UC Berkeley

Antineutrino detectors may be able to fill an important role in the global nuclear nonproliferation regime by discovering an undeclared nuclear reactor or monitoring a known reactor tens of kilometers away. However, such detectors are large capital investments, and a substantial portion of their cost is the photomultiplier tubes that collect the light from antineutrino interactions in the detector volume and convert it into a usable electronic signal. As such, maximizing the light collection efficiency, and thus performance, of each photomultiplier tube is paramount. Wavelength-shifting plates may be able to aid in this goal. Wavelength-shifting plates increase the amount of light collected by each photomultiplier tube, which translates into a greater energy resolution of the detector. At the same time, however, the wavelength-shifting plates smear out the collection of this light in time, as light captured by the plates is delayed for different amounts of time before reaching a photomultiplier tube. This reduces the ability of post-processing algorithms to successfully reconstruct the location at which an event occurred in the detector. This is expected to have a countervailing effect to the improved energy resolution provided by the wavelength-shifting plates. This work explores the trade-off between these two effects, utilizing Monte Carlo simulations validated against experiments in air and in a water-Cherenkov detector to predict the effects that wavelength-shifting plates will have on large-volume water-Cherenkov detector performance. It was found that wavelength shifting plates improve the overall signal-to-background ratio of a 16 meter diameter, 16 meter height cylindrical detector by 13%, allowing a 3000 MWth undeclared nuclear reactor to be discovered 22% faster, in 16 days rather than 20.4 days when a detector without wavelength-shifting plates is utilized. This improvement is significant as it demonstrates that the improvement wavelength-shifting plates provide to light collection outweighs their effects on timing, so that wavelength-shifting plates may be a cost-effective way to improve the overall performance of large-volume water-Cherenkov antineutrino detectors intended for nuclear nonproliferation missions.