Development of a 4π directional fast neutron detector using tensioned metastable fluids

The development of directional-position sensing neutron detector technologies has the potential to embody transformational impact on to the field of nuclear security and safeguards. Directional neutron detectors offer vastly superior background suppression enabling the detection of smaller quantities of special nuclear materials (SNM) at larger standoffs. Additionally, the ability to image the SNM neutron source directly would be particularly advantageous in active interrogation scenarios where one needs to discriminate interrogating neutrons from neutrons resulting from SNMs. A directional fast neutron detector utilizing the acoustic tensioned metastable fluid detector (ATMFD) system has been developed that is not only comparable in technical performance with competing directional fast neutron technologies but also offers a significant reduction in both cost and size while remaining completely insensitive to gamma photons and non-neutron cosmic background radiation. Past assessments by our group have shown that an ATMFD system (with a 6cm × 10cm cross-sectional area) would be capable of detecting a 8 kg Pu source at 25m standoff with a resolution of 11.2°, with 68% confidence within 60 s. While previous ATMFD system configurations were limited to determining angular resolution in 2π, a new ATMFD sensor system capable of ascertaining directionality in 4π fields is now presented. Characterization and validation of the AMTFD system in cylindrical and spherical geometries as developed includes Monte-Carlo based nuclear particle transport assessments using MCNP-PoliMi and multi-physics based assessments accounting for acoustic, structural, and electromagnetic coupling of the ATMFD system via COMSOL's multi-physics platform. A methodology based on geo-positioning-scheme (GPS) and a higher harmonic based scheme were successfully developed. The spherical (higher-harmonic) technology offers the tantalizing capability for rapid-fire (within tens of seconds) the direct visualization based directionality of incoming neutron radiation via line-of-sight tracks - effectively comprising multiple single detectors within the envelope of a single spherical ATMFD.