Efficiency and resolution of germanium detectors as a function of energy and incident geometry

The IEEE Germanium Test Standard (325-1996) is widely used for the specification of HPGe detectors, but only specifies detector performance in limited situations. Modeling programs such as MCNP are being used to predict the response of HPGe detectors in a wide variety of detector-source geometries. The accurate simulation of germanium detectors in response to incident gamma rays relies on knowledge of the performance of the detector in different detector-source geometries. The efficiency and resolution depend on the detector-source geometry. Other investigators have shown that the peak shape and efficiency change with position of the incident gamma ray on the detector. The efficiency as a function of incident pencil beam gamma rays has been measured for several different types of detectors at different energies. Based on these measurements, the dead layer on the sides (cylindrical surface) of the detector was estimated and used in the MCNP program to calculate the efficiency in the same geometry. Using detailed measurements, some agreement between experimental data and calculations was obtained.