Design optimization of a layered boron based solid state neutron spectrometer

The high thermal neutron capture cross section of boron provides a unique opportunity for the construction of solid state neutron detectors with application to the identification of nuclear fuels and non-proliferation activities. The ionization resulting from the neutron capture products, ¹⁰B+n→⁷Li+α, is deposited within a short range of the initial interaction, thus allowing the location of the capture event to be determined. Construction of a system consisting of alternating layers of thin sensitive detector and moderating materials in a geometrically regular shape that allows coordinate dependence of neutron thermalization to be determined results in a novel solid state neutron spectrometer. A GEANT4 model has been developed to study neutron thermalization as a function of detector geometry, neutron moderation materials, and radial sensitivity. The simulation results indicate that a unique relationship exists between the incident neutron energy and the statistical pattern of energy deposition. This allows the geometrical configuration and moderator identity to be tuned to match the neutron energy spectrum of interest. The detector response is found empirically to match the shape of the Fisher statistic distribution allowing the incident neutron spectrum to be extracted without unfolding. Furthermore, the pattern of energy deposition is strongly dependent on the direction of incidence of the neutrons. Ultimately, this instrument can be used to spectroscopically differentiate localized neutron sources from a cosmic background.