TRI{PP{96{10 Apr 1996 Dependence on neutron energy of neutron induced peaks in Ge detectors E. Gete, D.F. Measday B.A. Moftah, M.A. Saliba, T.J. Stocki TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., Canada V6T 2A3 and Dept. of Physics, University of British Columbia, Vancouver, B.C., Canada V6T 1Z1 Abstract We have studied the peak shapes at 596 and 691 kev resulting from fast neutron interactions inside germanium detectors. We have used neutrons from a 252 Cf source, as well as from the 28 Si(?,n), and 209 Bi(?,xn) reactions to compare the peaks and to check for dependence of peak shape on the incoming neutron energy. In our investigation, no dependence of these peak shapes on the neutron energy spectra has been observed. In a comparison of these peak shapes with other studies, we found similar results to ours except for monoenergetic neutron irradiations around 1 to 8 MeV. (submitted to Nuclear Instruments and Methods) Corresponding author: Dept. of Physics, 6224 Agricultural Road, Vancouver, B.C. Canada V6T 1Z1., Tel:(604)822-3853, Fax:(604)822-5324, E-mail: MEASDAY@TRIUMF.CA 1
1. Introduction When germanium detectors are irradiated with neutrons of energy of an MeV or more, triangular peaks result from inelastic neutron excitation of the nuclei of the various Ge isotopes within the detector itself. These peaks have a peculiar triangular shape of about 40 kev across, because the recoiling Ge nucleus deposits its energy inside the detector, which adds to the energy of the de-excitation -ray when it interacts in the active volume. The presence of these peaks was rst discussed by Chasman et al. [1], and a further investigation of these peaks was done by Bunting and Kraushaar [2]. Recently, Skoro et al. [3] claimed to have found a dierence if the energy spectrum of the neutron source is modied. In studies of -ray lineshapes using Ge detectors, a knowledge of these peak shapes is important if these peaks happen to be around the region of interest, since the background from these peaks could cause serious systematic errors. In addition knowledge of the dependence of the peak shape on the neutron energy spectrum could be important, since if there is no dependence; one could use the peak shape obtained from one data-set for another one, caused by a dierent spectrum of neutrons. This work was initiated when a measurement of a Doppler broadened line of 28 Al from? capture on 28 Si was made. The goal was to measure the induced pseudoscalar coupling constant of the weak interaction [4], [5]. Two Ge peaks happen to be in the region of interest (near 1204 and 1216 kev), and since these Ge peaks are relatively weak, a study of the more prominent peaks at 596 and 691 kev was performed. We exposed a 10% HPGe detectors to neutrons from pion absorption as well as from a 252 Cf source and compared the three spectra. The pion experiment was particularly interesting, since it was possible to discriminate neutron and -ray events using their time of ight. 2. Neutron energy spectra from? and? capture and 252 Cf The neutron energy spectra following? capture on nuclei have been studied by several authors [6,7,8]. The spectrum consists of low energy neutrons, peaking around 2 MeV, from de-excitation of the nucleus formed after the capture, but the neutron energy extends to 25 MeV or more for more direct events. Pion absorption also results in one or two fast neutrons as well as evaporation neutrons for heavy nuclei. The neutrons from pion absorption could have an energy as high as 60 MeV or more, but also peak at a few MeV for heavier elements [9]. The neutron energy spectrum from the spontaneous ssion of 252 Cf is peaked at 0.7 MeV and extends to about 10 MeV. This spectrum can be well approximated by a Maxwellian distribution given by: where T is about 1.3 MeV [10]. 3. Experimental Methods dn de = E1=2 e?e=t A schematic of the experimental setup for the pion run is shown in Fig. 1. S1, S2, S3 and S4 are thin plastic scintillators used to dene the pion stop signal in the 2 (1)
