Absolute α-induced thick-target gamma-ray yields for the elemental analysis of light elements

Nuclear Instruments and Methods 212 (1983) 441-444 North-Holland Publishing Company

ABSOLUTE a-INDUCED THICK-TARGET ANALYSIS OF LIGHT ELEMENTS

441

GAMMA-RAY

YIELDS FOR THE ELEMENTAL

R. L A P P A L A I N E N , A. A N T I ' I L A a n d J. R A I S A N E N Department of Physics, University of Helsinki, SF-O0170 Helsinki 17, Finland Received 21 June 1982 and in revised form 22 November 1982

A systematic study of the absolute thick-target yields of prompt 7-rays following 4He+ bombardment has been carried out at E~ = 2.4 MeV for the elements Z = 3-9, 11-14. The spectra of interest are depicted and a table of the most suitable y-rays for elemental analysis is given. The detection limits are given under practical measuring arrangements. The advantage of the method is its good sensitivity for the detection of Li, Be, B and F on the ppm level even in samples containing high concentrations of other light elements such as Na, Mg and AI.

1. Introduction By c o m b i n e d use of the different elemental analysis m e t h o d s developed for low energy accelerators, almost all elements can b e detected u n d e r favourable conditions. The methods applied most frequently are Rutherford backscattering (RBS), and p r o t o n induced X-ray a n d y-ray emission ( P I X E a n d PIGE, respectively). Naturally, all these methods have their particular limitations, but in addition they have one c o m m o n limitation, namely, that the elemental analysis of the sample is not normally possible if it contains even a few percent of the element for which the detection sensitivity of the m e t h o d used is highest. For example, in connection with recent (p, y) yield m e a s u r e m e n t s [1], the P I X E analysis of a Hf sample yielded no other elements t h a n Zr, a l t h o u g h with P I G E seven trace elements were observed. The inverse situation occurs, if the sample contains mainly light elements such as Li, B or F, for which P I G E is very sensitive but not PIXE. Furthermore, it is evident that, by extending the variety of b o m b a r d i n g particles a n d hence the reactions, the detection possibilities can be improved. The variety of particle-particle reactions available could be large a n d their sensitivity good. However, due to the fact that their use is more complicated than the radiative reactions, they are less favoured in practice. The advantages of the radiative reactions are that the high penetrability of the y-rays diminishes matrix effects a n d the use of external b e a m allows the m e a s u r e m e n t of volatile samples. In our earlier work [2] a systematic study of the p r o t o n induced y-ray yields for the elements Z = 3-9, 11-21 has been carried out. In the present work, a similar study has b e e n performed using a l p h a induced g a m m a - r a y emission (AIGE). The advantage of the 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d

alphas is that they belong also to the n o r m a l routine use of low energy accelerators and are therefore readily available. Earlier alpha-studies have been performed by Borderie a n d B a r r a n d o n [3], and Giles a n d Peisach [4] at E~ = 3.5 a n d 5 MeV, respectively. However, the decay modes and cross-sections at low and high energies are quite different, a n d in addition, neither paper can be considered complete. Borderie and B a r r a n d o n [3] measured only y-rays with energies below 2 MeV, except in the case of Be, where the intensity of the double escape peak at Ey = 3417 keV was determined. Giles a n d Peisach [4] measured only the relative yields and give no m e n t i o n of Be, which has the highest total reaction cross-section for alphas.

2. Experimental arrangements The 4He + b e a m was generated with the 2.5 M V Van de G r a a f f accelerator of the University of Helsinki. The 4He+ passed through adjustable slits and a 40 cm liquid nitrogen cold trap before striking the target, which could be cooled with either water or liquid nitrogen. T h e angle of incidence of the 4He+ b e a m on the target was 45 °. The y-rays were detected using a Princeton 110 cm 3 Ge(Li) detector with a n energy resolution of 1.9 keV at E v = 1.33 M e V a n d 3.0 keV at E v = 2.61 MeV a n d with an efficiency of 21.8%. The y-ray spectra were stored in the 4 k m e m o r y of a P D P - 9 computer, after which the spectra were analysed on the PDP-9 and a Burroughs B6700. In all measurements the detection was at an angle of 0 = 55 ° relative to the b e a m direction at a distance of 4 cm from the target. The diameter of the b e a m spot on the target was collimated to a b o u t 4 mm. The b e a m intensity, which was adjusted according

