Characterization of a prototype detector unit for fast neutron imaging and spectrometry

Neutron scatter camera, having wide field of view and spectral sensitivity, has been recent research efforts regarding passive fast neutron imaging technique. However, limited the sensitivity to special nuclear material (SUM) prevents it from practical applications. Recently reported Single Volume Scatter Camera (SVSC) [1-5] developed by Sandia National Laboratory demonstrated an order of magnitude increase in detection efficiency. We present a new design of single volume neutron scatter camera in order to improve the position resolution of n-p elastic scatters inside detector unit. The camera composes by ten layers of optically segmented plastic scintillator sheet. Each scintillator sheet consists of a plastic scintillator sheet of 10 cm × 10 cm × 1 cm dimensions and two groups (6×2) of wavelength-shifting fibers with orthogonal directions embedded into grooves on two opposite scintillator surfaces. Fiber scintillation signals were read out by silicon photomultipliers (SiPMs). A prototype detector unit has been characterized experimentally. Light output of the unidimensional six fiber channels from one side of the detector unit was calibrated to be 14.45 photoelectrons per MeVee. Position resolution of the detector unit was measured to be 0.35–0.44 times fiber pitch, corresponding to 5.48 mm for proton recoil energy interval of 1.63–2.60 MeV, and 4.60 mm for proton recoil energy interval of 4.82–5.50 MeV. Energy threshold for recoil proton localization was estimated to be 1.18 MeV. The potential performance optimization methods include reducing light signal loss by increasing the fiber diameter and decreasing the number of channels involved in position reconstruction by narrowing the scintillation signal distribution function of the detector unit.