Abstract
Shielded special nuclear material (SNM), especially highly enriched uranium, is exceptionally difficult to detect without the use of active interrogation (AI). We are investigating the potential use of low-dose active interrogation to realize simultaneous high-contrast imaging and photofission of SNM using energetic gamma-rays produced by low-energy nuclear reactions, such as [Formula: see text]B(d,n[Formula: see text]C and [Formula: see text]C(p,p[Formula: see text]C. Neutrons produced via fission are one reliable signature of the presence of SNM and are usually identified by their unique timing characteristics, such as the delayed neutron die-away. Fast neutron spectroscopy may provide additional useful discriminating characteristics for SNM detection. Spectroscopic measurements can be conducted by recoil-based or thermalization and capture-gated detectors; the latter may offer unique advantages since they facilitate low-statistics and event-by-event neutron energy measurements without spectrum unfolding. We describe the results of the development and characterization of a new type of capture-gated spectroscopic neutron detector based on a composite of scintillating polyvinyltoluene and lithium-doped scintillating glass in the form of millimeter-thick rods. The detector achieves >108 neutron–gamma discrimination resulting from its geometric properties and material selection. The design facilitates simultaneous pulse shape and pulse height discrimination, despite the fact that no materials intrinsically capable of pulse shape discrimination have been used to construct the detector. Accurate single-event measurements of neutron energy may be possible even when the energy is relatively low, such as with delayed fission neutrons. Simulation and preliminary measurements using the new composite detector are described, including those conducted using radioisotope sources and the low-dose active interrogation system based on low-energy nuclear reactions.
Highlights
Fast Neutron Detection in Active InterrogationDetection of special nuclear material (SNM), especially when shielded, is an exceptionally challenging problem in nuclear security
We study the use of the 11B(d,nγ)12C nuclear reaction, producing 4.4 MeV and 15.1 MeV gamma rays and simultaneously energetic neutrons as a prospective technology for active interrogation of shielded SNM
In conclusion, we developed a novel type of composite scintillator based on scintillating PVT and scintillating Li-doped glass which takes advantage of the physics of interaction and energy deposition in an optimized composite configuration to realize an exceptional level of gamma rejection (>108), even at low incident neutron energies
Summary
Detection of special nuclear material (SNM), especially when shielded, is an exceptionally challenging problem in nuclear security. Active interrogation by an external source of penetrating radiation is needed to perform radiography of a suspected object and to simultaneously enhance the rate of characteristic radiation emission by fission. This is an Open Access article published by World Scientific Publishing Company. One promising signature of fission we are studying is the emission of beta-delayed neutrons, which are emitted over timescales of seconds to minutes and exhibit very low background Their abundances and average energies (~0.5 MeV) are low, and they are immersed in a complex background of beta-delayed gammas and activation gammas. We summarize our recent progress in designing and testing novel composite scintillators for application in active interrogation, with special attention given to their spectroscopic capabilities
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