Development of an extruded plastic array for narrow scintillation pulse widths

Abstract

This work describes and shows data from a prototype instrument designed to sample transitory gamma rays resulting from stockpile stewardship testing. The required instrumentation needs to be large, sensitive, low cost, and have the ability to measure pulse widths as narrow as 5 ns (or less). The natural material for an instrument to meet these criteria is an organic scintillator. In order to cover a large area and reduce the overall cost of the detection system (material and electronics), one approach would be large pixel elements to cover an array of a few square meters. Unfortunately, this approach will not provide the accuracy required for these experiments as large volume scintillation detectors broaden the intrinsic full-width at half maximum (FWHM) of the scintillation light pulse due to delays introduced by internal reflections within the scintillator volume. We devised an approach to mitigate these broadening effects from large volume detectors, while remaining at a low cost. Our detectors consist of a bundle of extruded plastic scintillation bars, readout by wavelength shifting fibers that pipe the scintillation light to a fast light readout device. In this paper we describe the detector unit and assembly procedure, the fast photomultiplier tube (PMT) and readout electronics, as well as data from the laboratory with a radioactive source and cosmic-ray muons. Additionally, we show results from a detector unit tested at the NRL Mercury pulsed power facility. The concluding section discusses the path forward for this instrument and possible improvements for a field-deployable system.