Improving Germanium Detector Resolution and Reliability

High purity germanium (HPGe) detectors used in radionuclide assay systems such as Radionuclide Aerosol Sampler/Analyzer (RASA) systems must operate unattended for long periods of time. Currently, the performance and reliability of commercially available large (~100%) HPGe detectors are acceptable for the majority of laboratory applications. However, for remote systems designed for nuclear detonation detection and analysis, improved performance and reliability are required. The goals of this research include improving germanium detector reliability and resolution performance by developing new surface passivation techniques coupled with a unique detector contact arrangement. A major reliability improvement would be realized if germanium detectors were made less sensitive to storage and operational environments by improving the film coating used to passivate and protect the intercontact surface. This research is the investigation of two surface film techniques for passivating HPGe detector surfaces. Wet chemically grown oxide films and sputtered film coatings will in turn be applied to two standard P-type HPGe coaxial detectors. Each coating will be characterized to determine their surface chemistry passivating qualities as well as their inherent noise contribution to the overall system noise. The resolution of a germanium detector is a function of electrical noise and the inherent resolution of the detector element. The electrical noise components which reduce the signal to noise ratio of a detector system are associated with detector leakage current, intercontact surface noise, contact noise and noise sources associated with preamplifier electronics. In particular, those noise components associated with the intercontact surface are leakage current and surface noise. We believe these two significant sources of noise can be reduced by using novel surface passivation techniques resulting in improved detector resolution. The techniques developed by this research will be applicable to new detectors and can also be applied as a retrofit to existing RASA units. This will improve the ability to obtain more sensitive and reliable detections of remote nuclear activity.