dependence in these systems has not yet been thoroughly investigated, but it has been proved that, under carefully-chosen conditions, they can show colour changes directly proportional to dose.2 For this condition to be realized it is, however, necessary to deoxygenate them, which is in practice a disadvantage.The development of this field will be awaited with interest. The evolution of hydrochloric acid from organic halogen compounds irradi- ated by X-rays, originally studied by Minder and his co-workers,sl has been developed into actinometric procedures by Andrews and Shore,52 who have studied chloral hydrate solutions, and by Taplin and Douglas,53 who advocate two-phase systems of chloroform and water.Both these groups study the change in hydrogen ion concentration in the water-phase, the former group conducti- metrically and the latter colorirnetrically. Kanwisher 54 has recently described an ingenious conductimetric procedure applied to the chloroform-water system, by which he measures the absorption of energy from a radio-frequency field in the aqueous phase. Henley and Miller report the incorporation of an indicator into polyvinyl chloride sheet, to provide a coloured plastic sensitive to radiation.55 These systems have one great asset: they are much more sensitive to X- and y- rays than any others yet investigated, and for this reason their possible use for reasons of convenience under conditions where great precision is uncalled for, e.g.in civil defence, should be regarded seriously, but they suffer from many disadvantages. They depend on the measurement of hydrogen ion concentrations in unbuffered systems, and the results are therefore critically affected by the purity of the materials, while the observed chemical change per roentgen is not independent of the wavelength of the incident radiation, since the materials are not air-wall. The dependence of these systems on dose rate has in most cases not yet been adequately studied, particularly in regard to their possible uses in civil defence.In short, a great deal of work will have to be done before a standard procedure can be established, and in the meantime these systems can only be used to measure relative doses after direct calibration in each laboratory. Similar considerations apply to the sensitive systems suggested by Prevot,56 in which the polymerization of styrene or acrylonitrile is followed dilatometrically. Here again the purity of the material is a highly critical factor and it is improbable that the yield will remain independent of dose rate, while these systems have the added disadvantage that a somewhat involved experimental procedure is necessary both for making up the materials and for carrying out the analyses.N.MILLER AND J . WILKINSON 59 In connection with these last two systems it should be remembered that highly sensitive actinometric procedures are normally only called for in biological or biochemical work, and that, on the basis of chemical change per unit energy input, none of the systems mentioned earlier in this article compare very un- favourably with those in use for photo-actinometry. The above article deals with the progress which has been made in recent years towards the provision of an actinometric system suitable for ionizing radiation. Summarizing, such reactions as have been found to show a linear relation of concentration change to dose, independently of other variables, are complex reactions whose mechanisms are still only partly understood.Certainly they involve a chain of intermediate molecules and radicals whose steady-state con- centrations may under certain circumstances be appreciable, whereas the ideal actinometric reaction would be one involving the minimum number of inter- mediates. In principle the actinometric measurement of y-radiation has one important advantage over photo-actinometry in that the primary process of energy absorption, Compton scattering, is independent of the atomic number or of the chemical combination of the atoms in which the absorption is taking place. The authors would like to record their thanks to Messrs I.C.I. Ltd., for a grant in aid of this work, and to Dr. W. Wild and the staff of the Radiation Chemistry section at A.E.R.E., Hanvell, for the use of their Van de Graaff generator.They are also much indebted to the other workers on this subject whose free exchange of results and ideas prior to publication has been a happy feature of this field: and particularly so to Dr. T. J. Hardwick and Dr. J. Weiss for their consent in the quotation of so many of their hitherto unpublished data. One of them (J. W.) is indebted to D.S.T.R. for a research grant. 1 for a review, cf. Glasser, Radiology, 1941, 37, 221. 2Day and Stein, Nucleonics, 1951, 8 (2), 34; Dainton and Collinson, Ann. Rev. Physic. Chem., 1951, 2, 99. 3 These principles have recently been well summarized by Mayneord, " Applications of Nuclear Physics in Medicine" (Brit. J. Radiulogy, Suppl. 2, London, 1950), pp. 132 et seq. 4 Miller, J.Chem. Physics, 1950, 18, 79. 5 Fricke and Morse, Phil. Mag., 1929, 7, 129 ; Miller, Nature, 1948, 162,448. 6 Hardwick, Can. J. Chem., 1952, 30,23. 8 Stein and Weiss, J. Chem. Soc., 1949, 3245. 9 Wang, Nucleonics, 1950, 7 (2) 55. 10 Gerbes, Ann. Physik., 1935, 23, 648. 12 Pigge, Ann. Physik., 1934, 20, 233. 14 Hochanadel, private communication, 2nd August, 1951. 15 previously unreported results by the authors. 16 Todd and Whitcher, A.E.C. U.-458 (U.S. Atomic Energy Commission). 17 Rigg, Stein and Weiss, Proc. Roy. Soc. A (in press). 18 Hardwick, Can. J. Chem., 1952, 30, 39; Hawkings and Bayly, private corn- 19 Hardwick, private communication, 21st December, 1951. 20 Hart, A.E.C.U.-1534 (U.S. Atomic Energy Commission). 21 private communication, January, 1952.22 Clark and Coe, J. Chem. Physics, 1937, 5, 97. 23 Haissinsky, Lefort and Le Bail, J . Chim. Phys., 1951, 48, 208. 24 Dewhurst, private communication. 25 Brandt and Smith, Anal. Chem., 1949, 21, 1313. 26 computed from data in ref. (16). 27 Potterill, Walker and Weiss, Proc. Roy. SOC. A , 1936, 156, 561. 28 Medalia and Byrne, Anal. Chem., 1951, 23, 453. The authors are more in agree- ment with these figures than those of Hardwick (loc. cit.6). 29 Hardwick, private communication, 13th June, 1951. 7 Day and Stein, Nature, 1949, 164, 671. 11 Eisl, Ann. Physik., 1929, 3, 277. 13 Hardwick, Can. J . Chem., 1952, 30, 17. munica tions.60 DOSIMETRY I N THE P I L E 30 Carr, Nature, 1951, 167, 363. 32 Hart, J. Amer. Chem. Soc., 1951, 73, 68. 33 Fricke, Hart and Smith, J. Chem. Physics, 1938, 6, 229. 34 Hart, private communication, 22nd November, 1950. 35 Baxendale, Barb, George and Hargrave, Trans. Faraday Soc., 1951, 47, 462. 36 Dewhurst, J. Chem. Physics, 1951, 19, 1329. 37 Collins and Reiling, Physic. Rev., 1938, 54, 499. 38 Gray, Proc. Roy. Soc. A, 1937, 159,263. 39 Zlotowski, J. Physique Rad., 1935, 6, 242. 40 Fricke and Hart, J. Clzem. Physics, 1935, 3, 60. 41 cf. Allen, Hochanadel, Ghormley and Davis, A.E.C.U.-1413 (U.S. Atomic Energy Commission). 42 e.g. tyrosine decomposition by X-rays, Strenstrom and Lohmann, J. Biol. Chem., 1928, 79, 673; by cc-rays, Nurnberger, Proc. Nut. Acad. Sci., 1937, 23, 189; carboxy- peptidase deactivation by X-rays and x-rays, Dale, Meredith and Gray, Phil. Trans. Roy. Soc. A, 1949, 242, 33. 43 Dale, Meredith and Gray, loc. cit.42 44 Dee and Richards, Nature, 1951, 168, 736. 45 Nurnberger, J. Physic. Chem., 1934, 38, 47. 46 Sworski, private communication. 47 Loebl, Stein and Weiss, J. Chem. Suc., 1951, 405. 48 Stein and Weiss, J. Chem. Soc., 1949, 3254. 49 Day and Stein, Nature, 1950, 166, 146 ; Proctor and Goldblith, Nucleonics, 1950, 51 Minder, Radiol. Clin., 1947, 16, 339. Minder, Knuchel and Gurtner, Experientia, 53 Taplin et al., Nucleonics, 1950, 6 (6), 66 ; 1951, 9 (2), 73. 54 Kanwisher, U.R.-167 (US. Atomic Energy Commission). 55 Henley and Miller, Nucleonics, 1951, 9 (6), 62. 31 computed from data in ref. (30). 7 (2), 83. 1948, 4, 219. 50 Day, Stein and Schneider, Nature, 1951, 168, 644. 52 Andrews and Shore, J. Chem. Physics, 1950, 18, 1165, Note.-The doses in rep. quoted in this paper are too high by a factor of about 1000 due to an error in the computations. 56 Prevot, Compt. rend., 1950, 230, 288.

