Evaluation of the mean ionisation energy (W) of gas mixtures used in the NPL primary gas counting system
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The National Physical Laboratory (NPL) has well-established facilities for radioactivity standardisations of beta emitting gases, e.g. 3H, 85Kr, and more recently 11C, by internal proportional counting.
A Monte Carlo simulation has been developed to determine the corrections for counting losses during the standardisation of 11CO2 in P-10. It is important to validate this technique. Initially it is necessary to determine the mean energy required to generate a single ion pair (W value) in the gas P-10/CO2 mixture.
The W value of P-10 is well known, 26.0 eV, but values for the three or four part gas mixtures commonly used in the gas counting systems have not been reported.