Real-Time Shielded and Unshielded Moving SNM Detection Using Large-Array Tensioned Metastable Fluid Detectors

This article discusses the design and performance of a single array of centrifugally tensioned metastable fluid detectors (CTMFDs) for real-time detection of shielded and unshielded neutron-emitting special nuclear materials (SNMs), both while moving and stationary, and at variable standoff distances. With the goal to maximize the detection rate of 0.02 eV to 12+ MeV energy range neutrons, the sensor fluid in each CTMFD unit was formulated to include natural boron. At a tensioned negative pressure ( P_{\mathrm {neg}} ) state of −7 bar, the 16 cc CTMFD intrinsic detection efficiency was measured for both shielded (in 12^{\prime \prime } OD paraffin) and unshielded configurations as ~35% against a Pu-Be ( \alpha , n ) neutron source emitting 2\times 10^{6} n/s. An array of 13 CTMFDs stacked in a rectangular enclosure demonstrated conclusive real-time tracking of the Pu-Be source in the trunk of a car shielded in a 0.3-m ( 12^{\prime \prime } ) OD paraffin bucket, or unshielded at variable standoffs, and while speeding at 20 mph (~32 kph). The measured detection counts range from ~2 to 12 times over background at closest approach standoffs ranging from 20 m (~70 ft) to 4 m (~13 ft), respectively. The experimental scenario was modeled and simulated to explain the paradoxical and nonintuitive experimental findings pertaining to shielded and unshielded SNM cases, both displaying similar detection rates. The relative effects of neutron down-scattering for shielded cases, which increased detection probability-compensating for shield-absorbed reductions, along with shadowing effects from detector panel background shielding, were found to be dominant contributors.