ISSN:0366-9033    

DOI:10.1039/DF9521200050

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

60 DOSIMETRY I N THE P I L E THE PROBLEM OF DOSIMETRY IN THE PILE BY 3. WRIGHT Atomic Energy Research Establishment, Harwell, Didcot, Berks. Received 12th February, 1952 Since energy absorption from pile radiation is high enough for calorimetric measure- ment at reasonable pile powers, such measurements when complete should provide a basis of dosimetry for pile radiation chemistry. Monitors are required for each individual irradiation so that the dose can be related to that received in unit time in the calorimeter. Nuclear physical monitors are not entirely satisfactory, but it may be possible to use as monitors chemical systems whose behaviour has been studied under carefully controlled conditions. Experiments on the irradiation of various systems are described, and it is shown that the oxidation of ferrous sulphate in 0.8 N H2S04 provides a suitable monitor for short irradiations.Experiments in which this system is used to study transient pile phenomena provide information about one component of pile radiation-the gamma radiation resulting from decay of fission products. The results show the importance of attaining steady pile conditions and emphasize the fallibility of megawatt-hours as a measure of dose. Certain results suggest that the chemical effects of neutrons and of gamma rays are not always additive for a mixed source, but further confirmation by more direct experiments is being sought. The experiments described in the present paper have been designed and monitored in such a way that they can be related to the calori- metric measurements when these are complete.It will then be possible to derive G values for pile radiation for the various systems studied.J. WRIGHT 61 As a source of neutrons for radiation chemistry, a pile has many advantages over sources hitherto available. A high ratio of neutron to gamma energy is provided, and this ratio can be varied over a wide range; energy absorption from pile radiation is high enough to permit calorimetric measurements at readily obtainable pile power levels; and the large volume within which a high dose rate is available may be useful in recovering measurable amounts of a substance produced only in low yield. Unfortunately, the nature of the source raises several difficult problems associated principally with dosimetry and the adoption of suitable monitors.Some of the problems encountered in quantitative work in pile radiation chemistry are set out below, and some of the experimental work undertaken in an effort to find suitable monitors is described. Attention is confined to thermal neutron reactors and, though most of the remarks apply equally to other types of pile, they are directed in the first place to natural uranium, graphite moderated piles of the type available at Harwell.1 THE BASIS OF DosIMETRY.-Tn experiments with gases the ion pair yield M/N of a product is a convenient way of expressing the result of radiation chemical action. The number of ion pairs formed in unit volume of the gases by a certain quantity of radiation may often he determined by experiment and, in certain cases, the concept of ion pair yield has been most useful in the interpretation of radi- ation chemical effects in gases.2 The possibility of contributions to the yield from excited molecules makes the concept of ion pair yield in radiation chemistry of less value than the corresponding concept of quantum efficiency in photo- chemistry and, like the latter, it is capable of theoretical interpretation only if the secondary reactions are related to the primary act in a simple stoichiometric manner.Moreover, in condensed systems the ion pair formation cannot usually be measured directly. For such systems it is better to express the yields on the basis of energy absorbed by the system, and the G unit (molecules converted or produced per 100 eV absorbed) proposed by Burton 3 is convenient for this purpose.Few radiation sources are sufficiently strong to permit direct measurement of the energy absorbed, though Stahel4 and his collaborators have used micro- calorimetric techniques for this purpose. For most radiations it is possible to use the relationship between energy deposition in a condensed system and ion- ization produced in a small air-filled cavity within the system. I n practice this involves the construction of an ionization chamber such that most of the ioniia- tions are caused by particles having their origin in the chamber walls which must be of material equivalent to that of the condensed system. With neutron sources, though this is still possible in theory, it is no longer so accurate nor so convenient in practice.Because of the large and irregular differences in neutron scattering and absorption cross-sections between the elements, the relation between ioniiation in an air-filled chamber and energy deposition in a condensed system varies considerably with the nature of the latter. It varies also with neutron energy and hence with neutron spectrum of any non-homogeneous source. Further com- plications arise from variations in the quantity of gamma radiation present in any composite source. The theoretical basis for relating ioni7ation in a gas-filled chamber to energy deposition in biological tissue has been investigated, especially by Gray5 and by Aebersold6 and a more general relation applicable to solid systems rich in hydrogen has been derived by Gray.7 From this work it is clear that the materials used for the walls and the gas in the ionization chamber must be changed when the nature of the condensed phase is altered.To avoid these complications and uncertainties it is clearly desirable to measure directly the energy absorbed in a system whenever possible. Fortunately, jt is possible to make direct calorimetric measurement of energy absorbed in various materials in the pile at reasonable power levels. Measurements have been made in the Harwell pile, but the results so far obtained for the different materials are inconsistent with their known physical properties such as mass absorp- tion coefficients for gamma rays and neutron scattering cross-sections. The62 DOSIMETRY IN THE PILE inconsistencies are such as to cast doubt on some details of the experimental procedure and they emphasiire the difficulty of designing calorimetric apparatus suitable for radiation dosimetry.To avoid giving values which may be mislead- ing, the energy absorption values will not be quoted until these anomalies are removed, and in the present paper chemical yields are expressed in terms of thermal neutron dose rather than as G values. The experiments have been monitored in such a way that they can be related to the calorimetric measurements at a later stage. These calorimetric measurements are being made by members of the Pile Group at A.E.R.E., Harwell, and it is hoped that the results will be published later by Mr. F. W. Fenning. The methods used for monitoring irradiations are therefore considered.MONITORING.-The nature of the calorimeter and the necessity of taking measurements over a relatively long period of time make it unsuitable for monitor- ing individual short irradiations. Although calorimetric measurements provide the ultimate basis for calculating radiation chemical yield, we need a monitor whose functions are to ensure that the nature of the incident radiation is constant and to provide a means of relating the dose received in the individual irradiations with that received in unit time in the calorimeter. For reasons which will appear in discussing the experimental results, integrated megawatt-hours derived from the pile control instruments are a most unsuitable measure of dose ; the monitor must accompany the irradiation vessel itself.No completely satisfactory scheme of nuclear physical monitors has so far been devised for radiation chemical purposes. Thermal neutron monitors, depending usually upon activation by some (n, y) process, measure a component of the radi- ation which contributes little to energy absorption processes in many chemical systems (including especially water and organic compounds), “ Threshold ” monitors, depending on activation processes which occur only with neutrons of energy above a certain minimum, measure only the high energy neutrons of which there are relatively few in the normal pile spectrum. There are no monitors for neutrons of a few kilovolts energy which contribute most to the energy deposi- tion in hydrogenous materials, but by using “ resonance ” detectors it is possible to measure the density of neutrons having energies in a narrow energy band, usually in the region of a few electron volts.A combination of all three types of monitor provides the best assurance against changes in neutron spectrum and, in the work described below, gold was used as thermal neutron monitor, the 32s (12, p ) 32P reaction as threshold monitor for neutrons above about 2 MeV, and the cadmium ratio for gold provided a reasonable monitor for neutrons of 4.8 eV at which energy gold has a strong resonance absorption. None of these monitors gives any information about the gamma component of the radiation and, in spite of the complications mentioned earlier, it is desirable to supplement the nuclear physical measurements by ionization chamber measurements.Unfortunately, no suitable chambers are available for use in the pile at normal power levels, whilst at very low power levels the gamma to neutron ratio is different. Undoubtedly the best monitor for irradiations of a chemical system would be another chemical system similar in nature and of known behaviour under controlled conditions. It is in the hope of building up informa- tion about a few such systems that the present work was started. EXPERIMENTAL MATERIALS.