Efficiency and Resolution of Germanium Detectors as a Function of Energy and Incident Geometry

The use of germanium detectors for the identification and quantification of radionuclides in unknown and non-standard counting geometries has increased in the recent past with the need to clean up and verify sites used for radionuclide processing, smuggling detection and other purposes. The calculation of the amount of radionuclide present requires a knowledge of the efficiency of the detector in the counting geometry. Several methods of determining the efficiency in these unusual geometries have been developed over the years. These methods are used in waste measurement, field (in situ) measurements and material verification. One method currently being used is the modeling of the detector response using Monte Carlo simulation programs, especially Monte Carlo N-Particle Code (MCNP). The input to MCNP is a detailed knowledge of the detector construction details, detector crystal details and the source being measured. This paper addresses the difficulty in obtaining the required detector crystal details, even when the common specifications are well known. In addition, the Institute of Electrical and Electronic Engineers (IEEE) Germanium Test Standard (325-1996) is widely used for the specification of High Purity Germanium (HPGe) detectors. The two important specifications are the efficiency and the resolution of the detector. However, the IEEE 3251996 standard only specifies the Full Width at Half Maximum (FWHM) measurement at one geometry and two energies. Thus, this standard does not apply in most counting geometries used today, especially for environmental samples. Modeling programs, such as MCNP, use the physical dimensions of the detector crystal to predict the response of HPGe detectors on the assumption that the detector response can be related to the physical dimensions and that the detector crystal response is independent of the position of the interaction in the crystal. Other investigators, have shown that the peak resolution (FWHM, FW.1M and FW.02M) change with position of the incident gamma ray on the front of the detector. Such variability has possible implications for the accuracy of peak shape and area determination, since the calibration is potentially a function of angle of incidence. To demonstrate that the efficiency and resolution vary as a function of energy and gamma-ray point of incidence, measurements have been made on several coaxial detectors of various crystal types and sizes for different energies. The full-energy peaks from 60 keV to 2.6 MeV were used.