Development and performance of the Fast Neutron Imaging Telescope for SNM detection

FNIT (the Fast Neutron Imaging Telescope), a detector with both imaging and energy measurement capabilities, sensitive to neutrons in the range 0.8-20 MeV, was initially conceived to study solar neutrons as a candidate design for the Inner Heliosphere Sentinel (IHS) spacecraft of NASA’s Solar Sentinels program and successively reconfigured to locate fission neutron sources. By accurately identifying the position of the source with imaging techniques and reconstructing the Watt spectrum of fission neutrons, FNIT can detect samples of special nuclear material (SNM), including heavily shielded and masked ones. The detection principle is based on multiple elastic neutron-proton scatterings in organic scintillators. By reconstructing n-p event locations and sequence and measuring the recoil proton energies, the direction and energy spectrum of the primary neutron flux can be determined and neutron sources identified. We describe the design of the FNIT prototype and present its energy reconstruction and imaging performance, assessed by exposing FNIT to a neutron beam and to a Pu fission neutron source. Keywords: fast neutrons, neutron imaging, passive SNM search, container screening 1. INTRODUCTION A critical gap in national security is the inability to efficiently detect and identify problematic quantities of Special Nuclear Material (SNM). These materials, specifically uranium and transuranics, emit neutrons via spontaneous or induced fission. Unlike the other forms of radiation produced by SNM (e.g. -rays), copious and penetrating neutron emission is unique to fissionable material. From a practical point of view, shielding of fission neutrons ( e.g. in a cargo container) represents a far greater challenge and requires a considerably larger and heavier amount of passive material than shielding of other forms of radiation emitted by SNM. Neutron detection, therefore, is of particular interest for SNM identification for security and proliferation deterrence, as well as for nuclear waste detection and monitoring. While improvements in all forms of radiation detection are necessary to close the SNM security gap, there are unique problems associated with the detection and measurement of neutrons. Some of these are: • current neutron detectors used in the field ( e.g. Bonner spheres