Spectrally Matched Neutron Detectors Designed Using Computational Adjoint SN For Plug-In Replacement of 3He

2013 J. D. Williams Student Paper 2nd PlaceThe current very limited supply of 3He is attributed to a lack oftritium production for the nuclear weapons complex along with asignificantly increased demand for the gas in various neutron detectionapplications. Circa 2000 more than 200,000, liters (standardtemperature and pressure) were in the 3He stockpile, buttoday less than 45,000 liters remain, and the U.S. Departmentof Energy (DOE) is rationing the supply to only 8,000 liters peryear. A number of research efforts have been conducted to determineif existing materials could serve as an adequate substitutefor 3He and additional efforts have also evaluated new materialsthat might serve adequately as replacements. Regardless of theeffort, each study almost always focuses solely on “simple” detectioncases where the overall system efficiency for one specificsource (e.g., 252Cf) is the only concern (e.g., handheld devices,backpack units, and portal monitoring systems). In these cases,inserting additional detectors and/or materials can address theissue of cumulative counts, because the spectral response is essentiallyirrelevant. However, in many applications such as forsafeguards, nonproliferation efforts, and materials control andaccountability (MC&A) programs, including fissile material assessmentsfor plutonium and actinides, measurements are oftencalibrated to responses in a 3He proportional counter. In thesecases, a mismatch in the neutron response function can produceserious quantification errors and potentially dire consequences.The application of a simple detector addition approach in theseinstances is neither appropriate nor possible due to influences resultingfrom the complex nature of neutron scattering in moderators,cross-sections, gas pressures, geometries and structuralinterference. These more challenging circumstances require thata detailed computational transport analysis be performed for eachspecific application. A leveraged approach using computationaladjoint transport, validated by forward transport and Monte Carlocomputations and laboratory measurements can address thesecomplex scenarios. This paper will present novel designs that arespectrally matched to a baseline 3He detector that can directlyserve as a “plug-in” replacement with equivalent system efficiency.