Neutron Spectroscopy Development in Tensioned Metastable Fluid Detectors

This dissertation describes work conducted in pursuit of interests in adapting Tension Metastable Fluid Detectors (TMFDs) for dosimetry-related applications with the specific intent of engineering a neutron ambient dose spectrometer. TMFDs possess several charac- teristics desirable for neutron spectrometry, including high efficiencies, complete blindness to gamma and beta radiation, and tailorable-threshold response functions. Prior spectro- scopic work with TMFDs, aptly named Single Atom Spectroscopy (SAS), was constrained to a specific subset of detection fluids who’s composition includes hydrogen and only one other higher Z element (e.g. hydrocarbons), where only one element is assumed capable of initiating a cavitation detection event (CDE). The present work alleviates these restrictions, enabling spectroscopy in detection fluids with multiple constituent elements.

Simulating the detector’s response predicates knowledge of the energy necessary for ra- diation induced nucleation, which has been theoretically derived with nucleation theory for superheated fluids, but remains unbeknownst for tensioned metastable states. This limi- tation was overcome using MCNPX-PoliMI to model the spatial recoil nuclei spectra from isotope sources and coupled with SRIM to generate the ion energy deposition probabil- ity density within a critical length scale of each interaction event. Thereafter, the energy deposition threshold necessary to generate a detection event, and corresponding response matrix, was derived empirically by solving for the solution curve that minimizes the residual difference between the measured and simulated count rates.

The accuracy of the derived response matrix was evaluated through comparisons with a 6LiI Bonner Sphere Spectrometer in which, for 252Cf and 239PuBe/241AmBe isotope source neutron spectra, the two systems offered results within ±10% of each other for ambient equivalent fluences on the order of 100 μRem/hr fields. Notably, when under ultra-low (10 μRem/hr) fields the Bonner spectrometer and other traditional detectors proved impractical. In contrast, the TMFD system was capable of resolving underlying spectral features and corresponding ambient dose rates within ±5% of MCNP predictions.