218 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 38, NO. 2, APRIL 1991

Segmented Ge Detector Rejection of Internal Beta Activity Produced by Neutron Irradiation

L.S. Varnell, J.L. Callas, and W.A. Mahoney Jet Propulsion Laboratory, Pasadena, CA 91 109

R.H. Pehl, D.A. Landis, P.N. Luke, and N. W. Madden Lawrence Berkeley Laboratory, Berkeley, CA 94720

Abstract

Future Ge spectrometers flown in space to observe cosmic gamma-ray sources will incorporate segmented detectors to reduce the background from radloactivity produced by energetic particle reactions. To demonstrate the effectiveness of a segmented Ge detector in rejecting background events due to the beta decay of internal radioactivity, a laboratory experiment has been carried out in which radioactivity was produced in the detector by neutron irradiation. A 252Cf source of neutrons was used to produce, by neutron capture on 74Ge (36.5% of natural Ge) in the detector itself, 75Ge (ti12 = 82.78 min), which decays by beta emission with a maximum electron kinetic energy of 1188 keV. By requiring that an ionizing event deposit energy in two or more of the five segments of the detector, each about one centimeter thick, the beta particles, which have a range of about one millimeter, are rejected, while most external gamma rays incident on the detector are counted. Analysis of this experiment indicates that over 85% of the beta events from the decay of 75Ge are rejected, in good agreement with Monte Carlo calculations.

I. INTRODUCTION

To achieve better sensitivity to weak cosmic sources, future Ge spectrometers will incorporate segmented detectors to reduce the radioactive background produced in space by energetic particle reactions with the Ge detectors. Previous studies [11 have estimated that approximately 90% of the beta events from this radioactivity can be rejected by using an externally segmented detector inside a thick active shield and requiring that only events which deposit energy in two or more segments be selected. Beta particles, which have a range of the order of a millimeter, will be rejected, while gamma rays from 0.15 to 8 MeV, which typically interact by Compton interactions in two or more segments, will be selected. Although the externally segmented detector was introduced several years ago [ 11, there has been no balloon or space flight to demonstrate its effectiveness. Altemative tests involve the use of particle beams from accelerators or neutrons from radioactive sources to irradiate the detector and produce internal radioactivity, simulating the energetic particle environment of space. Although these laboratory

experiments cannot produce the complete range of background experienced in a balloon flight [ 2 ] , they can measure the effectiveness of the segmented detector in rejecting intemal background. We have carried out such an experiment by producing 75Ge by neutron capture reactions with the detector material (36.5% 74Ge). Simultaneous spectra are then taken of the activity in the detector under two conditions: a free spectrum in which all events in the detector are accumulated, and a gated spectrum in which events are accumulated only if they deposit energy in two or more segments. By comparing the spectra, the effectiveness of the detector in rejecting beta events can be measured.

11. SEGMENTED DETECTOR IRRADIATION EXPERIMENT

The five segment coaxial Ge detector used in this experiment has been described previously 133. It is a reverse electrode detector, 5.4 cm in diameter and 5.3 cm long, divided electronically by segmenting the outer electrode. The total energy signal is taken from the center electrode. Each segment has its own signal chain of preamplifier, amplifier and lower level discriminator taken from the external electrode. There is an output from the discriminator whenever energy larger than the threshold is deposited in that segment. The discriminator level was set at 45 keV for these experiments to avoid possible pile-up problems due to high counting rates during the neutron irradiations. The five discriminator outputs are fed to a coincidence unit, which can be set to give an output when 1 or more, 2 or more, etc, discriminator pulses are present simultaneously. During the measurements, two spectra are recorded using the total energy signal from the center electrode. One spectrum is the free spectrum of all events. The second spectrum is gated so that only events which deposit energy above the threshold in 2 or more segments are recorded. These spectra can then be compared to determine the effectiveness of the segmented detector at rejecting events due to beta decay from intemal radioactivity.

The neutron source for the irradiation was a sealed 252Cf source emitting 1.4 x lo6 neutrons/sec. The spectrum of fission neutrons from 252Cf peaks at 1 MeV with an average energy of about 2 MeV. For the first experiment, the

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Fig. 1 The free spectrum taken 75 min after the irradiation. The beta continuum is from the decay of 75Ge (t1/2 = 82.78 min), with an endpoint of 1188 keV, corresponding to channel 3332. The lines at channels 3075 and 3631 correspond to the 1097.3 keV and 1293.5 keV lines from the decay of '161n (ti12 = 54.15 min).

fast neutrons from the source were used to produce the inelastic scattering peaks from the Ge isotopes and other prompt activity. Then the source was removed and the radioactivity produced in the detector was measured. Spectra were recorded simultaneously for the free and gated modes of the detector during the irradiation and after the source was removed. The inelastic scattering lines in Ge and many lines from fission fragments were observed in the spectra obtained during irradiation, but any 75Ge radioactivity produced in the detector was too weak to be observed above room background following the irradiation.

For the second experiment, paraffin wax with a thickness of 13 cm was placed between the source and the detector to thermalize the neutrons, and a shield of lead bricks was built around the detector after the irradiation to lower the background so that weak radioactivity could be observed. The detector was irradiated for 3.5 hours, then placed inside the lead shield. Spectra were recorded in intervals of 15 to 90 minutes for 6.3 hours, then longer intervals were used to observe the longer lived activity. Fig. 1 shows the free spectrum taken in an interval of 30 minutes beginning 75 minutes after the end of the irradiation. The shape of the beta spectrum from the decay of 75Ge ( t1/2 = 82.78 min) is clearly seen.