442

R. Lappalainen et al. / Thick- target "/- ray yields

to the counting rate, varied from 0.5 nA to 10 ~tA. The Be-, C-, Mg-, AI- and Si-targets were 1 × 1 cm ~ plates with thickness approximately 1 mm. In other cases the sample were prepared from the powders, i.e. Li2SO 4, B, TaN, SnO2, CaFz and NaCI, by pressing them into pills, 1 mm thick and 13 mm in diameter.

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The measurements were performed on all Z~< 14 elements, except for hydrogen, helium and neon. In the hydrogen and helium cases the Q-values are too low for an observable y-yield to be possible. N e o n may yield

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R. Lappalainen et al./Thick-target v-ray yields Table 1 Thick target yields of alpha induced T-rays at E. = 2.4 MeV and at 8 = 55 °. The detection limits are based on the volcanic stone measurement. Element

Ey (keV)

Li Be

478 4439 170 3088 3684 3854 a 937 1041 1080 2125 2471 2542 4525 351 1634 110 197 1275 440 1130 1809 1273 1779 2839 2235 2230

B

C N

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Detection limit

9.6 X 105 1.6 X 106 2.5 X 104 3.1 x 103 4.9 x 104 3.1 x 104 5.9 5.6 13 4.5 4.4 7.8 5.9 15 16 9.2 x 102 1.6× 104 6.2x 103 1.7 X 103 15 610 8.8 15 0.6 15 0.3

0.45 ppm 0.42 ppm

10 ppm

1.1% 2.0% 22 ppm 260 ppm

1.3% 3.2% 65%

No separate v-ray was observable.

y-rays, b u t in the elemental analysis of non-gaseous samples it is of n o great importance. Because the y-yield is low in all cases other t h a n for Li, Be, B a n d F, the y-spectra were taken using only one b o m b a r d i n g energy E,, = 2.4 MeV. W h e n the sample was in c o m p o u n d form, the relevant spectra were also t a k e n from the other elements. In fig. 1 the spectra for all elements except for c a r b o n are shown. In the c a r b o n case, although a 3 m C spectrum was collected, n o y-peaks from the c a r b o n reaction were observable, only those due to the neutrons. The spectra illustrated in fig. 1, in addition to the y-peaks from the relevant reaction, include those from the reactions induced b y n e u t r o n s in the Ge-detector, b u t for clarity such b a c k g r o u n d or i m p u r i t y peaks were identified a n d removed, whenever possible. The energy values a n d the origin of each v-ray is given in the figures. The energy values are taken from the latest compilations [5,6]. All results have b e e n collected in table 1. The absolute yields correspond to those o b t a i n e d from the pure element and were deduced by comparison against the 24 + 2 eV strength of the Ep = 992 keV resonance of the 27Al(p,y)28Si reaction [7]. The y-ray yields from the samples in c o m p o u n d form were converted into those of the pure materials using the m e t h o d proposed by K e n n y et al. [8] a n d the stopping powers were o b t a i n e d from the compilations of Ziegler [9]. F o r the values given in table l, no error limits are quoted, b u t due to uncertainties arising from the b o m b a r d i n g energy, the geometry, the detection efficiency a n d a b s o r p t i o n effects, the error c a n be as m u c h as 20% in some cases. However, for accurate elemental d e t e r m i n a t i o n the s t a n d a r d spectra should be taken in the applied geometry in order to avoid those uncertainties.

