Pulse Shape Discrimination For Background Rejection In Germanium Gamma-Ray Detectors

ABSTRACT We report on the development of a pulse shape discrimination (PSD) technique to reject the 8-decay background resulting from activation of germanium (Ge) gamma-ray detectors by cosmic ray secondaries. These 6-decays are a major source of background at 0.2-2 MeV energies in well shielded Ge detector systems. The technique exploits the difference between the detected current pulse shapes of single- and multiple-site energy depositions within the detector: 6-decays are primarily single-site events while photons at these energies typically Compton scatter before being photoelectrically absorbed to produce multiple-site events. Algorithms have been developed to distinguish between single- and multiple-site pulse shapes. Depending upon the amount of background due to sources other than 6-decay, PSD can more than double the detector sensitivity. In addition, we report on tests of PSD by laboratory activation of a detector with a fast neutron source, and on the first direct measurement of the B-decay background at balloon float altitude using a Ge detector with PSD.1. INTRODUCTIONActively shielded high resolution Ge gamma-ray spectrometers flown on balloons or spacecraft to observe astrophysical sources are subject to several background components: atmospheric and cosmic gamma-rays that enter the detector aperture and leak through the shield; elastic scattering of energetic atmospheric neutrons off Ge atoms in the detector; and 6-decays of radioactive nuclei produced within the detector itself. Aperture flux and shield leakage can be reduced through improved collimation and shielding. Elastic neutron scattering is not significant for energies >200 keV. And 6-decays, which are significant in the energy range 0.2-2 MeV, can be reduced through the techniques described in this paper.The radioactive nuclei that 6-decay are produced through the interaction of cosmic ray secondary protons and neutrons with Ge atoms in the detector.1 If the daughter nucleus of a 6-decay is produced in its ground state, the energy is deposited in a small localized region because the decay electrons have a short mean free path in Ge (e.g., 0.08 cm at 1 MeV). If the daughter nucleus is produced in an excited state, it will emit additional gamma-rays that are most likely to deposit energy in anticoincidence shielding, thus eliminating those counts. A few of these gamma-rays may interact within the detector producing a multiple-site event; however, non-localized 6-decays occur more than an order of magnitude less frequently than localized ones.Gamma-rays in the same energy range (0.2-2 MeV) typically Compton scatter one or more times before being photoelectrically absorbed (Figure 1). Therefore, by distinguishing single- from multiple-site events within the detector, one can in principle eliminate those background counts that result from localized 6-decays.One way to recognize and subsequently eliminate the 6-decay background is to discriminate between the different