SPECTRALLY-MATCHED NEUTRON DETECTORS DESIGNED USING COMPUTATIONAL ADJOINT SN FOR PLUG-IN REPLACEMENT OF 3He

There is presently a very limited supply of 3He, attributed to a lack of tritium production for the nuclear weapons complex along with a significantly increased demand for the gas in various neutron detection applications. Circa 2000, there were more than 200,000 liters (standard temperature and pressure) in the 3He stockpile, but today less than 45,000 liters remain, and the DOE is rationing the supply to only 8,000 liters per year. A number of research efforts have been conducted to determine if existing materials could serve as an adequate substitute for 3He and additional efforts have also evaluated new materials that might serve adequately as replacements. Regardless of the effort, each study almost always focuses solely on “simple” detection cases where the overall system efficiency for one specific source (e.g. 252Cf) is the only concern (e.g. hand-held devices, backpack units, and portal monitoring systems). In these cases, inserting additional detectors and/or materials can address the issue of cumulative counts, because the spectral response is essentially irrelevant. However, in many applications such as for safeguards, non-proliferation efforts, and materials control and accountability (MC&A) programs, including fissile material assessments for plutonium and actinides, measurements are often calibrated to responses in a 3He proportional counter. In these cases, a mismatch in the neutron response function can produce serious quantification errors and potentially dire consequences. The application of a simple detector addition approach in these instances is neither appropriate nor possible due to influences resulting from the complex nature of neutron scattering in moderators, cross sections, gas pressures, geometries and structural interference. These more challenging circumstances require that a detailed computational transport analysis be performed for each specific application. A leveraged approach using computational adjoint transport, validated by forward transport and Monte Carlo computations and laboratory measurements can address these complex scenarios. This paper will present novel designs that are spectrally-matched to a baseline 3He detector that can directly serve as a “plug-in” replacement with equivalent system efficiency.