Pulse Shape Discrimination of CsI(Tl) with Photomultiplier Tube and MPPCs

Abstract

To study three-nucleon force in proton (p) and deuteron (d) channels of muon capture on 3He, we use CsI(Tl) scintillator and Multi-Pixel Photon Counters (MPPCs) to obtain energy spectra of p and d. We use Pulse Shape Discrimination (PSD) two-gate method to separate these spectra and reject the background mainly from β − γ. In this method, we measure the waveform, integrate charges collected in Short Gate (tail-part) and Long Gate (entire pulse), then take their Ratio to identify different particles. The main contribution of this research is to evaluate and optimize the PSD performance of both MPPCs and photomultiplier tube (PMT). Since the PSD performance of PMT has been studied before, we compare the performance of MPPCs with that of PMT in two test experiments, α − β and p − d separations. In the first test, using natural Thorium, we confirm α−β separation on both PMT and MPPCs. PSD Figure of Merit (FOM) is about 3 for PMT and 2 for MPPCs. In the second test, we detect p and d from 65 MeV proton beam hits a mixed target of polyethylene and deuterated polyethylene. The p − d separation is only confirmed on PMT. For MPPCs, Long Gate should be increased to give a good PSD performance as PMT does. Thus, we change the digitizer used in two test experiments, the WaveCatcher (digitized waveform ∼2.5 μs), to a new data acquisition system, the μTCA, with digitized waveform about 9.0 μs. Using MPPCs and μTCA, FOM α−β is about 3. We expect a good p − d separation with this setup.