Study of timing performance parameters for a SiPM-based digital positron annihilation lifetime spectrometer

Positron annihilation lifetime spectroscopy (PALS) is a popular method for studying defects in materials. With the development of fast digital oscilloscopes and digitizers, the conventional analog positron annihilation lifetime (PAL) spectrometer is being replaced by a digital PAL spectrometer. Recently, newly conducted research has developed a PAL spectrometer using a silicon photomultiplier (SiPM) instead of a photomultiplier tube (PMT). Timing resolution of PAL spectrometer needs to be improved to identify the lifetimes of positrons. Many parameters affect the timing performance of a SiPM-based digital PAL spectrometer: bias voltage, pulse smoothing, and the constant fraction value of digital constant fraction discrimination (dCFD). Timing resolution depends on the radiation energy, but only a few studies have considered the energy difference of 1274 keV and 511 keV. In this study, we optimized the timing performance parameters of the SiPM-based digital PAL spectrometer by conducting the timing resolution measurements using two γ-ray sources (²²Na and ⁶⁰Co). With a 32 V bias voltage, 5th degree polynomial fitting method, and optimal constant fraction value, we optimized the timing resolution of our SiPM-based digital PAL spectrometer. Furthermore, the optimal constant fraction value depended on the γ-ray energy and we verified it by timing resolution measurement using ²⁰⁷Bi. We studied the reason why performance parameters have optimal conditions and the optimal constant fraction value varies depending on the γ-ray energy. With optimized performance parameters, we measured the lifetime spectrum of pure silicon to confirm the SiPM-based digital PAL spectrometer. The timing resolution function of the SiPM-based digital PAL spectrometer was 185.6 ps.