-GO~~ Foils, 1 cm square were cut from Ash’s Cohesive Gold Foil No. 4, prepared by The Amalgamated Dental Co. Ltd., and had an average thickness of about 2-5 mg cm-2. Sulphur pellets, $ in. diam. and $ in, long were prepared by melting Philip Harris’ pure crystallized sulphur and pouring the liquid into moulds.The Isotope Division, A.E.R.E., have used this material for several years and found that the only important activity developed by pile irradiation, other than from the sulphur, is a small amount of 24Na. Similar foils, 3 CM square, were used for the larger doses.J . WRIGHT 63 Ferrous sulphate.-A.R. ferrous sulphate was recrystallized from 0.8 N H2SO4 twice, the second time being immediately before preparing solutions for irradiation. Stock solutions were not kept more than one week. o-~~zenunthroline.-Hopkin and Williams' redox indicator o-phenanthroline was recrystallized twice from water and the monohydrate so formed gave colourless solutions stable for several weeks in diffuse light. Benzene.-A.R. benzene was redistilled and the middle fraction used.Mundelic acid.-May and Baker's L-mandelic acid, specific rotation - 158" to - 160", was used without further purification. Wuter.-All solutions for irradiation were prepared from specially purified water. Laboratory distilled water was redistilled from alkaline potassium permanganate and then either from alkaline manganous hydroxide (in early work), or from potassium bi-sulphate. The product was again distilled in silica apparatus and the distillate stored in closed silica vessels. All other chemicals were of A.R. grade and were used without further purification. IRRADIATIONS.-AI~ irradiations, except those of solid mandelic acid, were carried out in silica vessels inside a polythene carrier. The arrangement is shown in fig. 1. The polythene carrier was driven c by compressed air from the laboratory to a fixed position in the reacting core of the Harwell pile, taking 7 sec for the outward journey and 8 sec to return.A total of about 9 sec was spent in travelling inside the pile. While this led to small E uncertainties in the times of the shortest irradiations, no appreciable correction was necessary for calcul- ations based on the activity of the gold foils. Light signals, operated by a switch at the irradiation position, were used for accurate timing of the irradiation periods. The silica vessel developed beta activity due to the formation of 31Si, but there was no accompanying gamma radiation and the vessel could be handled with gloves for long enough to empty, wash, and refill. The weighed gold foils, - which accompanied every irradiation, were held in FIG.1 .--Arrangement of irradiation folded filter paper wrapped around the outside of the silica vessel. Irradiations of solid mandelic acid were con- ducted in aluminium cans placed in an experimental hole in the Hanvell pile and were accompanied by cobalt metal foils weighing about 2 mg for thermal neutron monitoring. ANALYsrs.-~ervous ion.-AII solutions were diluted with 0.8 N HzS04 after irradi- ation to give a total iron concentration of 10-4 M. 5 ml of diluted solution was added to 3 ml of 2 M sodium acetate solution containing 0.56 g/l. of NH4F. To this mixture was added 2 ml of 10-2 M o-phenanthroline solution and the optical density was measured 15 rnin later in a 1 cm cell of a Spekker photoelectric absorptiometer using llford filter 603.The concentration of residual ferrous ion in the irradiated solution was derived from a calibration curve constructed for synthetic mixtures of ferrous and ferric sulphates of total concentration 10-4M. Addition of F- was found to increase the stability of the solutions after colour development by complexing the ferric ion and retarding its conversion to ferrous o-phenanthroline. Too high a concentration of fluoride ion re- sulted in curvature of the calibration graph at the pure ferrous end. PHENOL.-TO 10 ml of irradiated solution was added 5 ml of Folin and Ciocalteu's reagent 8 followed by 15 ml 10 % Na2C03 solution. The mixture was made up to 50 ml and stood for 20 min at 25" C . The optical density was then measured in a 1 cm cell of a Spekker absorptiometer using Ilford filter 607.The concentration was derived from a calibration curve constructed for standard solutions of A.R. phenol. SucRosE.-The optical rotation of the irradiated solution was measured at 25" C in a 2 drn cell, using a Hilger pohrimeter Model M I 1 3. B D f /hch vessel in polythene carrier. A, cotton wool packing B, spherical ground surfaces c, gold foil ,,, polythene carrier E, silica irradiation vessel.64 DOSIMETRY I N THE P I L E MANDELIC ACID.-A weighed quantity of irradiated solid was dissolved in ethyl alcohol and the solution was made up to known volume. The optical rotation was measured at 20°C. MONITORING.-~hel'rna~ neutro~ dose.-Weighed gold foils were irradiated with all solutions and were usually counted next day in a reproducible position 11 cm below an end-window G-M tube.Groups of gold foils were counted alternately with 204Tl standards. Decay of activity of the gold foils, followed over 5 half-lives, gave a value of 64.3 h compared with the accepted value of 64.6 h for the half-life. A small amount of short-lived activity from impurity was of negligible importance t h after irradiation. Absolute calibration for thermal neutron dose was made by irradiating weighed foils in the G.L.E.E.P. for a known time in a known thermal neutron density and counting the foils in the standard position together with 204Tl standards. Correction to this calibration was made for the measured difference in cadmium ratio for gold in the G.L.E.E.P. and in the normal irradiation position in the Harwell pile.Resonance neutron dose.-The cadmium ratio for gold was determined by irradiating a bare gold foil and a foil covered by 1 mm thickness of cadmium successively in the same position. Simultaneous irradiation of bare foils in another part of the pile in each case enabled corrections to be made for any variations of pile behaviour between the two successive irradiations. A brief study has been made of some factors influencing cadmium ratio so that the effect of changes in experimental conditions could be assessed. The irradiation position was in a part of the pile where changes of equipment were seldom made, and experience showed that it was sufficient to check the value of the cadmium ratio at intervals of about two weeks.Fast neutron dose.-Sulphur pellets were irradiated inside the irradiation vessel which was filled with water for these measurements. After irradiation they were dried, weighed and dissolved in concentrated HNO3. The solution was made up to 50 ml with water and a few days later, when the 24Na activity was negligible, the solution was counted in a liquid counter previously calibrated with a solution of known 32P activity. Thermal neutron reactions led to active species (e.g. 34s) with radiations too soft to register in the liquid counter, and covering the sulphur pellets with cadmium during irradiation did not alter appreciably the activity measured. The fast neutron dose was checked at intervals throughout the work. RESULTS AND DISCUSSION NUCLEAR PHYSICAL MoNIToRS.