Performance of a Low-Background High Purity Germanium Detector with a Novel EndCap

Low background HPGe detectors have been used for many years in the measurement of low activities of radionuclides in various materials. With the increased importance of nuclear forensics, low background detectors are in increasing demand. The endcap and other materials used in low background detector construction must be selected to have the lowest activity possible. In some cases, the materials with the lowest amounts of naturally occurring radionuclides are difficult to obtain and costly. To obtain the best efficiency at all energies, low atomic number materials are needed for the endcap to ensure maximum transmission. Aluminum and magnesium are the materials used most often, with beryllium and carbon fiber being used for the low energy entrance windows. Even specially selected aluminum, magnesium and beryllium contain measurable amounts of naturally occurring radiomaterials, and beryllium has an associated health hazard. In a new approach, a new endcap has been designed completely from carbon fiber materials. This material is not mined so it does not contain any naturally occurring radionuclides. It has a low average atomic number, resulting in good transmission of low energy photons. The transmission of entrance windows below 60 keV is further improved because no support material for the entrance window itself is required. The efficiency of a large diameter and length detector with the new endcap has been measured and shows improvement over conventional endcaps for extended geometry samples. The uniformity of the sensitivity for low energies has been measured. The low background spectrum has been measured and is compared to conventional low background endcap detectors.