Calibration and operation of the AAEC working standard of measurement for the activity of radionuclides, Part 1 - the measurement system.

A pressurised 4 pi gamma ionisation chamber is used by the AAEC as a working standard of measurement for the activity of radionuclides. The instrument has a high degree of stability and the uncertainty in its activity measurements is only marginally greater than that of the primary standards used in its calibration. This report details the method of calibration for a set of known standardised isotopes and extends the usage of the ion chamber to other isotopes by an interpolation technique. Particular attention is paid to eliminating changes in calibration due to instrumental variations with time. The measurement system is straightforward in operation giving best results for gamma emitting isotopes with energies above 200 keV and minimum activity of the order of 1 MBq