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

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 68 Ge 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.