MARCH, 1967 THE ANALYST Vol. 92, No. 1092 Analytical Applications of a O.5-MeV Cockrof t - Walton Set based on the Measurement of Prompt ?-Radiation r-Radiation Emitted during Proton Reactions BY T. B. PIERCE, P. F. PECK AND D. R. A. CUFF (Analytical Chemistry Group, Atomic Energy Research Establishment, Harwell, Didcot, Berkshire) Nuclear reactions are outlined which yield y-rays when the elements from lithium to chlorine, with the exception of neon, are irradiated with protons of an energy of 0.5 MeV or less. Energies of the principal y-rays are listed and analytical applications based on measurements of the y-rays are discussed. ANALYTICAL methods based on the measurement of radiation emitted during the decay of unstable nuclei are well known, and, in particular, radiative-capture reactions induced by thermal neutrons have enabled very sensitive activation techniques to be developed for many elements.Conventional activation analysis is usually concerned with the measurement of radiation associated with some relatively slow decay, which allows the sample to be removed from the place of irradiation and subsequently counted, but many nuclear transi- tions occur too rapidly to be detected by this technique. These rapid decays can provide much information of analytical interest, but determinations based on the measurement of this so-called “prompt radiation” have hitherto received little attention, as radiations from the samples must be counted in sit%, often while irradiation is taking place, and chemical separation cannot be used to isolate the activity to be assayed from all others, before measurement.Of particular interest is the measurement of the y-radiation emitted during the de- excitation of excited nuclear states, because the energy and yield of the y-rays provides information about the type and number of nuclei present in the sample without a radioactive nuclide being necessary as a product of the reaction, and measurement of y-radiation enables the determination to be carried out on the intact sample. Advantages and disadvantages of prompt y-radiation methods vis-h-vis conventional activation analysis have been discussed elsewherel and will not be considered further here. Both penetrating radiation and charged particles can be used to produce excited nuclear states and the two techniques, when applied to analysis, are to some extent complementary, penetrating radiation being capable of pro- viding information from a relatively large sample, while charged particles permit examination of a thin section of sample near to the surface.Low energy charged particles can be used with advantage to determine light elements in surfaces when suitable y-lines are emitted from excited nuclei, and when limited penetration and heat dissipation in the sample do not preclude their use, for the coulomb barrier restricts reaction to elements of low atomic number, thus enabling the emitted y-radiation to be measured relatively easily by simplifying spectral analysis. Several analytical determinations, based on the measurement of prompt y-radiation emitted during charged particle irradiation, have already been reported but these have been largely concerned with the measurement of y-lines excited by particles accelerated by Van de Graaff electrostatic generator^.^^^^^^^^^ The use of an 0-&MeV Cockcroft - Walton set for exciting prompt y-spectra that may be used for analytical determination is considered in this paper.Although the sensitivity attainable with this machine is lower at these lower particle energies, the cost of the accelerator is also less (under L20,OOO). The use of proton irradiations to produce excited nuclear states in the elements from lithium to chlorine, with the exception of neon, is discussed. Even when targets are bombarded with particles of 0.5 MeV or lower energy, highly excited states of residual nuclei are frequently formed by virtue of the large positive energy balance of many reactions, and decay of the resulting bound or virtual levels can often be 143144 PIERCE, PECK AND CUFF : ANALYTICAL APPLICATIONS [Analyst, \’ol.92 complex. Fortunately many transitions occur in relatively low yield, thus simplifying analysis of the prompt y-spectra. A great deal of information is available in nuclear physics literature that describes reactions of the light elements with protons, and the results have been sum- mari~ed.