ISOCS characterization of sodium iodide detectors for gamma-ray spectrometry

Activity measurements require knowledge of the detector efficiency for the specific measurement geometry. Often it is not possible to perform a source-based efficiency calibration for the geometry of interest. In such cases the calculation of a response function or a Monte Carlo simulation for the detector may be used. Such methods can be time prohibitive, however, particularly in the case of measurement situations requiring a variety of counting geometries. The ISOCS methodology provides a pre-existing detector response which already contains the Monte Carlo calculated detector efficiencies for points in all spatial directions over a wide range of energies. The efficiency for a particular counting geometry can then be calculated very rapidly for a variety of source distributions and measurement geometries. The ISOCS software was traditionally limited to use with germanium (Ge) detectors, but recently has been extended to use with sodium iodide (NaI) detectors as well. This work presents the results of using a Monte Carlo simulation to characterize the efficiency response of 3"×3" and 2"×2" NaI detectors at several energies. The results from the ISOCS calculation were compared with measurements demonstrating the level of agreement for point and distributed sources. In addition the difficulties encountered in using a multi-line spectrum to determine the accuracy of the mathematical efficiency calibration were studied and these results are presented as well.