Silicon Photomultipliers As An Alternative To Photomultiplier Tubes For Nuclear Safeguards Applications

Photomultiplier tubes (PMTs) have been used for decades as the default light detection technology for scintillator-based radiation monitors. PMTs come with a handful of disadvantages, however, including large volume usage, fragility, high-voltage requirements, and susceptibility to magnetic fields. Arrays of silicon photomultipliers (SiPMs) are a possible alternative to PMTs in that they offer similar performance while offering improvements in the areas listed. We focus on straightforward “drop-in replacement” evaluations, in that we impose additional requirements of a single channel output for the SiPM signal, as well as a backend electronics data acquisition and analysis chain identical to that used for PMTs. In the realm of nuclear safeguards, the primary performance metric for gamma spectroscopy is detector resolution, and for neutron detection the primary metric is pulse shape discrimination (PSD) to separate neutron and gamma signals. In this work we present the results of replacing a PMT with a 2” x 2” SiPM array when mounted, in turn, to the same 2” x 2” cylindrical sodium iodide (NaI) crystal. We compare resolutions, characterize the nonlinearity in response, and the effects of a high-rate environment. The ultimate comparison for this use of gamma spectrometry is confirmation of uranium enrichment standards, from depleted uranium to 93% highly enriched uranium (HEU), by analyzing the resulting spectra with the NaIGEM software package. In addition to the gamma spectroscopy work, we will present the latest results on comparing the PSD capabilities of a 4” x 4” SiPM array to a PMT. As with the gamma spectroscopy evaluations, the light detectors are mounted in turn to the same liter-scale organic scintillators. This neutron-focused work includes evaluation of prototype SiPM readout boards with a single output signal summed over 256 individual SiPM pixels, and evident tradeoffs between resolution and fast response. This work was supported by the U.S. Department of Energy, National Nuclear Security Administration, Office of Nonproliferation and Arms Control and the Human Capital Development Program. Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-802581.