~ *’ Consequently, nuclear processes occurring during reaction of light elements with protons will not be discussed in detail here; only salient aspects will be mentioned. EXPERIMENTAL The arrangement of equipment used for the measurement of prompt y-radiation emitted during charged particle irradiation has been outlined el~ewhere.~ The proton beam from the Cockcroft - Walton set was analysed by means of a 90” bending-magnet, and was subsequently passed through a series of slits, lenses and stops to produce a well defined beam on target.If necessary, the area of sample irradiated could be increased by “wobbling” the target (oscillating it in the beam). Secondary electron suppressors were inserted in the line if required. Targets were irradiated in single or multiple target holders, the latter being remotely controlled and capable of holding up to 40 samples. An air-tight valve on the beam tube of the largest target changer allowed samples to be transferred to the accelerator, after preparation in an inert atmosphere, or at reduced pressure if necessary.y-Rays emitted during the irradiation were detected with either a sodium iodide scintil- lator or a lithium-drift germanium diode, and the output from the detector, after amplification, was fed to an analysing system; single, 100, 512 or 1024-channel analysers were used as required and as available. When a sample was to be irradiated to a known particle dose, the current falling on the target was fed to a current integrator and the integrator used to control the analyser. Real and live times were always recorded when multi-channel analysers were in use. Targets of a variety of shapes and sizes could be accepted by target holders, but for convenience, discs of 20-mm diameter, about 2 to 3 mm thick, were usually irradiated.These discs were either cut from solid metal, or, when this was not possible, made by compressing powder or turnings with a 30-ton hydraulic press. y-RAYS EMITTED DURING REACTION OF PROTONS WITH LIGHT ELEMENTS- Reactions of light elements which yield y-rays with protons of an energy of 0.5 MeV or less are discussed in this section, and the energies of the emitted y-rays are summarised. The excitation functions for these reactions frequently exhibit sharp resonances, and when several resonances occur at proton energies of less than 0.5 MeV, for brevity only the y-yield from one or two of the most intense resonances is considered. Moreover, when the y-yield from a reaction is particularly complex, the weakest y-lines have not been included.A list of energies at which (p,y) resonances occur has already been compiled.8 Lithium-y-Rays may be emitted by lithium-6 during proton irradiation by decay of the broad 6.35-MeV level of beryllium-7 to ground or by cascade through the first excited level at 0-431 MeV.g Two high energy y-rays are emitted as a result of radiative proton capture by lithium-7 at the 0.441-MeV resonance, one with an energy of 17.6 MeV, the other less intense with an energy of 14.8 MeV.l0 y-Rays from the reaction of natural lithium (7.4 per cent. of lithium-6; 92.6 per cent. of lithium-7) with protons are dominated by the yield from lithium-7. Beryllium-The reaction 9Be (p,y) loB shows a resonance at 0.33 MeV, feeding the 6-88-MeV level in boron-10. Decay occurs by cascade through levels of 0.717,1.74 and 2-15 MeV.The most prominent y-rays occur at energies of 0.41, 0.72, 1-02, 4.71, 5.12, 6.14 and 6.86 MeV.l1S12 Boron-The y-radiation from irradiation of natural boron with low energy protons is dominated by the reaction llB (p,y) 12C (Q = 15.958), and a resonance at 0.16 MeV feeds the capture state of carbon-12 at 16.11 MeV. Decay is to ground or by cascade through the 4.4-MeV level. Thus y-rays having energies of 16.1, 11.7 and 4.4 MeV3 are emitted. Carbon-Although a resonance for the reaction 13C (p,y) 14N is reported at 0-448 MeV, the y-ray spectrum from the reaction of natural carbon with low energy protons is dominated by the yield from the reaction 12C + p. A resonance for capture radiation is observed a t a proton energy of E, = 0.459 MeV, and decay of the 2.36-MeV capture level in nitrogen-13 is to the ground state.l*March, 19671 OF A O.5-MEV COCKCROFT - WALTON SET 145 Nitrogen-A resonance for radiative capture of protons by nitrogen-14 is observed at a proton energy of E , = 0-28 MeV feeding the 7.56-MeV level in oxygen-15. Decay occurs by cascade through levels at 5.2, 6.1 and 6.7 MeV, and y-ray energies have been reported as being 0.75, 1.39, 2-38, 5-29, 6.21 and 6.84 MeV.15 A weak resonance for proton capture by nitrogen-15 occurs at E, = 0-338 MeV, and at the same proton energy a resonance is observed for the reaction 15N (p,.) 