M13 channel at TRIUMF. In order to slow down the pions coming from the channel, a 2 mm thick Al degrader was inserted between S1 and S2, and a 2.5 cm thick Al degrader was inserted between S2 and S3. A 10% HPGe with a resolution of 2 kev full width at half maximum (FWHM) at 1.33 MeV was used. The Ge detector was surrounded by a NaI annulus to detect -rays Compton scattered from the Ge detector. The Ge detector was placed at 50 cm from the target in order to have a time of ight separation between the neutrons and the -rays reaching the detector. Typical ight times over this distance for neutrons of 1 and 100 MeV are about 60 and 7 ns respectively compared with 1.7 ns for photons. The time resolution of the Ge-detector is about 7 ns FWHM so that the neutrons can eectively be separated in the time spectrum. The 252 Cf run was also performed using the same Ge detector. In the? - 28 Si experiment, two HPGe detectors (Ge1 and Ge2) with relative eciency of 40% and 20% were used to detect the -rays. Time of ight separation was not available in these two experiments. 4. Methods of Data Analysis A time spectrum representing the time between the? shown in Fig.2. The time spectrum consists of the following events: stop and a Ge signal is 1. A \prompt" peak which results from the pionic x-rays and capture -rays occurring almost immediately after the pion stop. 2. A tail-like continuum from neutron induced -rays, which take a few nanoseconds to reach the Ge-detector and its surroundings, after being produced from? absorption in the target. 3. A wavy continuum from delayed -rays, \slow" neutron and background events. The waviness is caused by the R.F. structure of the proton beam bursts every 42 ns. 4. The continuum to the left of the prompt peak which is due to random background events not related to the pion absorption. The neutron induced events were then selected from the \neutron window" and the neutron induced peaks in the Ge detector were analyzed. For the 252 Cf and? - 28 Si runs, it was not possible to separate the neutron induced events by energy, and the peaks obtained from the total -ray spectra were analyzed. 5. Result 5.1. Fitting procedures The 596 kev and the 691 kev peaks from the rst excited states of 74 Ge(n,n 0 ) 74 Ge and 72 Ge(n,n 0 ) 72 Ge have been studied, and a comparison of these peak shapes from the various spectra has been made. 3
This comparison was made by tting the peaks to the following function: Y (x) = a 0 ERF C + nx i=1 a i Exp "? (x? x o) # Exp o "? 1 2 2 i (x? x i ) 2 # "? (x? x o) # + F (2) In the above equation, the rst expression corresponds to a complementary error function which basically determines the edge of the Ge(n,n 0 ) peak. The second expression determines the tail of the peak which was assumed to be exponential. The expression inside the summation is a Gaussian function which corresponds to any symmetric peak which might be sitting in the region of interest with about 0.8 kev. The last parameter F corresponds to a background which was assumed to be at for that region. A least squares t was done by allowing a o, a i, x o, x i, o, i, and to vary in the above expression. The parameter which was compared is since it determines the characteristic of the decaying exponential tail. The peaks obtained form the various spectra and their ts are given in Figs. 3-11. Table 1 and 2 give the values of for the 596 and 691 peaks for the various cases as discussed in sections 5.2 and 5.3. The rst error given is the statistical error obtained from the t while the second one is the systematic error. The sources of the systematic error considered are: 1. The lack of knowledge of the precise shape and level of the background. 2. Weak -ray peaks which could be lying in the exponential region. 