Simulation and Optimization of a Neutron Scatter Imager Based on Silicon Photomultipliers and Signal Multiplexing

A number of scenarios in nuclear nonproliferation, safeguards, security and disarmament would benefit from the development of a truly portable, rugged and efficient neutron imager and spectrometer. To aid in the realization and deployment of such a device, it must be made cost-effective and ideally be based on commercially available components. To best achieve this in the near future, an array of scintillators grouped together and read out by arrays of silicon photomultipliers seems to be the most technologically feasible option. Image reconstruction and event-by-event energy measurement can then be performed based on neutron scattering kinematics between the different scintillator volumes. Toward a cost-effective imager, EJ 299 plastic scintillators from Eljen Technology and silicon photomultiplier arrays from SensL will be used with a mixed-sinusoid multiplexing scheme developed in our group. The multiplexing is crucial to minimizing cost by dramatically reducing the number of physical readout channels to be digitized and is the difference between allowing an efficient imager to be built or being prohibitively expensive. However, the number and placement of detector channels and how many and which channels should be combined together requires careful consideration and optimization. This work uses Geant4 simulations to characterize imager performance for different imager compositions, geometries, and multiplexing schemes, and will result in a determination of the optimum layout of our imager. Design choices to be analyzed include the number of scintillator pillars to be used (i.e. 32 or 64), the number of channels to be multiplexed together (i.e. 8 per channel or 16 per channel), and which scintillators should be read out through the same digitization channel. Design-dependent quantities used to evaluate performance and to be optimized include efficiency, the fraction of events lost due to multiplexing, and neutron-gamma discrimination capability based on timing. The purity of sinusoids for our multiplexing will also inform our optimal multiplexing layout, and so the construction and evaluation of sinusoid-producing circuits will also take place.