12C, producing the 4.4-MeV level in carbon-12.However, a more intense resonance is observed at 0.429 MeV,lG but even at this energy the total y-yield from natural nitrogen (99.6 per cent.of nitrogen-14; 0.4 per cent. of nitro- gen-15) is low. Oxygen-The cross-section for the reaction l60 (p,y) l7F is low up to O.5-MeV proton energy, although radiative capture of protons has been reported.17 The first resonance in the reaction l80 + p is found to be at E , = 0.560 MeV.6 Fluorine-Reaction of fluorine with protons is dominated by the reaction lgF (p,.) 1 6 0 , for which three resonances are observed for proton energies at 0.222, 0-340 and 0.486 MeV. The resonance at 0-340 MeV is the most intense (cross-section = 160 mb) and the oxygen-16 de-excites largely by emission of y-rays of an energy of 6.14 MeV (96 per cent.) and 7.12 MeV (4 per cent.) .18 Sodium-The most intense resonance for the reaction of sodium with protons occurs at 0-308 MeV.More than 20 y-rays have been detected from the reaction 23Na + p at this energy, of which the most intense from proton capture have energies of 1.37, 2.86, 3.83, 4.23, 6.77, 7.75 and 10.61 MeV. In addition, a 1.63-MeV y-ray is observed from the reaction 23Na (p,.) *ONe.19 Magnesium-Two resonances in the reaction 24Mg (p,y) 25Al are observed at E, < 0.5 MeV, at E , = 0.22 and 0.42 MeV, corresponding to excitation energies of aluminium-25 of 2-50 and 2-69 MeV. De-excitation of aluminium-25 occurs by a number of different modes, giving y-rays of 0.46, 0.95, 1-54 and 2.04 MeV at the lower resonance and 0.45, 0.85, 0.88, 0-95, 1.33, 1.80, 2-24 and 2.69 MeV at the higher resonance.20$21 Resonances in the reaction 25Mg (p,y) 26A1 occur at 0.316, 0.392, 0.437 and 0.496 MeV.7 y-Rays at the resonance at 0.436 MeV are observed with energies of 0.415,1-33,1+34,3-82,4-70, 6-28 and 6.77 MeV.22 Three resonances for the reaction 26Mg (p,y) 27Al are found at E, < 0.5 MeV, at E, = 0.292, 0.338 and 0.454 MeV;7 y-rays emitted at the 0.454-MeV resonance have energies of 0.84, 1-01, 2-92, 3.18, 4.65, 5-79, 7-73 and 7.87 MeV.23 The most intense resonance for reaction of protons with natural magnesium (78.6 per cent.of magnesium-24, 10.11 per cent. of magnesium-25 and 11-29 per cent. of magnesium-26) at proton energies of less than 0.5 MeV is the one at E , = 0.454 from the reaction z6Mg (p,y) 27Al. AZuminium-Five resonances occur for the reaction 27Al (p,y) 28Si at proton energies of E , = 0-225, 0-294, 0.326, 0.405 and 0.442 MeV.7 The most intense resonance is a t E, = 0.405 MeV,Z4 and emitted y-rays have energies of 1.79, 2-84, 3.40, 4.60, 5.10, 6-80, 7.36 and 10.2 MeV.25 Silicon-The y-yield from the reaction of natural silicon with protons is not high. Re- sonances for the reaction 28Si (p,y) 29P occur at E , 7 0.369 MeV, for the reaction 29Si (p,y) 30P at E , = 0.326 and 0.414 MeV, and for the reaction 30Si (p,y).31P at E, = 0.5 MeV. The 2% (p,y) Z9P reaction at the 0-369-MeV resonance yields y-rays with energies of 1-14, 1-37, 1.72 and 1-95 MeV,26 while at the 0.414-MeV resonance the 29Si (p,y) 30P gives y-rays at E y = 0.67, 1.46, 2.28, 2-99, 4.50, 5.30 and 5-94 MeV.27 When a thick target of natural silicon is irradiated with protons of an energy greater than 0.414 MeV, the 5.30 and 0.67-MeV y-rays are found to be the most intense.Phosphorus-Resonances in the 3lP ( p , ~ ) 3 ~ S are observed at a proton energy of E , = 0.355 and 0.440 MeV. y-Rays emitted include those with energies of Ey = 2.23, 2.24, 4.47, 4.80, 7.03 and 9.27 MeV.28 Sulphur-A resonance for the reaction of protons with naturally occurring sulphur is found at 0.446 MeV, and is ascribed to the reaction 33S (p,y) 34Cl. The three most intense y-lines from this reaction have energies of 2.02, 3.41 and 5-41 MeV.29146 PIERCE, PECK AND CUFF ANALYTICAL APPLICATIONS [Anahst, VOl. 92 ChZorine-Irradiation of natural chlorine with protons provides one resonance below 0.5 MeV, at 0.446 MeV, which is ascribed to the reaction 35Cl + p. De-excitation of the residual argon-36 nucleus occurs primarily through the first excited state giving y-lines at 1.95 and 7-09 MeV.30 The y-lines mentioned above are collected together in Table I in the order of increasing energy, but it should be noted that many of the y-rays are emitted in low yield and are only observed after irradiation of a sample containing a large proportion of the target nuclides, to a high particle dose.Table I does not include y-rays emitted after decay of the ground state of the nuclide produced by the (p,y) or ( p , ~ ) reaction [eg., 13N (p+) 