Estimation of photopeak efficiency for IAEA gamma standards of different geometries using HPGe and 3 x 3 NaI(Tl) gamma spectrometry systems

IAEA certified reference gamma standards of primordial radionuclides like K-40, U-238 and Th-232 are counted using the gamma spectrometry systems viz., High Purity Germanium (HPGe) and 3 x 3 NaI(Tl) scintillation detectors to estimate the Full Energy Photopeak Efficiencies (FEPE) for the respective gamma emission energies. Additionally, in order to observe the effect of sample geometry and volume on FEPEs of the detectors of interest, the study has been extended to gamma counting of the above-mentioned IAEA standards in two different sample geometries: flat-cylindrical (~ 150 ml) and elongated-cylindrical (~ 250 ml). Furthermore, the minimum detectable activities (MDA) of the two detectors are calculated and compared for primordial radionuclides of interest present in the above-mentioned sample geometries. It is inferred from the study that for both the detectors, the calculated efficiency values are more for flat-cylindrical geometry than those of elongated-cylindrical geometry. Unlike FEPE, the MDA values are observed to be more for elongated-cylindrical geometry. The results also indicate that the efficiency and MDA of 3 x 3 NaI(Tl) scintillation detector are found to be higher than those of HPGe detector for both the geometries as expected. (author)