Neutron spectroscopy & H*10 dosimetry with tensioned metastable fluid detectors

Abstract This article discusses outcome of research for a novel, fast-to-thermal, continuous and pulsed neutron-spectroscopy enabled H*10 dosimetry capable (gamma-beta blind), high intrinsic efficiency sensor system utilizing the tensioned metastable fluid detector (TMFD) architecture. In our past studies the response matrix was developed in single-atom-spectroscopy mode due to which TMFD sensing fluids were limited to one with molecules for which only one atom was considered as causing detection for fast energy neutrons (>0.1 MeV). In this paper, we describe how these limitations were overcome using a new approach for determination of the response matrix – resulting in a generalized development of a neutron spectroscopy system for any arbitrary fluid, and for spectrometric detection of thermal and fast energy neutrons. Unlike for gamma-beta radiation where the dose weight-factor is invariant with energy, for neutrons it is highly non-linear with the induced dose per unit neutron flux increasing ∼ 100 times with neutron energy from eV to MeV; this must be accounted for to avoid significant conservatisms inherent in non-spectroscopic general purpose neutron detectors. The neutron spectrometry enablement for TMFDs was utilized to develop H*10-TMFD rapid, high efficiency neutron dosimetry for ultra-low ( ∼ 5 μ Rem/h) to high (potentially >10 Rem/h) environments, while remaining 100% gamma-beta blind. Comparisons were performed against the mainstay standard Bonner Sphere Spectrometer (BSS) in which, for 252Cf and Pu-Be/Am-Be isotope source neutron spectra the two systems offered results within +/- 10% of each other in >100 μ Rem/h fields. Notably, when under ultra-low ( ∼ 10 μ Rem/h) fields the BSS and other traditional detectors proved impractical (effectively dysfunctional); whereas, the H*TMFD system measured hard and soft neutron spectra to within +/-5% of those from MCNP predictions of the same experiments. Comparisons were also made for monoenergetic D-D (2.45 MeV) neutron based radiation fields ( ∼ 20-30 mRem/h) for which the H*TMFD system successfully predicted both the H*-10 dose and spectrum nuance characteristics (including for the effect of tritium contamination caused 14 MeV component) whereas, the BSS system was unable to do so appropriately.