13C produced by the reaction 1% (p,y) 13N] and first and second escape peaks have not been included where pair production is possible. Examples of two of the more complex spectra excited by low energy protons in thick targets are shown in Figs.1 and 2 and are in general accord with results already published for thin targets. Y-Ray energy, MeV I 50 I 50 Channel I0 Fig. 1. Prompt spectrum from sodium irradiated with 0.48-MeV protons. Angle of observation O", target 7.5 per cent. sodium as sodium bromide in iron powder, dose 100,000 PC APPLICATIONS- Analytical methods may be based on the quantitative measurement of the yield of prompt y-rays emitted during irradiation of a sample with charged particles, with beam current integration or some other form of standardisation to monitor the particle dose falling on the target. Interference with the method will be caused by the same excited levels being populated by reaction with nuclei of elements other than the one to be determined, or by y-rays from other elements being of an energy that is so similar to the one being measured that they cannot be distinguished by analysis of the y-spectra.In certain cases it is possible to calculate the magnitude of a particular interfering y-peak from others in the spectrum, and when a sharp resonance is observed in the excitation function for the reaction upon which the analytical method is based, the y-yield from that specific resonance may be obtained by irradiating the sample at energies above and below the resonance and calculating the difference. However, from Table I it can be seen that the reactions induced by O.5-MeVMarch, 19671 OF A O.5-MEV COCKCROFT - WALTON SET 147 protons are usually radiative capture and only rarely is the same nuclide produced from different target elements.One of the exceptions is the 4.43-MeV level in carbon-12 which is formed both by the (p,y) reaction on boron-11 and the (p,.) reaction on nitrogen-15. When boron is the target element the 4.43-MeV line is accompanied by high energy y-rays at 11.7 and 16.1 MeV. TABLE I ENERGIES OF Y-RAYS EMITTED BY LIGHT ELEMENTS IRRADIATED WITH PROTONS OF ENERGY LESS THAN 0.5MeV Ey, MeV 0.41 0.42 0.45 0.46 0.67 0.72 0.75 0.84 0.85 0.88 0-95 1.01 1.02 1.14 1-33 1.33 1.37 1-37 1.39 1.46 1.54 1-63 1-72 1.79 1-80 1-84 1.95 1.95 2.02 Ey, MeV 2.04 2.23 2.24 2.24 2.28 2.36 2.38 2.69 2.84 2.86 2-92 2.99 3.18 3.40 3.41 3-82 3-83 4.23 4.43 4.43 4.47 4.50 4.60 4.65 4.70 4.71 4.80 5.10 5.12 Ey, MeV 5.29 5.30 5-41 5.79 5.94 6.14 6-14 6.21 6.28 6.77 6.77 6.80 6.84 6.86 7-03 7.07 7.12 7.36 7.73 7.75 7.87 9.27 10.2 10.61 11.7 14.8 16.1 17.6 - The depth of penetration that can be achieved with charged particles accelerated by an O.5-MeV Cockcroft - Walton set is low and, in addition, the variation in cross-section that often occurs as the energy of the particles is degraded in a thick target may result in the y-yield being emitted from only a small proportion of this depth.Therefore, any analytical method based on the measurement of prompt y-radiation emitted during charged particle irradiation will be governed by these characteristics and will only yield information about the composition of a thin section of a thick target near to the surface. If the sample is homogeneous, this yield may be considered to be representative of the sample as a whole; if the sample is inhomogeneous the distribution of certain elements through the sample may be investigated, either by scanning the surface with a finely collimated particle beam, or by irradiating separate sections of the sample that have been prepared mechanically. Specific analytical applications of the measurement of prompt y-radiation emitted during charged particle irradiation will not be considered in any detail here.However, to illustrate the uses of the technique three examples are given in Fig. 3, none of which requires any chemical pre-separation of the constituents of the sample. Each time the y-yield from a particular reaction is plotted against the known amount of target element present in the sample. Results obtained for the determination of carbon in steel, based on measurement of the 2-3-MeV y-ray emitted during decay of excited states of nitrogen-13 formed by the reaction 12C (p,y) 13N, are shown in Fig.3 (a). Determinations based on this reaction have already been described with O.8-MeV protons,2 but as the resonance feeding the 2-3-MeV state of nitrogen-13 occurs at E , = 0.459 MeV, it should be possible to use an O-5-MeV Cock- croft - Walton set for similar determinations. Results