Standardisation of positron-emitters in gas with the NPL primary gas counting system

A primary standard for positron-emitters in gas has been developed. The method involves internal gas proportional counting and the use of the PENELOPE Monte Carlo code to determine corrections for counting losses. The development work was carried out using 11C, although the method can be applied to other positron emitters. The results were compared with measurements of 11C (in solution) carried out using a secondary standard re-entrant ionisation chamber previously calibrated with reference to absolute counting techniques.