Pulse Shape Discrimination of CsI(Tl) with a Photomultiplier Tube and Multi-Pixel Photon Counters

In this study, we evaluated and compared the pulse shape discrimination (PSD) performance of a typical photomultiplier tube (PMT) with that of multipixel photon counters (MPPCs), also known as silicon photomultipliers (SiPMs). We used CsI(Tl) scintillators for both setups and mainly evaluated the PSD performance in the $\alpha $ – $\beta $ separation. We used the charge comparison method, that is, identifying the particle type based on the ratio of the charges collected in two time gates: Short gate (SG) for the delayed part and long gate (LG) for the entire digitized waveform. With the same LG = 2250 ns, the PMT setup showed a better PSD performance with the figure of merit (FOM) being 3.33 (in $\alpha $ – $\beta $ separation), whereas the FOM of the MPPC setup was 2.26. This inferior performance of the MPPC setup was due to its slow response, mainly the slow decay time in the waveform of one photoelectron (p.e.) of the MPPCs. This study aimed to determine the relationships of the PSD performance with the number of p.e. collected and the time characteristics of the scintillator–photosensor coupling. From these relationships, we estimated the SG to obtain $\text {FOM}_{\text {max}}$ and afterward confirmed a method to further increase the PSD performance. The estimated SG gave a comparable FOM without an extensive search for the optimized SG for obtaining the actual $\text {FOM}_{\text {max}}$ in analysis. To further increase the PSD performance of the MPPC setup, we used a new data acquisition (DAQ) system and extended the LG to 8000 ns to collect more p.e. and mitigate the slow response of the MPPCs. Using the new DAQ system, we obtained an FOM of 3.91 for the MPPC setup, that is, an increase by a factor of 1.7.