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444

R. Lappalainen et al. / Thick - target T-ray yields

Due to the short lifetime of the de-exciting state and the high recoil of the nucleus, a clear Doppler-broadening occurs in many of the y-peaks shown in fig. 1. The broadening of the y-peak is useful in identifying the origin of the peak, especially in the cases where only one y-ray peak occurs, such as in the cases of Li and Be. In fig. 2 the spectrum taken from volcanic stone (Kilimanjaro) is illustrated. In the insert of the figure the weight percents of the components observed are given. The stopping power value needed in the analysis was calculated by supposing that the stone contains mainly light elements, i.e. those which were observed. The weight percents deduced agree fairly well with the average values of volcanic stone [10]. The spectrum shown in fig. 2 illustrates the possibility of measuring low Be abundances. The energy of the y-peak from Be is the highest and well above the normal background. The detection limits given in table 1 are based also on the stone measurement; i.e. the minimum detectable peak was assumed to be three times the square root of the background in the energy window of interest. Due to the high energy and high intensity of the y-peaks from Be in the case of the volcanic stone, the sensitivity limits of other elements is expected to be somewhat lower in samples not containing Be.

0.01 0.5 ppm can be given. Although A I G E is relatively insensitive as regards the detection of O and N, it is nevertheless useful in O and N determinations, because the samples often contain these elements in high concentrations. The sensitivity comparison for the data obtained with high energy alphas is difficult, although the same standard samples could be used, as the sensitivity depends strongly on the composition of the sample. However, it is evident that the sensitivity is somewhat better with higher alpha energies. On the other hand, a clear advantage in the low energy measurements, in addition to the fact that the accelerator investment is low, is that due to the low neutron fluxes even a large volume Ge(Li) detector can withstand continuous use without a decrease of its resolving power for many years (the one in our laboratory has withstood nine years of use). In conclusion, in view of the above and the points raised in the introduction, A I G E can be considered to be a useful complementary nuclear method for elemental analysis. It may be possible that, by use of other light projectiles, e.g. deuterium, tritium and 3He, somewhat lower detection limits could be achieved, but in the deuterium and tritium cases, at least, their use would be difficult due to the high neutron fluxes involved.

4. Discussion References

In a comparison of the data collected in table 1, it can be seen that the y-yield in the Li, Be, B and F cases is clearly higher than for the other elements. Furthermore, in comparison with the P I G E data given in our earlier paper [2], the y-yield for A I G E is higher only in the Be case, where it is about 100 times higher and due to the high background occurring normally in the (p, y) reactions the sensitivity of A I G E will be about 10 3 times better. Accordingly, one advantage of A I G E is its high sensitivity in the detection of Be. Another important advantage is that trace quantities of Li, Be, B and F can be determined from samples containing any other elements whatever. Such determinations with P I G E are not possible, if the sample contains, e.g., Na, Mg or AI as major components. The sensitivity of A I G E for the detection of Li, B, Be and F is indeed so good that in the spectra taken from our " p u r e " Na, Mg, A1 and Si samples we could detect the y-peaks from Li, B and F, and also, for Be, rather low upper limit of

[1] J. Rais~inen and R. H~inninen, Nucl. Instr. and Meth. 205 (1983) 259. [2] A. Anttila, R. H~inninen and J. Rais~inen, J. Radioanal. Chem. 62 (1981) 293. [3] B. Borderie and J.N. Barrandon, Nucl. Instr. and Meth. 156 (1978) 483. [4] J.S. Giles and M. Peisach, J. Radioanal. Chem. 50 (1979) 307. [5] F. Ajzenberg-Selove, Nucl. Phys. A281, 300, 320, 336 and 360 (1977-1981) 1. [6] P.M. Endt and C. Van der Leun, Nucl. Phys. A310 (1978) 1. [7] J. Keinonen and A. Anttila, Comm. Phys.-Math. 46 (1976) 61. [8] M.J. Kenny, J.R. Bird and E. Clayton, Nucl. Instr. and Meth. 168 (1980) 115. [9] J.F. Ziegler, Helium: stopping powers and ranges in all elemental matter (Pergamon, New York, 1977). [10] R.C. Weast (ed.), Handbook of chemistry and physics, 61st ed. (CRC Press, Cleveland, Ohio 1980 1981).