Comparison of SiPM and PMT Performance Using a ${\rm Cs}_{2}{\rm LiYCl}_{6}:{\rm Ce}^{3+}$ (CLYC) Scintillator With Two Optical Windows

Measurements of thermal neutrons and gamma rays using multiple Cs
₂
LiYCl
₆
: Ce
³⁺
(CLYC) crystals with photomultiplier tube (PMT) and silicon photomultiplier (SiPM) are compared. The first set of measurements used three single-sided crystals (two made from 95% enriched
⁶
Li and one made with 99% enriched
⁷
Li) mated with the PMT and SiPM to compare the energy resolutions and figures of merit (FOM) for the pulse shape discrimination of gamma rays and neutrons. All crystals and photomultipliers were able to resolve full-energy photopeaks for
¹³⁷
Cs and
⁶⁰
Co and distinguish gamma-ray interactions from neutron interactions (FOM > 1 in all cases) while operating in a thermal neutron environment. Measurements made with the PMT had better energy resolution, and switching to the SiPM degraded the energy resolution by an average of 34.0 ± 0.7% (at 662 keV). The second set of measurements used a CLYC crystal with two optical windows to enable simultaneous measurements with the PMT and SiPM. Once again, both photomultipliers were able to distinguish gamma rays from neutrons, and the PMT had the best energy resolution. However, in this case, the SiPM was not able to resolve the
¹³⁷
Cs full-energy photopeak, and neither photomultiplier could resolve any photopeaks if
⁶⁰
Co was used simultaneously. The degradation in energy resolution was due to the relative splitting of scintillation light intensity between each photomultiplier depending on the position of the interaction site within the crystal. Each pair of waveforms recorded from the PMT and SiPM, corresponding to a single interaction within the double-sided crystal, were combined to improve the energy resolution at 662 keV from 15.9 ± 0.9% to 12.7 ± 0.3% and the FOM from 1.850±0.004 to 2.117±0.003 when compared to the data just from the PMT.