-The absolute flux calibration of the gold foils showed that a foil giving 1000 counts min-1 mg-1 under standard counting con- ditions, corresponded to a thermal neutron dose of 5.42 x 1012 neutrons cm-2 in the G.L.E.E.P. and to 6-06 x 1012 cm-2 in the normal irradiation position in the Harwell pile.With the pile in a steady state, variations of thermal neutron flux were seldom more than 5 % from the mean value over a period of about 1 h. Results expressed in terms of gold activity always gave a smaller standard error than those on a time basis. On the other hand, variations of flux of 15 % or more were observed from day to day for the same nominal pile power. The con- stancy for short periods was due to the permanence of equipment in the vicinity of the irradiation position while the larger variations over a long period resulted from differences in pile loading or local disturbances near the controlling ionization chambers.The cadmium ratio for gold under the conditions of irradiation in the Harwell pile was found to be 2.70 (activity of bare foil divided by activity of cadmium covered foil). In the absence of the water-filled irradiation vessel the value fell to 2.61. No significant changes in these values occurred during the present series of experiments. The cadmium ratio for gold in the G.L.E.E.P. was 2.26. The high energy neutron flux under the conditions of irradiation in the Harwell pile was such that a thermal neutron dose of 1016 neutrons cm-2 corresponded to a 32P activity of 8-5 pc/g sulphur. In the absence of the water-filled irradiation vessel the value was 9.4 pc/g sulphur. OXIDATION OF FERROUS IoN.-Irradiations of ferrous sulphate were made in air-equilibrated 0.8 N H2SO4 solution and a series of irradiations of increasingJ .WRIGHT 65 duration were conducted up to complete conversion to ferric at each concentration. The results are summarized in table 1. TABLE RA RATE OF OXIDATION OF FERROUS SULPHATE ( 1 2 = thermal neutrons) (Fez numbers of ferrous ions) rate of oxidation _.__ - _ - ~ dose rate -- ~ after oxygen number of ('1 ~~~~~~ I in presence of air depletion irradiations (Fe2+1n]-l,~-l,-~2 number Of iiridations (Fe2+rnl-ln-km* x 10-3) x10 3) 7.04 6.7 1 6.75 6.82 7.06 6-82 6.85 6.77 6.73 7.98 7-79 4.06 2.10 1.05 0.62 0.27 FIG. 2.-Oxidation of 2-5 x 10-3 M ferrous sulphate solution ; dose rate, 6.70 x 1011 n cm-2 sec-1.3.34 L 0.10 3-35 1 0.04 3.31 1 0.06 3.24 0.07 3.21 0.21 3.10 - 0-10 3.47 - 0.15 3.03 - 0.21 - 3.14 J 0.10 3.14 0.07 3.44 I- 0.05 3.02 y 0.14 3-35 f 0.04 3.06 2 0-13 3.17 4 0-03 76 17 1s 25 10 6 15 9 7 7 7 16 7 8 I0 - b 80.5 9 ,d 100 4 f Y / For initial Fez+ concentrations from 1 0 - 4 to 10-3 M, a linear relation between dose and Fe2+/ml oxidized was observed up to complete conversion to ferric, but at higher initial Fe2f concentrations the relation was represented by two straight lines (fig. 2). The point of intersection of these two lines occurred at approxim- ately the same dose in all cases and was identified with the "oxygen break" where all the oxygen in the solution had been used and the aerated rate of oxidation gave place to the lower oxygen-free rate.When oxygen-equilibrated solutions66 DOSIMETRY I N THE PILE were used, the break did not occur till much higher doses were reached, whilst with nitrogen-equilibrated solutions, the break did not occur and the lower rate of oxidation was obtained from the start. Within the limits of experimental error, this rate was the same as that at higher doses with air-equilibrated solutions. The rate of oxidation of ferrous sulphate was independent of initial concen tra- tion of Fez+ within the limits of experimental error from 10-4 M to 3 x 10-3 M. The linearity of the graphs for an initial concentration of 10-4 M Fezi- to well beyond 90 %’ oxidation (fig. 3) enable extension of the lower limit of concentration independence to 18-5 M.The second part of tabIe 1 relates to experiments at different dose rates and in all cases the pile was allowed to reach steady state conditions before the series of irradiations was carried out. This is most important at low pile powers if changes in the incident neutron to gamma energy ratio are to be avoided. The P n x 16’~ 2 FIG. 3.-Oxidation of 10-4 M ferrous sulphate solution ; dose rate, 7-04 x 1011 n cm-2 sec-1. results show that the oxidation for unit dose was independent of the dose rate from about 8.3 x 1011 n cm-2 sec-1 to about 8 x 1011 IZ cm-2 sec-1. The mean value for the rate of oxidation in the presence of oxygen is 3.24 0.1 1 x 103 Fe2f ml-1 I T - 1 cm2, and in the absence of oxygen, I -88 + 049 i; 103 Fez+ 1111-1 12-1 cm2.When calorimetric measurements are complete it should be possible to derive G values for pile radiation which may be compared with the published values for gamma radiation 9 and for alpha radiation.10 5 x 1016 Fez+/mI have been oxidized. From the amount of oxygen present initially in aerated 0.8 N H2SO4,11 it may be calculated that this corresponds to approximately four Fez+ oxidized per oxygen molecule used. The number of ferrous ions oxidized per ml at the oxygen break agrees quite closely with the value obtained by Fricke and Morse 12 for X-rays and with Miller’s result 9 for gamma radiation (54 x 1016 Fe2+/ml) despite differences in experimental conditions, type of radiation, ratio of oxygenated to oxygen-free rates and dose rate. The oxygen break occurs when 60J .WRIGHT 67 These experiments showed that the system FeS04 in 0.8 N HzSO4 was suitable for studying pile characteristics. The amount of oxidation provided a simple integration of the effects of " normal " pile radiation * as measured by the neutron dose, and any deviations from this behaviour could be attributed to changes in the nature of the radiation (for instance, to changes in neutron to gamma energy ratio). The rate of oxidation was high enough to permit accurate measurements with irradiation of a few seconds at the higher dose rates. This was valuable in studying transient phenomena. For monitoring purposes it has been found convenient to use aerated solutions 10-4 M or 2 x 10-4 M in ferrous ion, and to irradiate to about 70 % of complete oxidation whenever possible. Reasonably reliable results can be obtained for 25 % conversion of an aerated 10-4 M ferrous solution and for 80 % conversion of an aerated 10-3 M ferrous solution, giving a factor of about 30 in dose.For FIG. 4.-Decrease of gamma radiation following shu t-down. A, shut-down from dose rate of 6.8 x 1011 n cm-2 sec-1 B, shut-down from dose rate C , shut-down froin dose ratc of of 0.62 x 1011 n cm-2 sec-1 0.27 x 1011 IZ cm-2 sec-1. higher doses, the upper limit can be extended by using nitrogen equilibrated or de-aerated solutions. PILE cmRAcTmIsrrcs.-The gamma component of pile radiation is derived from three sources-from fission, from (n, 7) processes, and from decay of radio- active species (particularly fission products).For a given irradiation position, energy from the first two sources will be proportional to thermal neutron flux in that region of the pile, but energy from the third source will vary in a complex manner with the past history of neutron flux in that region. The magnitude of this component is therefore of interest in considering the behaviour during pile power variations. It has been studied by irradiating ferrous sulphate solutions at intervals after shut-down of the pile. Some results are shown in fig. 4. In irradiations made a few seconds after the shut-off ro s had been lowered, an appreciable neutron dose due to delayed neutrons was observed. The * The term " normal '' pile radiation is used in the remainder of this paper to refer to the particular mixture of neutron and gamma radiation used in the ferrous sulphate studies described above.It constitutes a useful reference point in discussing variations in composition of the radiation but has no special significance or applicability to other piles.68 DOSIMETRY IN THE PILE observed chemical effect has been corrected for the contribution of normal pile radiation which, it may be assumed, accompanied these neutrons. The correction becomes negligible within a few minutes of shut-down, and only the first points on each curve of fig. 4 are affected. The three curves of fig. 4 show the fall in oxidation rate as the fission product gamma radiation decreased when the pile was shut down from three different power levels. Curve A was obtained after steady running at the power level at which the pile normally operates and therefore represents the decay of fission products whose initial intensity was characteristic of that power level.Curves B and C, on the other hand, were obtained after running at much lower power levels and are of different shape from A. Although the pile had been running steadily at these low powers for many hours the longer-lived fission products were still of intensity corresponding to much earlier high power running. By subtracting from curves B and C the contribution made by this earlier running, values can be derived which represent the contribution to the shut-down gamma radiation made by running at the lower power. The resultant curves are then of the same shape as A and may be superimposed by adjusting the ordinate scales in proportion to the dose rates before shut-down. The steep slope of the curves FIG.5.