Abstract
Composite materials consisting of 6Li scintillator particles in an organic matrix can enable thermal neutron detectors with excellent rejection of gamma-ray backgrounds. The efficiency of transporting scintillation light through such a composite is critical to the detector performance. This optical raytracing study of a composite thermal neutron detector quantifies the various sources of scintillation light loss and identifies favorable photomultiplier tube (PMT) readout schemes. The composite material consisted of scintillator cubes within an organic matrix shaped as a right cylinder. The cylinder surface was surrounded by an optical reflector, and the light was detected by PMTs attached to the cylinder end faces. A reflector in direct contact with the composite caused 53% loss of scintillation light. This loss was reduced 8-fold by creating an air gap between the composite and the reflector to allow a fraction of the scintillation light to propagate by total internal reflection. Replacing a liquid mineral oil matrix with a solid acrylic matrix decreased the light transport efficiency by only ∼10% for the benefit of creating an all-solid-state device. The light propagation loss was found to scale exponentially with the distance between the scintillation event and the PMT along the cylinder main axis. This enabled a PMT readout scheme that corrects for light propagation loss on an event-by-event basis and achieved a 4.0% energy resolution that approached Poisson-limited performance. These results demonstrate that composite materials can enable practical thermal neutron detectors for a wide range of nuclear non-proliferation and safeguard applications.
Highlights
The resulting need for lower-cost neutron detectors with high efficiency has spurred research into detectors based on solidstate thermal neutron scintillators
The light transport efficiency from a GS20 scintillator cube to a photomultiplier tube (PMT) photocathode is defined as η(Ti) 1⁄4 P(PiM) T=P0, where P(PiM) T is the scintillation light power absorbed by the photocathode of PMT #i, and P0 is the scintillation light power emitted within the GS20 scintillator cube for one neutron capture event
Providing an air gap between the scintillator and the reflector allows a large fraction of rays to undergo lossless total internal reflection, while all rays suffer attenuation by a reflector that is in direct contact with the scintillator
Summary
Deployment of a large-scale neutron detector infrastructure. The resulting need for lower-cost neutron detectors with high efficiency has spurred research into detectors based on solidstate thermal neutron scintillators. Us and others, n/γ discrimination is achieved by reducing the physical dimensions of the scintillator (e.g., use of particles, thin plates, or thin rods as opposed to bulk scintillators) in order to limit the energy deposition of γ interactions while largely preserving the energy deposition of the α and triton reaction products in the 6Li-containing scintillator.. Us and others, n/γ discrimination is achieved by reducing the physical dimensions of the scintillator (e.g., use of particles, thin plates, or thin rods as opposed to bulk scintillators) in order to limit the energy deposition of γ interactions while largely preserving the energy deposition of the α and triton reaction products in the 6Li-containing scintillator.9,12 This creates a gap between the gamma and neutron distributions in the pulse-height spectrum that allows for excellent n/ γ discrimination by setting an appropriate pulseheight threshold akin to 3He detector readouts..
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