Detector Arrays for Nuclear Security and Safeguards Applications

Over the last two decades it has become increasingly apparent that there is a need for new technologies capable of reliable and efficient fast neutron detection. As national and international stockpiles of 3He continue to dwindle, the need to find a new gold standard of neutron detection becomes more critical. Moreover, neutrons are generated typically in the MeV energy range. The use of 3He detectors for fast neutron measurements requires the use of moderators. Detector arrays capable of fast neutron detection are one solution to this problem. To this end, the focus of this work was the study of detector arrays for fast neutron detection applications, particularly as they relate to nuclear security and safeguards. The detector array study was carried out in three stages. The first stage focused on the identification and comparison of potential scintillator mediums for use in fast neutron detection. EJ-299-33A, CLLB, and CLYC were selected for initial modelling. Each material was identified through exhaustive literature surveys to be capable neutron/photon pulse shape discrimination and selected primarily for this feature. Single cell models of homogeneous compositions were developed using each of these materials. Simulations were done using the MCNP6 code. Additional simulations were performed for heterogeneous models consisting of a cell of the plastic scintillator EJ-299-33A filled with several layers of crystalline scintillator material. The efficiency of these heterogeneous samples was considered in relation to the size of the crystal components used. The second stage focused on simulation and investigation of detector arrays, their susceptibility to cross-talk between detector pixels and their potential in radiation imaging applications. The occurrence of cross-talk was studied for three cases 1) each pixel was unshielded and in direct contact with its neighbors, 2) lead shielding was placed between the pixels of the detector array, 3) pixels were staggered across two rows to avoid direct contact between neighbors. Imaging simulations were carried out to study the feasibility of using 2.5-MeV and 14-MeV neutron sources to identify targets or hidden materials without physically inspecting the contents of a container or object under scrutiny. The final phase of this work focused on experimental testing of the pulse shape discrimination capabilities of an EJ-299-33A plastic scintillator sample. Emphasis was placed on the material’s suitability for fast neutron detection particularly when employed in mixed neutron/photon fluxes. Measurements were done with photon sources and a PuBe source. Measurement data was analyzed to determine the figure of merit for the scintillator and identify its suitability for pulse shape discrimination applications