Response calculation for standard ionization chambers in the APMP using EGS4 Monte Carlo code

In the APMP radioactivity group (TCRI), a series of intercomparisons were conducted between member laboratories, with the aim to improve the calibration of ionization chambers used as secondary standards for radioactivity measurement. Several sources were sent to the participating institutes and the measurement results of calibration factors have been compared. Good agreement between laboratories was found for radioisotopes where the major contribution to the ionization current stems from high-energy gamma rays. It was expected in this energy region that these ratios would become unity for any type of chamber by normalizing with the same type of reference sources. However, where the dominating contribution to the measured ionization current is due to the presence of low-energy gamma rays, there were some serious discrepancies between participating institutes. In order to understand these differences, Monte Carlo simulation has been utilized by studying the effects of changing the wall thickness of the re-entrant well, the materials of the sample holder and the composition of the ionizing gas.