Triple-coincidence positron annihilation lifetime spectrometer based on ultrafast discrimination

In this paper, we establish a triple-coincidence positron annihilation lifetime (PAL) spectrometer based on fast-fast coincidence. It can be used to accurately characterize atomic-scale defects in radioactive samples. The PAL spectrometer consists of three BaF2 detectors and a fast-fast coincidence system, which is composed of analogic electronic modules. The timing resolution of the spectrometer is approximately 198 ps, and the coincidence counting rate can reach 44 s-1 for a 0.9 MBq 22Na source. The results demonstrate that with 22Na activity at 0.9 MBq, the triple-coincidence PAL spectrometer can operate with high precision at 60Co activity levels of up to 4.3 MBq. However, when the activity of 60Co exceeds this threshold, the lifetime spectrum experiences noticeable distortion and an increasing background. As a result, the reliability of the measurement results is compromised. Despite this limitation, the spectrometer was proved successful in characterizing defects in neutron-irradiated RPV steels.