-Variation in oxi- dation rate of ferrous sul- phate as the pile attains steady - state conditions ; dose rate decreased from 7.7 x 1011 n cm-2 sec-1 to 0.53 x 1011 n cm-2 sec-1 at zero time. at the start precludes extrapolation to zero time, but the first observed values are about 10 % of the oxidation rates obtained before shut-down. As an illustration of the practical effect of the gamma component due to residual fission product decay, we may consider the effect of changing pile power. Fig. 5 shows the rate of oxidation of ferrous sulphate at various times after the pile power had been reduced. The reduction of power took only 2 min and was complete at zero time on the scale: subsequently the pile power was maintained constant according to the control instruments.The decrease in oxidation rate with time after the power change is due to the decrease of gamma radiation as the fission product activities decay from the high level corresponding to high power running to the new, lower level. This residual gamma radiation is addi- tional to the normal pile radiation corresponding to the low power level and its value may be obtained from shut-down curves like A, fig. 4. Not only is the total energy deposition changing after alteration of pile power, but the proportion of that energy due to neutrons is also changing. The time required to reach stable conditions clearly depends on the magnitude of the power level change and may be 24 h or more in extreme cases.The broken curve in fig. 5 was calculated on the assumption that the chemical effects of normal pile radiation and of the residual fission product gamma radiation were additive, The observed values are lower than the calculated ones in allJ . WRIGHT 69 cases and this suggests that the chemical effects of normal pile radiation and of residual gamma radiation are not additive. Several other data obtained in work with pile radiation lead to the same conclusion but none is sufficiently well substantiated to be decisive. If the non-additivity of chemical effects is confirmed, it will imply that the yield of ferrous oxidation is liable to vary with changes in the proportion of neutron and gamma energy contributions.The rate of oxidation quoted in the preceding section would then be representative only of the particular mixture constituting normal pile radiation and would not apply to other piles or even to other positions in the same pile. This point is being studied at present by experiments in which the proportions of different types of radiation are being changed deliberately. The results of these experiments are clearly important in considering the use of chemical systems as monitors for pile radiation. More direct tests are planned in a search for evidence for or against additivity of chemical effects of neutron and gamma radiation. The result has practical as well as theoretical significance since correction is often made by subtracting a " blank " value derived from separate irradiations with one of the components of a mixed source (e.g., the gamma component of a Ra/Be neutron source).r" FIG. 6.-Diagram showing the re- lative positions of experimental hole and uranium channels in the Har- well pile. graphite ; uranium metal ; experiment a1 hole. The ratio of neutron to gamma energy deposition in a system varies according to its position in the lattice of the pile, being higher at positions marked A in fig. 6 than in position B. This is due to the varying thickness of graphite moderator between the irradiation position and the uranium metal. The magnitude of these changes and their chemical effects have been studied. The ratio of neutron to gamma energy deposition may be changed deliberately, being increased by surrounding the specimen with uranium and decreased by enclosing it in hydrogenous material.Studies may also be made at very low pile powers where the residual fission product gamma radiation from previous high power running is an appreciable fraction of the total incident energy. During experiments on transient pile phenomena, effects were noticed which illustrate the fallibility of megawatt-hours recorded by control instruments as a measure of dose. For several hours after a change of pile power, the thermal neutron flux at the irradiation position was found to be drifting although the power level registered by the control instruments was quite steady within 1-2 min of the power change. A drift in thermal neutron flux of 10 % in 3 h was often observed. This effect is partly due to changes in the " shadow " cast by the control rods on the control ionization chambers.When pile power is reduced, the gradual70 DOSIMETRY IN THE PILE reduction of temperature and of fission product “poisoning” result in an in- creasing reactivity of the pile which must be compensated by a gradual movement of the control rods inwards. In the Harwell pile the control rods enter on the same face as the ionization chambers and, as the rods move into the reacting core, they partly screen the chambers from thermal neutrons in that direction. Since the current output from these chambers is being held steady for control purposes, the flux in other parts of the pile less affected by the control rods must increase so long as the rods are moving inwards. The results shown in fig.5 have been corrected for this effect by allowing for the thermal neutron dose recorded by the gold monitor. All other experiments were carried out under steady state conditions when this effect was negligible. The differences in control rod setting for the same nominal pile power probably account for most of the day-to-day variation in thermal neutron dose rate already noted. We must conclude that megawatt-hours recorded by the control instruments are unreliable as a measure of dose. I 5 1 FIG. 7.-Production of phenol in aerated solutions of benzene in water ; dose rate, 8.4 x 1011 n cm-2 sec-1 IRRADIATION OF BENZENE IN AQUEOUS SOLUTION.-h a search for alternative systems for monitoring purposes, a study is being made of the production of phenol by irradiation of saturated solutions of benzene in water first described by Weiss and his co-workers.13 A graph showing the behaviour of saturated solutions of benzene in air-equilibrated water is shown in fig.7. The same type of behaviour is observed as in the more concentrated solutions of ferrous sulphate. The oxygen break occurs at the same number of phenol molecules per ml as in Weiss’ experiments with X-rays and corresponds to about six phenol molecules per oxygen molecule originally in the water. Much more work is required on this system, particularly in de-aerated solution, before it can be recommended for use in monitoring pile irradiations. The other products, of the irradiation of bemene in water are also being studied, particularly the aldehyde which Weiss found after neutron irradiation.14 sucrose are irradiated in the pile a slow change occurs in the optical rotatjon of the solution.A graph showing the change in the specific rotation of a 10-2 M solution with time of irradiation in the pile is given in fig. 8. No detailed work on this system has been undertaken so far, and the chemical changes which give rise to this effect may be quite complex, but it may prove of value as a monitor for large doses and in cases where a result is required within a short time of the end of irradiation. IRRADIATION OF SUCROSE IN AQUEOUS SoLuTIoN.-When aerated Solutions OfJ . WRIGHT 71 IRRADIATION OF SOLID MANDELIC ACID.-AS an example of quantitative measure- ments made on a solid system irradiated in the piIe, fig. 9 shows the change which FIG. S.-Decrease of optical rotation of sucrose solution ; dose rate, 8.0 x 1011 n cm-2 sec-1. FIG. 9.-Change in optical rotation of solid mandelic acid. occurs in the specific rotation of I-niandelic acid when this material is irradiated in the solid state in the pile. The principal chemical product of this irradiation is benzaldehyde, but the steps leading to its production are not known.72 IRRADIATED PURE WATER The author wishes to express his thanks to Mr. F. W. Fenning for helpful discussions and advice on pile physics and to Mr. H. Morley and colleagues in the pile radiation chemistry group for assistance in the experimental work. This paper is being published by permission of the Director, A.E.R.E. 1 Harwell ; the British Atomic Energy Researcli Establishmetit (H. M.S.O., 1 952), appendix B. 2 Eyring, Hirschfelder and Taylor, J. Cliein. Physics, 1936, 4, 479, 570 ; Hirschfelder and Taylor, J . Chem. Physics, 1938, 6, 783 ; Lind, Tlie Chemical Efects of Alpha Particles and Electroiis (Chem. Catalog. Co., Inc., 1928). 3 Burton, J. Physic. Chem., 1947, 51, 613. 4 Stahel, Strahlentherapie, 1929, 31, 502 ; 33, 296. 5 Gray, Proc. Roy. SOC. A, 1936, 156, 578 ; Gray and Read, Nature, 1939, 144, 439. 6 Aebersold and Anslow, Physic. Rev., 1946, 69, 1. 7 Gray, Proc. Camb. Phil. Soc., 1944, 40, 72. 8 Folin and Ciocalteu, J . Bid. Clzem., 1927, 73, 627. 9 Miller, J . Chem. Physics, 1950, 18, 79. 10 Nurnberger, J. Physic. Chem., 1934, 38, 47. 11 Bohr, 2. physik. Chern., 1910, 71,47. 12 Fricke and Morse, Phil. Mag., 1929, (7), 7, 129. 13 Stein and Weiss, J. Chern. Soc., 1949, 3245. 14 Stein and Weiss, J . Chem. Soc., 1949, 3254.

