Abstract
The possibility of gaseous emissions from cyclotron sites during the production of positron emitters for use in nuclear imaging is a public safety issue, and operators are required to measure any such emissions accurately. Despite the fact that stack effluent monitor instrumentation has been available for several years, a suitable calibration facility does not exist. Traceability to a primary standard is essential if such measurements are to be accurate. This project focused on the development of a primary standard based on a system already established at NPL for the standardisation of beta-emitters such as 85Kr and 133xe. Previous standardisations of beta+-emitters have been carried out using liquid or solid sources. Unlike the previously mentioned standardisation methods for positron emitters (which have been carried out using liquid or solid sources), a primary standard for positron emitters in gas has been developed, which can be used to calibrate the instrumentation used at PET/cyclotron sites.11C was chosen as the radionuclide to be standardised, 18F, 15O and 13N could not be used because of experimental factors. To provide an active gas sample suitable for counting and to provide a comparison with an existing standard based on measurement of a liquid, a method has been developed for conversion of sodium bicarbonate to carbon dioxide with subsequent drying and trapping of the gas. A sample of 11C liberated in this way was measured by internal gas proportional counting using a system of three counters. This document describes the standardisation of 11C by gas counting and the comparison of the primary standardisation results with an independent measurement of a sample of the same sodium bicarbonate solution that was used to generate the active gas. The independent measurement was carried out using a secondary standard ion chamber. The experimental determination of counting losses in internal gas proportional counting was difficult. Some of the losses for standardizing 11C,13N, 15O 18F have been calculated using the PENELOPE Monte Carlo Code. The PENELOPE code was selected for the study of the counting losses because it was the most suitable code for positron interaction simulation at the time of this work. The correction factors calculated with PENELOPE were less than 0.6% for the system of the three counters used as the primary standard. A transfer instrument for calibration of local instruments at sites of positron emitting radionuclides can be carried out based on the primary standard.