Optimization study of a new β ⁺-γ coincidence positron annihilation lifetime spectrometer using Geant4

Abstract

Positron annihilation lifetime (PAL) spectroscopy is a unique method for characterizing atomic-scale defects and ultramicropores in materials. The conventional PAL spectrometer adopts the γ-γ coincidence principle, and its performance, especially the coincidence counting rate (CCR), can hardly be further increased. Another coincidence principle, β +-γ coincidence, has the potential to simultaneously improve the CCR and coincidence time resolution (CTR) of PAL spectrometers. However, early β +-γ coincidence PAL spectrometers have not been widely applied due to the considerable room for improvement in their performance. In this work, we proposed a new β +-γ coincidence PAL spectrometer utilizing silicon photomultiplier (SiPM) array as the positron detector and conducted a comprehensive optimization of its structure with the aim of achieving a breakthrough in performance. The effects of start signal threshold and structure parameters on its CTR, CCR, and proportion of source contribution (P SC) were studied using Geant4. The simulation results show that, with a 68Ge positron source of 30 μCi, the optimized β +-γ coincidence PAL spectrometer can achieve an extremely high CCR exceeding 10000 counts per second (cps) and an outstanding CTR below 160 picoseconds (ps) while maintaining a low P SC below 12%. This study provides valuable guidance for constructing high-performance β +-γ coincidence PAL spectrometers.