3. Contribution of other possible weak Ge(n,n') bump in the region of interest. The systematic error in each case was estimated by tting the function to dierent regions as well as by xing some parameters and monitoring the change in as the xed value is being varied within a certain range. An example is given in Fig. 3 where a t was done using dierent regions of the same peak and a dierence of 20% in the value of was observed. In addition, it was observed that including weak peaks which exist in the region of the t could give results which are dierent by more than 10% (Fig. 4). For the peak at 596 kev, the 608 and the 630 kev peaks which result from the second excited states of 74 Ge and 72 Ge were included in the ts for the Si run since the statistics in those runs is superior to the other runs. Including the peak at 608 kev in the t gave negative value for the amplitude of the peak, and this is probably caused by the -ray peak at 609 kev (Fig. 5). However we xed the amplitude to a certain value and made a t. This was done for dierent values of the amplitude, and a change of less than 6% in the value of was observed for both detectors. The triangle at 630 kev was also included in the t, and the dierence in the value of with and without is also less than 5%. There are several features of the 596 and 691 kev peaks which should be explained. For the 596 kev feature, there could be an additional triangle at 608 kev, from the cascade of the 1204 kev level in 74 Ge, if the subsequent 596 kev -ray escapes 4
from the detector. However, the excitation of the 1204 kev level is about half of that of the 596 kev level [11] and most of the time both -rays appear to be absorbed in the detector because we nd no need for a 608 kev triangle. There was however a gaussian peak at 609 kev which we cannot identify. (If there are thermal neutrons around, one can get a 608.4 kev line from 73 Ge(n,), but then one also gets a stronger peak at 595.7 kev which is not observed in this spectrum.) At 630 kev, there is a clear need for another triangle. This is strange because this is also a cascade, in this case from the 1465 kev level in 72 Ge, but the amplitude is small, about 1% for Ge1 and 4% for Ge2 of the 596 kev triangle. There also might be a structure around 645 kev. The 691 kev triangle is particularly unusual. This comes from excitation of the rst excited state of 72 Ge, a 0 + which does not decay via -rays but by emission of a conversion electron, and its detection probability is practically 100%. However, the level has a measurable lifetime of 0.64 s [12]; thus the main pulse is separated from the nuclear recoil pulse and the energy deposited by this recoil is not fully added to the pulse caused by the conversion electron. The actual reduction depends on the electronic circuits used; in our case, the spectroscopic amplier had time constants of 3 s for the? and 252 Cf runs and 3 s for Ge1 and 5 s for Ge2 for the? 28 Si run. The width of this peak is noticeably narrower in most experiments, and of course depends on the details of the timing of the gates etc. 5.2. Comparison of neutron peaks from?,?, and 252 Cf runs In this section, we discuss the comparison between the peaks from 28 Si(?,n), 252 Cf and 209 Bi(?,xn), then we discuss the comparison for the 596 kev peak of 209 Bi(?,xn) spectra for dierent neutron energies. 691 kev The values of found for the 252 Cf and the? 209 Bi spectra are very similar (15.2 and 15.4 kev respectively), (Table 1) whereas the values for? 28 Si are dierent by about 15%. For the peaks obtained from 28 Si (Figs. 7,8), a peak at 691 kev is observed; this peak is probably due to the fact that the lifetime of the state (0.64 s) is comparable to the average charge collection time of the Ge detector. Hence the energy deposited by the recoil is deposited separately from the energy deposited by the conversion electrons. However, we cannot exclude the possibility that there is a 691 kev line being observed in the reaction 28 Si(?,n) itself. The peak is less pronounced in Ge2, i.e. Fig 8, because the energy resolution is slightly worse and the neutron triangle seems relatively more intense. In addition there were two more peaks at 718 and 731 kev. On the other hand, the 209 Bi and 252 Cf spectra are cleaner (Figs 4,6), and no peak at 691 kev is observed, probably because the timing was dierent in the electronics. 596 kev The values for the three cases agree well within the errors. Due to a complex background, it was not possible to t the 596 kev peak from the 252 Cf run. These values are also consistently larger than the values of for the 691 kev peak by about 20%. This is probably because of the long lifetime of the 72 Ge rst excited state, as explained above. 5
