Development of a ZnS (Ag)-Silicon Photomultiplier Fast Neutron Detector for the Neutron Hodoscope at TREAT

The Transient Reactor Test (TREAT) Facility at Idaho National Laboratory is being restored to support the mission of advanced reactor and fuel development in the United States. A key part of this restoration is the fast neutron hodoscope which images the fuel tested during extreme transient conditions. Traditionally, this testing has been done with ZnS(Ag)-based proton recoil scintillators (PRS) coupled to photomultiplier tubes (PMTs). Recently, silicon photomultipliers (SiPMs) have emerged as a new candidate to replace PMTs in this application. This work develops, tests, and evaluates a SiPM-based PRS detector for incorporation into the updated fast neutron hodoscope, also known as the Fuel Motion Monitoring System (FMMS). The primary characteristics to be optimized are the gamma rejection and neutron detection efficiency of the detector, which will be done using a pulse shape discrimination (PSD) technique. Five types of SiPMs are tested and compared using standard PSD figures of merit, and the Hamamatsu 6075 is selected as the ideal SiPM. PSD performance for the Hamamatsu 6075 SiPM with the PRS detector is also characterized as a function of temperature. Finally, the gamma rejection and neutron detection efficiency are characterized for this SiPM and benchmarked to the traditional PMT-based PRS detectors. Both light sensors exhibit excellent gamma rejection and neutron detection efficiency. However, the SiPM-based detectors exhibit a greater ability to discriminate neutrons from gammas based only on pulse height which is important in operating at high count rates. This motivates the development of a SiPM-based PRS detector in a fully self-contained assembly to test directly with TREAT or in similar environments.