Development of an absolute method for efficiency calibration of a coaxial HPGe detector for large volume sources

Abstract

In this work an absolute method for the determination of the full energy peak efficiency of a gamma spectroscopy system for voluminous sources is presented. The method was tested for a high-resolution coaxial HPGe detector and cylindrical homogeneous volume source. The volume source is represented by a set of point sources filling its volume. We found that the absolute efficiency of a volume source can be determined as the average over its volume of the absolute efficiency of each point source.

Experimentally, we measure the intrinsic efficiency as a function upon source-detector position. Then, considering the solid angle and the attenuations of the gamma rays emitted to the detector by each point source, considered as embedded in the source matrix, the absolute efficiency for each point source inside of the volume was determined. The factor associate with the solid angle and the self-attenuation of photons in the sample was deduced from first principles without any mathematical approximation.

The method was tested by determining the specific activity of 137Cs in cylindrical homogeneous sources, using IAEA reference materials with specific activities between 14.2 Bq/kg and 9640 Bq/kg at the moment of the experimentation. The results obtained shown a good agreement with the expected values. The relative difference was less than 7% in most of the cases.

The main advantage of this method is that it does not require of the use of expensive and hard to produce standard materials. In addition it does not require of matrix effect corrections, which are the main cause of error in this type of measurements, and it is easy to implement in any nuclear physics laboratory.