Characterization of Mechanically Cooled High Purity Germanium (HPGe) Detectors at Elevated Temperatures

High resolution gamma spectroscopy is a tool used in nuclear security applications due to its achievable energy resolution and associated ability to identify special nuclear material. This identification ability is achieved by identifying the characteristic gamma-rays of a material. The challenges that have confronted industry concerning the use of hand-held high purity germanium (HPGe) in homeland security applications have centered on weight, geometry, and cool-down time. Typical liquid nitrogen cooled detectors ranging in size from 10% to 150% detectors will cool down sufficiently within 2-6 hours of filling. The cool-down time achieved in this research ranges from 45 min on the smallest detector to six hours on the largest 180 cm3 detector; which is consistent with typical hand held HPGe devices. The weight and package geometry for HPGe-based designs is driven by the need to cool the HPGe detector to cryogenic temperatures. This is due to small bandgap (~0.7 eV) of HPGe. Liquid nitrogen or mechanical cooling is required to achieve such temperatures.

This dissertation presents work performed to characterize energy resolution performance as a function of temperature in a new mechanically cooled HPGe detector design based upon a split-Stirling cryocooler. This research also quantifies the microphonic noise contribution from this cryocooler. Measurements have been taken on detector sizes ranging from 6.75 cubic centimeters to 180 cubic centimeters. Focus has been placed on determining volume dependence on energy resolution at elevated temperatures. Microphonic noise contribution from the cooler has also been studied over the same temperature range. This energy resolution degradation was most pronounced at low temperatures (<110ºKelvin) and has been shown to be a function of cooler drive voltage. This research shows that in some cases the energy resolution degradation observed can be as much as 1.5 kiloelectronvolts.

This differs from previous studies where detectors were liquid nitrogen cooled. This research is also an expansion of previous research in that the size of the detectors studied is larger than previous. Previously identified research is limited to 75 cubic centimeter volume detectors whereas detectors up to 180 cubic centimeters will be reviewed.