Estimation of photopeak efficiency for IAEA gamma standards of different geometries using HPGe and 3″x3″ NaI(Tl) gamma spectrometry systems

Abstract

The IAEA certified reference gamma standards of primordial radionuclides, such as K-40, U-238, and Th-232, are analysed using gamma spectrometry systems, namely High Purity Germanium (HPGe) and 3″x3″ NaI(Tl) scintillation detectors. The analysis aims to estimate and compare the Full Energy Photopeak Efficiencies (FEPE) of these detectors for the respective gamma emission energies. Additionally, to assess the impact of sample geometry and volume on the FEPEs of the detectors, the study is extended to gamma counting of the aforementioned IAEA standards using two commonly employed sample geometries: flat-cylindrical (∼ 150 ml) and elongated-cylindrical (∼ 250 ml), usually chosen based on the sample quantity. Furthermore, the study calculates and compares the Minimum Detectable Activities (MDA) of the two detectors for the primordial radionuclides in these sample geometries. The study reveals that for both detectors, the calculated efficiency values are higher for the flat-cylindrical geometry compared to the elongated-cylindrical geometry. In contrast to FEPE, the MDA values are observed to be higher for the elongated-cylindrical geometry. The results also indicate that the values of FEPE and MDA for the 3″x3″ NaI(Tl) scintillation detector are higher than those of the HPGe detector for both geometries, in line with the expectations. This study on FEPEs and MDA of gamma detectors are crucial because precise measurements using efficient detectors with low MDAs are vital for obtaining accurate data and drawing valid conclusions.