The operation of an ionization chamber with depleted gas for radioactivity measurement: Calibration procedure and utilising normalized manufacturer's radionuclide factors

At the National Metrology Institute of South Africa (NMISA) absolute activity measurements of γ-emitting radionuclides are maintained for longevity on a secondary standard ionization chamber. Because the ionization chamber has lower gas pressure than that normally produced by the manufacturer, this paper focuses attention on explaining a normalization scheme devised to allow NMISA to make use of the manufacturer's supplied radionuclide calibration factors when necessary. The applicability of the procedure is justified through analysis of some results from a comprehensive simulation of the chamber undertaken in an independent study. Comparisons of the derived normalized calibration factors against those obtained through measurement at NMISA are made for a variety of radionuclides. •The NMISA secondary standard ionization chamber calibration process is described.•An explanation of a normalization scheme applied to manufacturer's radionuclide calibration factors is provided.•The effect of reduced current output due to depleted gas within the chamber is analyzed using simulated data.•Comparisons of the derived normalized calibration factors against those obtained through measurement for a variety of radionuclides.•Most secondary standards produced are of radionuclides for dissemination to the nuclear medicine community.