CHARACTERIZATION AND PERFORMANCE EVALUATION OF AN HPXe DETECTOR FOR NUCLEAR EXPLOSION MONITORING APPLICATIONS

Expanding missions in nuclear explosion monitoring (NEM) and nuclear security have highlighted the need for high-resolution ambient-temperature gamma detectors that can provide radionuclide-specific monitoring under demanding field conditions. Recent improvements in high-pressure xenon (HPXe) detectors indicate that this technology has potential to provide rugged, large volume ambient temperature gamma detectors with adequate resolution for radionuclide analysis to meet needs in several mission areas. The purpose of this Phase I study was to evaluate the feasibility of HPXe-based monitoring systems for meeting required detection sensitivity limits for Ba for specified NEM sampling and counting conditions. An HPXe detector was selected and characterized for the NEM application. A series of experimental measurements with a custom NIST-traceable 9-radionuclide source were conducted to define the energy, efficiency and resolution performance of the detector, and to compare the performance with sodium iodide and germanium detectors. Monte Carlo (MCNP) simulation was used to select optimum air filter geometries (concentric cylinder), to examine efficiency improvements for aluminum vs. steel detector wall material (aluminum ~50% more efficient), and to estimate optimum shield dimensions for an HPXe based nuclear explosion monitor. MCNP modeling was also used to estimate the detection sensitivity of the HPXe detector for the nuclear explosion fission product indicator,Ba. Background spectra for the HPXe detector were calculated with MCNP by using input activity levels as measured in routine NEM runs at Pacific Northwest National Laboratory (PNNL). Analysis of the composite spectra indicates that the required detection sensitivity for Ba can likely be met using the 537 keV gamma peak in the composite spectrum of the HPXe detector. Based on the Phase I project results, a design concept was defined for a NEM Prototype to be developed in the Phase II program. HPXe appears to be a candidate for applications requiring simple, high reliability ambient temperature radionuclide measurement systems with slightly less energy resolution capability than that of cooled germanium systems. Unfulfilled applications exist now for these performance capabilities in nuclear non-proliferation, homeland security, nuclear site cleanup, nuclear power, and portable instruments for health physics needs. 29th Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies