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 ( $\sim 5\times 10^{{\mathbf {7}}}$ n/s) and lightweight ( 3 , 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 cm 3 to up to 40 cm 3 and rejection ratios of >10 4 :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 >10 2 :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).