ISSN:0366-9033    

DOI:10.1039/DF9521200060

出版商:RSC    

年代:1952

数据来源: RSC

摘要:

72 IRRADIATED PURE WATER CHEMICAL PHENOMENA IN IRRADIATED PURE WATER BY P. BONET-MAURY Institut du Radium, Facult6 des Sciences de Paris, Laboratoire Curie, 11 Rue Pierre Curie, Paris 5e, France Received 12th March, 1952 Densely ionizing radiations (natural a-particles) decompose water, following a simple mechanism in which secondary reactions interfere only to a small extent. The action of sparsely ionizing radiation on water, for example X-rays, leads to a very small radiolysis. Dynamic equilibria between the formation and decomposition of products are observed and establish an equilibrium concentration, independent of dose, within certain limits. The heavy accelerated particles behave as if they contained an X-ray and an a-particle component; the radicals of the X-ray component can react with the H202 or H2 from the a-component and this picture is perhaps applicable to all ionizing particles.For a linear ion density of the order of 200 ion pairslp (corresponding to the equidistance of the radicals H and OH) the yield per particle passes through a maximum and this critical density appears to mark the transition of the r-particle chemistry to the X-ray chemistry. The radiochemical reactions depend without doubt on the initial distribution of ion- ization in the water, no matter what the ionizing particle is, and this fact plays a major part in the chemistry of radiations. The numerous biological effects depend in the same manner on the ion density and the different biological efficacy of certain radiations can be explained by the difference in the primary chemical effects.The irradiation of a medium containing a high percentage of water (aqueous solutions, living organisms) primarily brings about chemical processes in the aqueous phase. The great importance of a satisfactorory knowledge of the phenomena for radiation chemistry and for radiobiology is thus apparent. Ir- radiated pure water * constitutes a particularly simple radiochemical system both * We mean by the term pure water, water distilled at least twice (preferably in a quartz apparatus) and deoxygenated by degassing under vacuum. Aerated water is a solution of oxygen and constitutes one of the simplest radiochemical system in aqueous solution.P . BONET-MAURY 73 cxperimentally and in the interpretation of the phenomena.The radiolysis produces only three molecular species, Hz02, H2 and 0 2 , and only the first two arise in the primary effect, oxygen appearing only slowly due no doubt to the secondary decomposition of H202.1 Experiment shows that the formation of oxygenated water and of hydrogen follows very different laws when the water is irradiated with densely ionizing rays as, for example, K-rays from radium, or with sparsely ionizing rays, for example, when X- or y-rays are used.14 5 Before examining the experimental results it is convenient to define the term radio- chemical yield. THE RADIOCHEMICAL YIELD.-The formation of H202 or of hydrogen is character- ized by its radiochemical yield or the number of molecules produced by the absorption in the water of a certain quantity of energy, q. Taking q as 100 eV the yield is defined as G ; if q corresponds to the energy of formation of an ion pair the ionic yield M/lV is obtained.Now the energy of ionization in air varies 2 from 31 eV to 36 eV depend- ing on the nature of the radiation and, for water, the yield is not accurately known, but the differences in values obtained nevertheless do not exceed 10 %. We there- fore propose a mean value of q = 33.3 eV, which is convenient because M/N =- G/3. Also, the yield per particle has been used recently for a-rays, deuterons and protons.3 In the publications on radiolysis of water the term “yield” has been given a variety of meanings and this has resulted in a certain confusion in the comparision of numerical results. The ionic yield M/N can nave, following various authors, five distinct meanings.This is the number of water molecules decomposed per ion pair; the calculation supposes a knowledge of the primary reaction in the radiolysis and the value of the yield depends on the choice of this reaction. (ii) YIELD IN TERMS OF FORMATION.-((I) Of oxygenated water or (M/N)H,O,.---The formation of hydrogen peroxide appears to depend in certain cases on the concentration of hydrogen in the irradiated water ; 3 this concentration, determined by Henry’s law, varies with the hydrogen pressure in the gas phase above the water. The yields of H202 obtained by diRerent authors are only comparable for identical hydrogen concentrations and these depend on the conditions of the experiments, e.g. thickness of the water layer, agitation (mechanical, or by gas), irradiation in a closed vessel or one open to the air, under vacuum or under pressure of hydrogen, relative volume of the liquid and gas phases, etc.(b) Of total gas or (hf/N) H,+O,.-Certain authors have measured the total volume of the gas formed without analyzing it. This yield, like those following, depends on the pressure of the gas phase above the water and a description of the experimental arrange- ment is essential. (c) Of hydrogen or (h.f/Jv)H2.--It appears that at the beginning of the radjolysis, the yield is equal to that in the former case (b), the oxygen appearing only as a secondary product. (i) YIELD IN TERMS OF DECOMPOSITION OF WATER OR (M/N)H,o. (4 Of oxygen (WN)o2. (A) PRIMARY REACTIONS IN THE RADIOLYSIS.--AS has already been shown, these reactions depend principally on the density of ionislation caused by radiation * and the phenomena may be contrasted if the radiolysis by natural a-rays and by X-rays is considered first.(i) FORMATION OF HzOz IN PURE WATER.-(^) Raysfrom Rn (6.5 MeV).-The quantities of hydrogen peroxide formed are proportional to the dose, with a constant yield of fornia- tion, i.e. ( M / N ) H ~ o , = 0.3 or G = 0.90. Within certain experimental limits this yield does not vary in practice with the radiation intensity, the temperature, the pH or the introduction of oxygen.5 (h) X-rays.-For doses up to 2 x 1011 ion pairs/ml the observed yields are very small 3 or 78i-0.5* 6 With a sensitive method we have not been successful in detecting HzOz * The mean linear ion density is onlv a crude estimate of the true distribution of ion- izations.These ionizations are grouped as 2, 3 or more and the mean spacing diminishes rapidly towards the end of the ionizing trajectory; for evaluating this spacing a mean grouping of 3 ion pairs is taken. The distribution is further complicated by the presence of &rays often representing more than half of the ionizations.29 4 -f The H202 is complexed with titanic sulphate and the peroxide is determined by means of ferrous sulphate and thiocyanate. The sensitivity of the method is 0.1 y/ml or 1.8 < 1015 molecules/ml. C’74 I R R 12 D1 A T E D P U R E W A 7 F K in water irradiated under vacuum ; 6 with 1 x 108 r (0.87 x 1018 ion pairs/nil). Cal- culation shows that the formation yield is, in this case, less than 0.02.The introduction of oxygen largely increases the yield which varies with a number of factors, e.g. dose, intensity, temperature and pH.79 3 (c) Accelerated heavy particles.-With protons (1 a 8 to 7.5 MeV) the yield decreases with dose as for X-rays and oxygenated solutions. The initial yield for small energies (1.8 MeV) is similar to that for natural cx-particles, i.e. Mihr-9-3. The initial yield FIG. 1. falls when the energy of the protons is increased (0.12 for 7-4 MeV) and the yieldper particle passes through a maximum for an energy of the order of 6 to 7 MeV (fig. 1). This important phenomenon, discovered by Toulis, is also found for dei:terons (maximum around 13 MeV) but has not been observed for a-particles, at least up to energies of 34 r"" MeV.Table 1 and fig. 2 clearly show the correlation between the initial yield * and ion density, although the experimental results are very limited in number and new experiments are very desirable. particles from Rn.-It has been known for a long time that the formation of hydrogen is proportional to the dose and the yield is constant but this agreement is not borne out by the values which vary, according to the authors (table l), from 0.8 to 0.6. (b) X-rays.-As for oxygenated water the observed yields are very small (3.6) and this is ofien attributed to presence of impurities in the water. Even when carefully purified, it can still contain con- centrations of the same order as that of the products of rddiolysis. We have undertaken, with Patti,s a series of experiments for measuring the liberation of hydrogen, with the apparatus (ii) FORMATION OF HYDROGEN.-(a) CC- * When the formation yield remains constant with increasing dose it can be considered to deal with the primary reaction.When the yield diminishes with the dose, its deter- mination at the beginning of the irradiation for doses as small as possible is of particular interest, because recombination reactions are also small and the primary formation re- action can be observed almost without complication ; this is termed the initial yield in table 1.P. BONET-MAURY TABLE 1 ion energy initial, ionic yield MJN in MeV density radiation (p.i./p.) 4600 fission of' boron c( = 1.522 a-particles and Li =0.88 H 2 0 ~ H2 yield con- - stant with Li nucleus) Rn 4000 a-particles froin 750 protons part ides deuterons protons 200 -deuterons ,&rays from .electrons I (tritium) 160 protons 109 X-rays I SO X-rays 10 electrons 6.5 1.8 31 8 12-14 6-7 0.005 7.5 0.2 0.017 1.0 dose 0.3 0.3 0.18 - 0.12 0.1 1 ._ 0.12 3.000 -0.0 < 0.02 - 0.8-0.6 - 0.25 0.18 - __ 0.1 3-0.03 0.15 0-4 0.03 0.13-0.03 0.17-0.07 shown in fig. 3. This apparatus has a volunie of 50 rnl 75 authors Bonet-Maury Duane and and Lefort 5 Scheuer 1 Nurnberger 1 Lanning and tind 1 Lefort 1 Toiilis 3 - 7 ) Toulis 3 9 9 Allen 1 2 7 Allen 1 7 9 Toulis 3 Fricke 5 Fricke 5 Toulis 3 and Patti 3 Bonet-Maury Bonet-Maury and Patti 8 I Allen 1 for the liberated gas and enables the irradiat~on of 3 1n1 of pure water contained in a quartz vessel to be carried out through FlG.3. an aluminium window F. the degassing and irradiation. The water is vigorously stirred by a magnetic stirrer M during The water vapour is condensed by a liquid nitrogen , _. - F~G. 4.76 IRRADIATED PURE WATER trap P and the final pressure is measured by means of a McLeod gauge reading to 10-4 niin Hg. The detachable vessel A is provided for analysis of the gas by chemical ab- sorption and the electrodes E for obtaining an electric spark to allow of the recombination of hydrogen and oxygen. We have observed, as have preceding authors, a small but quite measurable liberation of gas and a reasonable proportionality with dose up to 1 x 1018 ion pairs/ml(1.6 x 106 r). The yield (M/N)H, = 0.03 (fig. 4). Since the water, after high doses, becomes “ purified ” by the irradiation, this observed yield may be attributed to the radiolysis of water.to ‘8 Hz ;yield I I (c) Accelercrted particles.- The observed yield for protons of energy 7.4 MeV is pro- portional to the dose up to 13 x 1019 ion pairs/ ml, and is of the order 0.15. For x-particles of 31 MeV, MIN -= 0.25 compatible with values found using radon. For the formation of hydrogen the same correlation is found between the yield and the ionization density of the irradi- ation (fig. 5 and table 1). DISCUSSION Over all, these results show that the “X-ray chemistry” of water is very different from the “ ct-particle chemistry ”, the latter being apparently much simpler. In effect, not only does the a-yield remain constant but also the concentrations of radiolysis products obtained, even with small doses, are easy to measure without par- ticular precautions.On the contrary, even with high doses of X-rays (several million r) concentrations of the order of those of the impurities in the carefully purified water, are obtained, that is < 10-6. As the most sensitive analytical methods detect with difficulty less than 0-1 ylml the use of much more powerful sources of X-rays must be envisaged for further progress. The contrast between the u.- and the X-ray chemistry has already been inter- preted in a reasonable manner in terms of the chemical theory of Weiss 11 and the physical theory of Lea and Gray.4.12 If, following these theories, the H and OH radicals are distributed originally in the same way as the ions from which they were derived the initial concentrations of these radicals will be very different when the cc-particles and the X-rays are used.The ct-particle chemistry is characterized principally by the initial separation of the H and OH radicals, leading to the combination processes : OHi-OH - Hz02 4 51 kcal (initial OH concentration, 1 M) ; H+H = H2 + 103 kcal (initial H concentration, 8-7 Y 10-3 M) ; whereas the X-ray chemistry corresponds to a uniform distribution of both radicals favouring the recombination : H -t OH H20 + 11 8 kcal (initial concentration 7 ‘<, 10 5 M for an electron of 60 kV). Calculation shows in addition 4,12 that the reactions in cc-particle chemistry are effected principally inside each column of ions which constitutes an isolated system and the negligible influence of intensity of the a-radiation confirms the small importance of reactions between columns of ions.This chemical indi- viduality of the ionized trajectory must disappear for X-rays and the reactions occur throughout the bulk of the water, facilitating decomposition reactions because of the excess of radicals. The influence of radiation intensity in X-rayP. BONET-MAURY 77 chemistry confirms this because the mean primary concentrations of radicals depends on the mean ionic concentration, that is to say, the intensity. In spite of this simplicity, without doubt exaggerated, this interpretation allows equally well an explanation of the phenomena observed with accelerated heavy particles ; these latter form a progressive transition between a- and X-ray chemistry.In fact, a-particles of several MeV give yields of the same order as those from natural cr.-particles whereas for high energies the appearance of phenomena charac- teristic of X-ray chemistry is observed. Now this increase of energy involves a progressive spacing of the radicals, the initial concentration of which is lowered, whilst the separation of H and OH disappears. It is striking to find that the yield per particle passes through a maximum for an energy corresponding to a mean ion density of 200 ion pairs/p and a mean spacing of 15 mp for the OH radicals, that is to say, precisely the distance which separates them from the H radicals in a column of a-ionization. This is close to the value expected for the transition of columnar distribution to a sensibly homogeneous distribution of radicals produced by the absorption of X-rays in water.This interpretation involves, as a first approximation, a constant distribution of radicals along the length of the ionized trajectory and it certainly minimizes the part played by the 6-rays or the ends of the electronic trajectories. In order to take this into account more reasonably without entering into the details of the true distribution, as yet not well known, it is possible to represent schematically the trajectory of any ionized particle as composed of two parts : (i) " X-ray part ", where the loss of energy and the radicals are distributed in a sensibly uniform manner ; in the main they react to produce water by recombination.