Off-center efficiency of HPGe detectors

The detection efficiency of an HPGe detector for a gamma emitting radioactive point source positioned at different places on the detector cap plane varies strongly due to the geometry and absorption factors. Therefore it would be inaccurate to relate the detection efficiency for large planar samples placed on the detector surface with the efficiency of a point source at the center of the detector. A possible solution might be to count the sample at large enough distance from the detector surface to allow considering the sample as a point source. This solution while possible for highly active sample is undesirable for samples with low activity because of the low counting rate resulting from the large distance. To get a maximum counting rate the sample should be counted on the detector surface and hence correction has to be done for the varying efficiencies. One way to correct for the changing efficiencies is to experimentally map the detector surface for off center detector efficiencies by standard gamma ray point source and integrate numerically over the whole area of the sample. A preferable way to numerical integration is to find an interpolation equation, which describes the varying of the efficiency with the off-center distance