Uranium Isotopic and Quantitative Analysis Using a Mechanically-Cooled HPGe Detector

For several decades there has been a demand for a portable high-resolution gamma-spectroscopy instrument for characterizing samples containing uranium under field conditions. Historically these systems have been based on high-purity germanium (HPGe) crystals cooled with liquid nitrogen. Until recently, mechanically-cooled HPGe detectors did not provide sufficient resolution for this type of measurement because of vibrations inherent to this cooling technique. Recently, the ORTEC trans-SPEC, a new portable high-resolution HPGe-detector gamma-spectroscopy system with a miniature Stirling-cycle cooler, has become available. The use of a new long-life mechanical cooling system and noise reduction algorithms has significantly improved detector resolution and eliminated the need for liquid nitrogen. This paper describes the results of a study designed to assess the applicability of this new instrument for isotopic and quantitative analyses of uranium. Two software packages, PC/FRAM (Fixed-Energy Response Function Analysis with Multiple Efficiency) and ORTEC ISOTOPIC, were chosen to test the trans-SPEC’s performance. PC/FRAM was used for uranium isotopic analysis, and ISOTOPIC, for quantitative analysis. PC/FRAM has strict gain-setting and energycalibration requirements, which are different from the “standard” factory parameters for trans-SPEC and the ones that are typically used by ISOTOPIC. Performance of ISOTOPIC with a set of PC/FRAM parameters was investigated and shown to provide better results than the standard set of parameters. The use of uranium isotopic ratios determined by PC/FRAM as inputs for the ISOTOPIC estimates of mass under different geometry and shielding conditions also yielded estimates of 238U mass with improved accuracy and precision.