Simulations of pulse shape discrimination (PSD) techniques for background reduction in germanium detectors

For modern, actively shielded, narrow aperture germanium detector (GeD) spectrometers at balloon or spacecraft altitudes, the continuum background in the 0.2 to 2 MeV energy range is dominated by the β− decay of radioactive nuclei produced by the interaction of cosmic rays and secondary protons and neutrons within the detector. PSD techniques distinguish single-site (primarily β−-decay) from multiple-site (primarily photon) events by analyzing the shape of the current pulse formed when the electrons and holes resulting from these energy depositions propagate through the detector. Through numerical simulation of the charge collection process, we demonstrate that the effectiveness of PSD can be enhanced by optimizing detector geometry and external electronics. In particular, PSD results for closed-end coaxial detectors with standard (1.2 cm effective diameter) and narrow (0.6 cm diameter) inner bore, and for a true coaxial narrow-bore detector, are presented. With a newly developed PSD algorithm, improvements in sensitivity at 1 MeV of factors of 2.8 and 3.6 for the standard-bore and narrow-bore closed-end coaxial detectors, respectively, and 4.0 for the true coaxial detector, are predicted.