Performance and Characteristics of a Low-Background Germanium Well Detector for Low-Energy Gamma-Ray Nuclides

Abstract

Many environmental samples are low activity and small, typically a few grams, with the typical nuclides of interest emitting gamma rays in the range of 10 to 50 keV. These samples are most efficiently counted with a well-type detector, where the sample is inserted into a well or hole in the detector. This geometry almost surrounds the sample with active detection volume giving a subtended solid angle of 4-π steradians. A Hyper Pure Germanium (HPGe) well detector has been constructed with a well tube of carbon fiber material. The carbon fiber is naturally low background, mechanically strong, and has good transmission (65%) down to 10 keV. Previously, low-background aluminium was used for the well tube, but the transmission is less than 1% at 10 keV for the thicknesses required for mechanical strength. The IEEE 325-2010 standard describes the method to characterize detectors for efficiency and resolution, including the well detector, over the energy range from 50 keV to 2 MeV. The source-detector geometry defined in IEEE 325 for well detectors is the most common geometry for small samples. The HPGe detector was installed in a low-background shield of 15 cm of lead, 1 mm tin, and 6 mm OFHC copper. The shield is approximately 275 m above sea level. The 4π solid angle increases cascade summing (TCS) which reduces the net areas of the full energy gamma ray peaks for all nuclides with cascade decays, such as 60 Co and 88 Y. These nuclides are commonly used for calibration. To correct for the cascade summing, the efficiency calibration was done using the TCC methods of GammaVision. The efficiency, both uncorrected and corrected for TCC, and resolution from 50 keV to 2 MeV are presented. The background from 10 keV to 3 MeV will be shown. The minimum detectable activities for common nuclides based on the measured efficiency, background, and different counting times will also be presented.