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 (140)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 tilde 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, (140)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 (140)Ba can likely be met using the 537 keV gamma peak in the composite spectrum of the HPXe detector. Presented at the Monitoring Research Review (29th): Ground-Based Nuclear Explosion Monitoring Technologies, held in Denver, CO on 25-27 September 2007. Published in the Proceedings of the Monitoring Research Review (29th): Ground-Based Nuclear Explosion Monitoring Technologies, p787-797, 2007. Sponsored by the National Nuclear Security Administration (NNSA) and the Air Force Research Laboratory (AFRL). The original document contains color images.