(ii) " a-part ", where the loss of energy is large; the two radicals are separated and give the compounds Hz and H202. The critical loss of energy corresponding to the transition from X-ray to a-particle chemistry, is of the order of 70 MeV/cm, i.e. about 210 ion pairs/p. If the %rays are considered, it is of the order of 140 MeV for a-particles (420 ion pairslp) and 64 MeV/cm (192 ion pairslp) for deuterons and protons. Now for natural a-particles the X-ray part is negligible but becomes important for a-particles of large energy. Conversely, the a-part plays only a small role for ordinary X-rays whereas it becomes the principal effect for electrons of small energy; the liberation of H2 which we have observed with X-rays is due to this a-component." For accelerated heavy particles the relative importance of these two effects varies with the energy; the yield per particle grows with the energy, then more slowly and passes through a maximum; it decreases subse- quently and then tends to the low X-ray yields (fig.6 ) . primary molecules (€3202 and H2) as their concentration increases with dose leads to reaction either between themselves or with the free radicals. Some secondary decomposition reactions will occur which in a general way will bring about a lowering of yield with dose. The parallel increase of amounts formed and de- stroyed leads finally to a dynamic equilibrium which manifests itself as a constant concentration independent of the dose. This equilibrium concentration will depend on a number of factors such as intensity and concentrations in the gas and liquid phases. The dynamics of these phenomena appear quite complex and the published experimental data are again few.It seems certain nevertheless that the equilibria of H202 and of H2 are established in a distinct manner, for different doses ; the equilibrium concentration of oxygenated water appears first. H202 EQUILIBRIA.-(~) NaturaZ a-particles.-No equilibrium has hitherto been reported, the concentration increasing linearly with the dose. * The corresponding yield of Hz02 will be of the order of 0.015, that is to say, at the limit of sensitivity of our method of chemical analysis. (B) SECONDARY REACTIONS-RADIOCHEMICAL EQUILIBRIUM.-The diffusion Of78 IRRADIATED PURE WATER (6) X-rays.-The experiments are difficult on account of the small primary yields.Toulis claims to have observed an equilibrium concentration of the order of 3 x 1015 molecules/ml (0.17 y/ml) which is greater than expected and which would correspond to a yield of the order of 43,000 molecules of H202 per ion pair. Allen, however, reports an equilibrium concentration of the same order for 1 MeV electrons. Our own cxperiments with Patti 8 have given a negative result, that is to say, less than lo15 molecules/ml for 1.8 x 1018 ion pairs/ml. (c) Heavy accelel.atectparticles.-For protons of 7.4 MeV the curve of forma- tion as a function of dose leads to an expected equilibrium concentration of 2 or 4 X 1019 molecules/ml or 600 ylrnl.3 (ii) H2 EQUILIBRTA.-(~) Natrrval a-particles.-The use of conccntrated solu- tions of radium salts for the preparation of Rn ampoules has led to thc old 0's- servation of the presence of this equilibrium for a pressure of the order of 706 inm m.The secondary appearance of oxygen in the gas mixture indicates a dccom- position of the H202 formed, which perhaps can be attributed to one of the three reactions : (i) H202 + H2 = 2H20, (ii) H202 4- OH = H20 -t "02, (iii) H202 + H = H2O 4- OH or HO2 1 H:. The first reaction is improbable; this is confirmed by the fact that elimination of hydrogen by evacuation does not modify the formation of H202.6 The second initiates the chain H202 + HO2 = 0 2 + H20 -:- OH, OH 4- H02 --- H20 + 0 2 (termination). (b) X-rays.-The equilibrium observed by Fricke is attributed by him to impurities in the water, but Allen has observed an equilibrium pressure for 1 MeV electrons (1-2 cm) and for 5 keV (10-20 cm) due probably to reactions of the or.-part of the electron trajectory (hot spots).Our experiments with Patti do not give any evidence of an equilibrium in the range of doses used (fig. 4). (c) Accelerated heavy particles.-An equilibrium is not expected for 7 MeV protons at pressures lower 3 than 1 atm. but 8 MeV deuterons lead to an equilib- rium pressure of 60 cm. The experimental study of these equilibria is again not very advanced and it seems most desirable to develop it further. INTERPRETATIoN.-The interpretation of the results is as yet not very advanced because these equilibria result from competition between several possible decom- position reactions, of which the respective probabilities depend on numerous factors.Physical factors determining the probability of erzcounter are potential barriers and, particularly, the number of collisions per second ; this number depends on the temperature, the concentration and the time of sojourn in the solvent cage, etc. The chemical factors, determining the probability of reaction at the moment of collision, are the state of excitation and the free energy change in the reaction.P . BONET-MAURY 79 The calculation is difficult and it can only be hoped that approximate solutions That given by Toulis leads to the division of the reacting species can be obtained. into three categories, in decreasing order of their probability of reaction : (1) probability about unity-interaction of radicals, H + H = H 2 H + OH 7 H20 OH + OH = H202 H02 + HO2 -- H202 f 0 2 H02 + OH = H2O + 0 2 . (2) probability about 10-3-interactions of the radical H with 0 2 and HO2, H + 0 2 = HOz H + H202 = H2O -1- OH; (3) probability about 10-5-interactions of the radical OH with H2 and H202, OH + H202 = HzO f HOz OH 1 H2 = H2O + H. 1 (a) Kernbaum, Radiiinr, 1909, 6, 225. (h) Duane and Scheuer, Radium, 1913, 10, ( d ) Allen, (e) Laming, and Lind, J. Physic. Chern., 1938, 42, 33. J . Physic. Chin., 1948,52, 479. 1229. (c) Nurnberger, J . Physic. Chem., 1937, 41, 431; 1934, 38, 47. 2 Fano, Nat. Bur. Stand., Report No. 1002, 1950. 3 Toulis,Tlie decomposition of water by radiation ; Berkeley and Toulis, The influence 4 Lea, Action of radiation on livirrg cells (Cambridge, 1946). 5 Bonet-Maury and Lefort, Compt. rend., 1948, 225, 1353, 1445 ; Nature, 1948, 162, 381. Bonet-Maury, Bull. Soc. Chim., 1951, 18, 333. 6 Fricke, Hart and Smith, J. Chenz. Physics, 1938, 10, 229. Fricke, Chemistry and Physics of Radiation Dosimetry, Sj~mposiim, 4, 1950. 7 Bonet-Maury and Deysine, Compt. rend., 1951, 232, 1101. * Bonet-Maury and Patti (in course of publication). 9 Gray, J . Chim. Pliys., 1951, 48, 472. 10 Risse, 2. physik. CIiem., 1929, 140, 133. 11 Weiss, Nature, 1944, 153, 743. 12 Dale, Gray and Meredith, Phil. Trans. Roy. Soc., 1949, 242, 33. of density of ionization on the decomposition of water and biological survival.

ISSN:0366-9033    

DOI:10.1039/DF9521200072

出版商:RSC    

年代:1952

数据来源: RSC