The decay branching ratio to the ground state of 75As is 0.871 with a maximum beta energy of 1188 keV, corresponding to channel 3332. The gamma-ray lines at channels 3075 and 3631 correspond to the 1097.3 keV and 1293.5 keV transitions in l16Sn from the decay of l l 61n (t1/2 = 54.15 min). The In radioactivity is produced by neutron capture in the In metal used to mount the detector.

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416.9, 1097.3, and 1293.5 keV from the decay of 116In are labeled.

Fig. 2 shows the gated spectrum taken simultaneously with Fig. 1 with the lines from the decay of 1161n labeled. The ordinate scales of the two figures are equal, but the energy scales are not quite equal because different multichannel analyzers with slightly different conversion gains were used. The reduction in the beta spectrum by requiring interactions in two or more segments is apparent.

From the accumulated spectra, the beta continuum was obtained by subtracting the counts from the gamma-ray lines and from the background measured in the shield before irradiation of the detector. Over the energy range of the beta particles, 85% of the beta activity is rejected. This is in good agreement with the expected value. In 87.1% of 75Ge decays, the transition is by beta decay to the ground state of 75As, and calculations show that 96% of these betas will be rejected. In 11.5% of the decays, the transition is to the 264.6 keV level of 75As, followed by a gamma ray of 264.6 keV. Monte Carlo calculations indicate that 50% of these transitions will be rejected by the segmented detector. Most of the transitions not rejected by the detector wouId be rejected by an active shield. The remaining 1.4% of beta transitions are followed by gamma emission and a similar fraction would be rejected. The sum of 96% of 87.1% and 50% of 12.9% gives 90%, slightly higher than the 85% rejection measured. A slight correction will be made to the experimental spectra for continuum counts due to Compton scattering of the In gamma rays by a measurement with an irradiated In foil or a Monte Carlo calculation. The gamma- ray efficiency of the segmented detector in the mode requiring interactions greater than 45 keV in two or more segments varies from 0.40 at 417 keV to 0.65 at 1293 keV relative to an ordinary detector, in good agreement with our previous measurement [3]. The result for that measurement, for a discriminator setting of 50 keV, is shown in Fig. 3, together

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Fig. 3 Relative full energy peak (FEP) efficiency versus energy for the five segment coaxial detector. The relative efficiency is obtained by dividing the peak area of a given line in the spectrum requiring interactions in two or more segments by the peak area in the free spectrum (one or more segments with interactions). The points calculated by the ACCEPT code are joined by line segments.

with Monte Carlo calculations for discriminator settings of 10 and 50 keV made using the code ACCEPT [41.

The segmented detector will provide a significant improvement in sensitivity for energies greater than 400 keV. Taking into account the reduction of beta decay background by almost a factor of ten, the segmented detector in this mode has a factor greater than two better sensitivity at these energies than an ordinary detector of the same size. At energies below 400 keV, the efficiency of the mode of interactions in two or more segments decreases significantly. In this energy range, the front segment only mode of the segmented detector is used to improve sensitivity. In this mode, events are counted if there is an interaction only in the front segment. For gamma rays of energy up to a few hundred keV, there is a high probability of complete absorption in the front segment (nearly 100% from 15 to 100 keV). The front segment only mode for low energy gamma rays gives a high efficiency with a background for a five- segment detector only 1/5 that of an ordinary detector. Again, the sensitivity will be improved by a factor greater than two compared to an ordinary detector. The presence of the gamma rays from 116111 decay is significant for the construction of detectors for space. Although only an estimated 0.1 gram of indium was used in mounting the detector, the large cross section for neutron activation results in the strong gamma-ray lines. The intensity of these prominent lines demonstrates the importance of strict control over the materials used to build gamma-ray detectors for use in space. Preparations are being made for a balloon flight in the Fall of 1991 or the Spring of 1992 with a larger segmented detector in a rugged mount and low mass cryostat to measure the detector performance in a space environment.

The research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

REFERENCES

L.S. Varnell, J.C. Ling, W.A. Mahoney, A.S. Jacobson, R.H. Pehl, F.S. Goulding, D.A. Landis, P.L. Luke, and N.W. Madden, "A Position-Sensitive Germanium Detector for Gamma-Ray Astronomy," IEEE Trans. Nucf. Sci. 31, pp. 300-306, 1984.

N. Gehrels, "Instrumental Background in Balloon-Borne Gamma-Ray Spectrometers and Techniques for Its Reduction," Nucf. Instrum. Methods, A239, pp. 324- 349, 1985.

L.S. Varnell, J.C. Ling, W.A. Mahoney, R.H. Pehl, C.P. Cork, D.A. Landis, P.L. Luke, N.W. Madden, and D.F. Malone, "Performance of a Five-Segment Coaxial N-Type Germanium Detector," in Nuclear Spectroscopy of Astrophysical Sources, ed. N. Gehrels and G. Share, New York:AIP, 1988, pp. 490-497.

J.A. Halbleib and T.A. Mehlhom, "ITS: The Integrated TIGER Series of Coupled Electron/Photon Monte Carlo Transport Codes," Nucl. Sci. Eng., 92, No. 2, pp. 338- 339,1986. ACCEPT is one of of the ITS codes, available from the Radiation Shielding Information Center, ORNL, P.O. Box 2008, Oak Ridge, TN, 37831 USA. ITS Version 2.1 is CCC-467.