Light propagation in a neutron detector based on 6Li glass scintillator particles in an organic matrix

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.

Performance assessment of a compact neutron detector module based on scintillating composites

“…The voltage determined for the second PMT was 800.3 V. Following the gain matching procedure, each PMT was coupled to an end face of the composite, and the exposed lateral face was surrounded with an ESR reflector. It was determined previously that an air gap between the composite and reflector provides a more optimal configuration for optical transport by enabling total internal reflection to occur between the composite-air boundary; thus optical losses are minimized for photons which are incident at the surface within the critical angle condition [8]. To achieve this configuration with the dual-PMT assembly, the ESR film was wrapped around the lateral wall of the PMT windows and spanned the separation distance between the two PMTs.…”

“…In the present case, there is an index mismatch of 0.009 between the GS20 cubes (n = 1.551) and the NOA71 optical adhesive (n = 1.560) and an index mismatch of 0.053 between the NOA71 optical adhesive and the SUVT matrix (n = 1.507), resulting in a partial reflection of 0.0008% and 0.03%, respectively, for normal incidence and a wavelength of 397 nm. Index values at 397 nm for the GS20 and acrylic were determined based on those reported in [8], and the index for the optical adhesive is reported as specified by Norland Products.…”

“…Several solid-state materials doped with luminescent ions and containing the 6 Li isotope have been developed for scintillation-based detection mechanisms [4][5][6][7]. In the last decade, there has been substantial progress in the development of detector capabilities utilizing 6 Li-enriched glass composite materials, which comprise glass scintillator elements within an organic matrix [8][9][10][11][12][13][14].…”

Simulation of the response of the Solid State Neutron Detector for the European Spallation Source

“…Figure 3: Optical properties employed in the simulation: (a) the quantum efficiency of the bialkalai cathode of the H12700 PMT [9], (b) the scintillation emission spectrum of GS20 [22], (c) refractive indices of GS20 [22] and borosilicate glass [23], (d) 1/e attenuation lengths of GS20 [22] and borosilicate glass [23].…”

Simulation of the Response of the Solid State Neutron Detector for the European Spallation Source

“…Figure 3: Optical properties employed in the simulation: (a) the quantum efficiency of the bialkalai cathode of the H12700 PMT [3], (b) the scintillation emission spectrum of GS20 [14], (c) refractive indices of GS20 [14] and borosilicate glass [15], (d) 1/e attenuation lengths of GS20 [14] and borosilicate glass [15].…”