5.3. Comparison for dierent neutron energies The neutron time of ight spectrum for the? Bi run was divided into two and the peaks at 596 kev corresponding to the two time windows were compared (Figs 9-12). The results obtained are given in Table 2. The slower neutrons correspond to an energy of about 1 MeV whereas the faster neutrons have an average energy of about 20 MeV. Since there was a slight change in the timing of the electronics during the course of the experiment, the runs before and after the change were analyzed separately. As given in Table 2, the values of for the rst set of runs agree within the statistical error given. While for the second set of runs, the value of for the high energy neutron spectra is signicantly larger. Although we cannot fully explain these discrepancies, it is clear from Fig. 12 that the region between the iodine -rays (about 625 kev) is being tted very poorly due to an apparent excess of counts above 630 kev. We have presented these data to act as a warning that, with poor statistics, it is easy to be confused by unidentied background. 6. Discussion In the work of Skoro et al. [3], they tted the 691 kev peak from the following neutron sources using a function similar to ours. 1. Environmental neutrons using a 10 cm lead shield. 2. Environmental neutrons using 20 cm lead shield. 3. Neutrons from a 252 Cf source. The values of they found are 20.8 1.6, 12.0 1.0 and 20.4 2.0 kev respectively. They suggested that the signicantly dierent result in the second case was caused by a dierent neutron energy spectrum resulting from the dierent thickness of the shielding used. However visual inspection of their data clearly indicates that these dierences could be due to systematic errors due to an energy dependent background, subsidiary peaks etc. In addition, the eects of the electronic gating is not discussed, and there may have been dierences there too. Thus we believe that there are other possible explanations for why they observed dierent values of b i.e. (1/) in our notation. Other publications also give useful information on this problem. For example Vonach et al. [13] studied -rays from neutron interactions on isotopic targets of 207 Pb and 208 Pb. The neutron energies varied from 3 to 200 MeV using the WNR facility at LAMPF. Their Fig. 2 shows the spectrum for 208 Pb for incident neutrons of 13-15 MeV and for 100-200 MeV. Now, the germanium detector is not in the beam, but clearly the scattered neutrons must be higher in energy for the second case. Note that the 1040 kev triangle is relatively stronger for the higher energy neutrons. The width of the 596 kev triangle is very similar however. Another experiment giving useful data is the study of pionic x-rays at PSI by de Laat et al. [14], and by Taal et al. [15]. In particular Fig. 1 of de Laat et al. shows strong neutron induced peaks which are clearly separated from -rays and the statistics are excellent. The 691 kev triangle appears in the \random background" and 6
is noticeably thinner because the selected events have the conversion electron events well separated from the recoil. Notice also that the neutron triangles at 596 kev, 834 kev, 1041 kev, 1204 kev, and 1465 kev are very clear, as are smaller ones at 563 kev and 1109 kev from the rare isotope of 76 Ge (only 7.76% abundant). We have analyzed their data from the gures in the publications. This clearly is dicult and entails errors of at least 20%, but the results are very interesting. We obtain values of of 16,15, 17, 21 kev for the peaks at 596, 834, 1041, and 1204 kev respectively; the value for the 596 kev peak is in reasonable accord with our own results. However when one refers to measurements with monoenergetic neutrons the situation is very dierent. The measurement of Chasman et al. [1] used monoenergetic neutrons of 1.2, 2.2, 4.7, and 16.3 MeV. The triangles for E n =1.2 MeV are noticeably narrower than those