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.