Advancements in tension metastable fluid detector (TMFD) technology have led to an extension in detector sensitivity to now also detect and distinguish thermal energy (∼.02 eV) neutrons in addition to fast neutrons — spanning 109 orders of magnitude. The unique nature of detector operation and inherent detection mechanism in TMFDs offers a distinct advantage over conventional systems. TMFDs now posses the capabilities for simultaneous sensitivity to fast and thermal neutrons with high intrinsic efficiency, ascertaining directional and spectroscopic source information, all while remaining completely blind to background gamma and beta irradiation. The additional of thermal energy sensitivity was enabled via inclusion of boron in the detection fluid mixture; a compound composed of decaflouropentane (DFP), trimethyl borate (TMB) and methanol. Experimental benchmarking studies were conducted using the spontaneous fission based neutron source 252Cf, in conjunction with theoretical assessments using the nuclear particle transport package MCNP. Source neutron thermalization was accomplished through submersion of the source in a block of ice, such that the moderated spectrum contained an approximate 1:1 ratio between the fast and thermal flux magnitudes. Experimental results show that borated detection fluids yielded up to 6x improvements in the detection rate over their non-borated counterparts. Implications of the current results in regards to the applicability of TMFDs in the field of special nuclear material (SNM) interrogation and detection are discussed.

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