presented in Fig. 3 (a) were obtained by irradiating samples with 0.48-MeV protons. Fig. 3 ( b ) shows results obtained from the148 PIERCE, PECK AND CUFF: ANALYTICAL APPLICATIONS Y -Ray energy, MeV [Analyst, Vol. 92 0 5 I 1.78 I I 1 3.40 460510 736 I 0 2 1 I I I I Fig. 2. Prompt spectrum from aluminium metal Angle of observation go", irradiated with 0-48-MeV protons.dose 50,000 pC irradiation of magnesium - aluminium alloys with 0.48-MeV protons, measuring the 0.84-MeV aluminium-27 line emitted as a result of the reaction 2sMg (p,y) 27Al. Fig. 3 (c) is a plot of the yield of 6.1-MeV y-rays emitted during irradiation of mixtures of iron and calcium fluoride. In the absence of previous chemical processing of the sample, prompt-radiation techniques must rely on purely instrumental methods to isolate the activity to be measured from all others emitted by the sample. Under these conditions, the smallest amount of any element that can be determined quantitatively is governed by a number of factors, including the I I I I I 0 2 4 6 8 I( Magnesium, per cent.L I00 200 3W 400 I luorine, p.p.m. Fig. 3. Plot of y-line yield against the content of emitting element for a number of samples: (a), carbon in plain carbon steels; ( b ) , magnesium in magnesium - aluminium alloys; (c), fluorine in calcium fluoride - iron mixturesMarch, 19671 OF A 0.5-MEV COCKCROFT - WALTON SET 149 dose to which the sample can be irradiated, the efficiency of y-ray detection, the method of spectral analysis and the intensity of interfering y-radiation. Consequently, the approximate sensitivities, given in Table 11, are recorded as y-yields of targets containing a single reacting element, as detected by a 3 x 3-inch thallium-activated sodium iodide scintillator, 3.5 cm from the target at 90” angle of observation. The total dose to which samples could conveniently be irradiated depended on the rate at which the dose could be given, that is, the beam current during the irradiation.For samples that were easily damaged by heating, the beam current was usually limited to 1 pA or less, but many metallic samples were subjected to beam currents of 100,uA. The dose of 105 microcoulombs given in Table I1 corresponds to an irradiation approaching 17 minutes at a beam current of 100pA. Where y-ray energies were sufficiently high for pair-production to occur, counts in the first and second escape peaks were totalled together with the photopeak. As sensitivity depends on stopping power, the type of target used is also recorded in Table 11. TABLE I1 Y-YIELDS FROM THICK TARGETS OF SOXE LIGHT ELEMENTS BOMBARDED WITH 0.475-MeV PROTONS Element Beryllium .. Boron . . Carbon . . Fluorine . . Sodium . . Magnesium . . Aluminium . . Target . . 2 per cent. Be - Cu alloy . . Be - Fe . . C - Fe .. CaF, - Fe . . NaBr - Fe .. Mg- Fe . . A1 - Fe y-Ray energy, Sensitivity, 0.72 2.4 4.43 4.2 2.36 6.1 6-14 950 1.37 2.2 0.84 + 1.01 2.5 1-79 0-3 Ey, MeV c per p.p.m. per lo5 pC Prompt y-rays must be measured in the presence of two types of background; that from any natural or artificially formed radioisotope in the vicinity of the target and prompt radiation produced in the machine. The intensity of the y-yield from radioisotopes will depend on the previous history of the irradiation position, which, for example, may have been used for neutron generation. Energies of y-rays emitted by long-lived radioisotopes are of a lower energy than much of the radiation from proton capture, and interference occurs only when prompt y-rays of low energy are being measured. The contribution of delayed radiation can be assessed by counting before or after irradiation, or eliminated by using a pulsed beam, and subtracting counts detected between pulses from counts detected during pulses5 The most troublesome form of machine background has been found to arise from contaminants in the target assembly or accelerator flight-tube.In particular, y-contributions from carbon and fluorine are difficult to remove when low energy protons are used to irradiate the samples. By careful cleaning of flight-tube, stops and target assembly, the fluorine background could be reduced to about 1 p.p.m. for these experiments.1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. REFERENCES Pierce, T. B., Peck, P. F., and Henry, W. M., AnaZyst, 1965, 90, 339. Pierce, T. B., in “Practical Aspects of Activation Analysis with Charged Particles,” European Morgan, I. L., 09. cit., p. 149. Pierce, T. B., and Peck, P. F., in Shallis, P. 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