A preliminary investigation into a simple method for the determination of the mean ionisation energy of gas mixtures used in the NPL primary gas counting system

The activity concentration of gaseous beta-emitting radionuclides such as 3H, 85Kr and, more recently, 11C, is measured at NPL using a set of length-compensated proportional counters. The active gas is mixed with argon–methane (P-10) and passed to the counters. Adding gases to P-10 changes the mean ionisation energy, W, of the gas mixture. Estimation of the counting losses using the Monte Carlo model requires a knowledge of W. Unfortunately, only a limited amount of published data is available.This paper describes the initial experimental studies performed to enable the extension of the MC model based loss correction method to gases other than carbon dioxide in P-10. Preliminary measurements have been made to determine the W value for a gas mixture containing 85Kr in nitrogen and P-10. The DC current through the counters is measured; the counters are also operated in the normal way with pulse amplifiers, discriminators and scalers. The value of W is derived from a knowledge of activity, counter current and mean beta energy.

► Preliminary measurements made to determine W value for 85Kr in nitrogen and P-10.
► IGPC operated in both proportional counter and ionisation chamber mode.
► Response ‘ideal’ counter (long−medium counter volumes) used in W value calculation.
► W value 32.4±3.4 eV (k=1) for the gas mixture 6% nitrogen in P-10 determined.