for 2.2 MeV; unfortunately the spectra for the higher energy neutrons show only weak eects. We warn the reader that visual impressions are very deceptive because of the changing background levels. Gujrathi and D'Auria [16] used 2.6 MeV neutrons and we nd a value of of 47 kev from their published gure for the 596 kev triangle. The one at 691 kev is noticeably narrower ( 30 kev), but we attribute this to the timing of the electronics. Bunting and Kraushaar used about 8 MeV neutrons, and we obtain 60 kev for their 596 kev triangle, and again the 691 kev triangle is narrower ( 40 kev), probably due to the electronics. Our conclusion is that there is no apparent dierence between these neutron triangles in several experiments, which had a broad spectrum of neutron energies. Intuitively one might expect a broader triangle for higher energy neutrons, and this is observed When one compares the spectrum of Gujrathi and D'Auria with the spectra of Chasman et al. The excitation of the 596 kev and 691 kev levels is likely to have a broad peak in cross-section between 1 and 3 MeV [11] and fall o fast by two orders of magnitude by 100 MeV [13]. Thus the 1 to 3 MeV neutrons dominate the eect, and we are observing the average triangle induced by this energy band. To make a denitive measurement will require excellent statistics and careful control of the energy of the incident neutron. 7. Acknowledgment We wish to thank Prof. J. Konijn for sending us some of the PSI data. Our research was made possible by the support of the Natural Sciences and Engineering Research Council of Canada, and also the National Research Council of Canada which we gratefully acknowledge. References [1] C. Chasman et al., Nucl. Instrum. Methods, 37:1, (1965). [2] R.L. Bunting and J.J. Kraushaar, Nucl. Instrum. Methods, 118:565, (1974). [3] G.P. Skoro et al., Nucl. Instrum. Methods, A316:333, (1992). [4] D. F. Measday et al., Proceedings of the IV Conference on Weak and Electromagnetic Interactions in Nuclei (1995). [5] V. Brundanin et al., Nucl. Phys., A587:577, (1995). 7
[6] T. Kozlowski et al., Nucl. Phys., A436:717, (1985). [7] R.M. Sundelin et al., Phys. Rev. Lett., 20:1198, (1968) [8] M. Lifshitz and P. Singer, Phys. Rev. C, 22:2135, (1980). [9] H.L. Anderson et al., Phys. Rev., 133:B392, (1964). [10] E.A. Lorch, Appl. Rad. Isotop., 24:585, (1973). [11] K.C. Chung et al., Phys. Rev. C, 2:139, (1970). [12] G. Braun et al., Nucl. Instrum. Methods, 224:112, (1984). [13] H. Vonach et al., Phys. Rev. C, 50:1952, (1994). [14] C.T.A.M. de Laat et al., Nucl. Phys., A523:453, (1991). [15] A. Taal et al., Nucl. Phys., A511:573, (1990). [16] S.C. Gujrathi and J.M. D'Auria, Nucl. Instrum. Methods, 100:445, (1972). Table I Values of for the 596 and 691 kev lines. (Note:The rst error is the statistical error from the t while the second is the systematic error.) Neutron Source (596 kev) (691 kev) (kev) (kev) 252 Cf * 15.2 1.0 0.9 209 Bi(?,xn) 209?x Pb 19.2 1.1 2.5 15.4 1.0 2.0 28 Si(?,xn) 28?x Al (Ge1) 22.4 0.5 2.5 18.8 0.5 2.0 28 Si(?,xn) 28?x Al (Ge2) 22.0 0.4 2.5 17.5 0.4 2.0 Not tted due to background and poor statistics. Table II Values of for the 596 line for dierent neutron energy spectra from the? run. (Note: Errors include statistical errors from the t only.) Runs (low energy) (high energy) (kev) (kev) 1st set 14.3 1.8 12.4 1.6 2nd set 15.3 2.6 23.3 5.3 8
FIGURE CAPTIONS 1. Experimental set up for the? 209 Bi experiment. 2. Time spectrum for? on 209 Bi. 3. 74 Ge(n,n 0 ) line from the? 209 Bi run. 4. 72 Ge(n,n 0 ) line from the 252 Cf run, illustrating ts with and without a weak line at 703 kev. 5. 74 Ge(n,n 0 ) line from the? Si run (Ge2). 6. 72 Ge(n,n 0 ) line from the? Bi run. 7. 72 Ge(n,n 0 ) line from the? Si run (Ge1). 8. 72 Ge(n,n 0 ) line from the? Si run (Ge2). 9. 74 Ge(n,n 0 ) from the? run (low energy neutrons, rst set). 10. 74 Ge(n,n 0 ) from the? run (high energy neutrons, second set). 11. 74 Ge(n,n 0 ) from the? run (low energy neutrons, rst set). 12. 74 Ge(n,n 0 ) from the? run (high energy neutrons, second set). Fig. 1. 9
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