Neutron spectrometry and source location using solid-state detectors

Semiconductor neutron detectors (SCND) devices have undergone dramatic improvements over the last ten years, with reported single detector thermal intrinsic efficiencies approaching 30%. The proposed spectrometer design consists of several alternating layers of SCND´s and neutron absorbers/reflectors. A 3-D array of SCND´s is embedded into a hydrogenous media, creating a single detection layer that moderates fast neutrons to thermal energies, where they are subsequently detected. The absorption/separation layers, consisting of 1 mm of cadmium and 2 mm of bismuth, provide increased separation of detector layer responses to mono-energetic incident neutrons. The GEANT4 toolkit was used to extensively model and optimize the proposed design. The devices in each detection layer were arranged in a radial-arm array; simulations of this configuration resulted in an azimuthal angular resolution of 0.75 degrees, while polar angular resolution was found to be dependent on the number of detection layers present. A response library was built by simulating mono-energetic neutrons, ranging from 0.01 eV to 12 MeV, incident to the face of the first detection layer. This library was then used with a maximum likelihood expectation maximization (MLEM) algorithm to unfold the spectrometer response to several mono- and poly-energetic neutron sources. The unfolded spectra exhibited excellent agreement with the simulated sources, demonstrating discrimination between different neutron source types, but also between sources of the same type.