Threshold Rejection Mode Active Interrogation of SNMs Using Continuous Beam DD Neutrons With Centrifugal and Acoustic Tensioned Metastable Fluid Detectors
A threshold-mode based, first of its kind, portable, lightweight SNM active interrogation mode assessment system is under development by our group with the goal of detecting 1 kg HEU at 1 m under 2-min scan time. This system is comprised of a lightweight (<30 kg) inertial electrostatic confinement DD neutron source (emitting  $\sim 5\times 10^{{\mathbf {7}}}$  n/s) and lightweight (<30 kg) panels of tensioned metastable fluid detector (TMFD) sensors. This paper presents results of studies conducted for detecting special nuclear materials (SNMs) such as HEU and Pu via active interrogation with a continuous source of 2.45 MeV neutrons from a DD accelerator system. The detector system utilizes either the centrifugally tensioned metastable fluid detector (CTMFD) or the acoustically tensioned metastable fluid detector (ATMFD). Evidence for TMFD sensors to be used in threshold mode monitoring mode for detecting SNMs while rejecting the vast majority of interrogating DD source neutrons is shown and discussed. Both TMFD sensors achieve this via tailoring the tensioned metastable negative pressure (Pneg) state so as to enable selective detection based on incoming neutron energy. Intrinsic neutron detection efficiencies of over 60% for fast (MeV) neutrons previously were attainable in CTMFDs with detection volumes of about 40 cm3, all the while remaining 100% blind to gamma photons from fission and DD neutron interactions with surrounding elements. We report results of assessments with CTMFDs of various sizes ranging from 4 cm3 to up to 40 cm3 and rejection ratios of >104:1 (i.e., over 15,000 DD neutrons rejected for each 252 Cf neutron detected). Similar experiments were also conducted with the ATMFD architecture (in which the sensing state oscillates at over 40 kHz from active to inactive). It was found that the rejection ratio rises from about 1:1 at a drive powers above ~7 W, towards ~10:1 at ~3 W, and to >102:1 below 1 W. In sharp contrast, using a 100cc NE-213 liquid scintillation detector under identical conditions, the rejection ratio varied from a high of ~12 (with 0% gamma rejection) down to only ~1.4 (with 99.9% gamma rejection).