Speaker
Description
Two newly implemented primary standardisation techniques (4π(LS)-γ live-timed anti-coincidence and 4π(HPPC)-4πγ) were cross-validated against their respective established laboratory methods (4π(LS)-γ coincidence and analogue 4π(PC)-γ), using Cobalt-60. ANSTO has also recently developed a more robust ionisation chamber measurement and analysis strategy that traces current measurements to the national standard for direct current, thereby reducing reliance on ageing 226Ra reference sources. This measurement campaign was utilised to confirm and document best practice for both the new and established techniques, which contributed towards efforts to formalise the laboratory's quality assurance system and obtain ISO 17025 accreditation. 60Co was selected because of its simple decay scheme which allows highly accurate standardisation, its long half-life facilitating submission to the SIR, and its availability. However, the initial solution used provided inconsistent results.
Preliminary results indicated a 2% discrepancy between the secondary standard ionisation chamber, previously calibrated for 60Co by intercomparison, and the primary measurements, which agreed to within standard uncertainties. As HPGe spectroscopy confirmed that the solution did not contain any gamma emitting impurities, it was hypothesised that low energy electron or X-ray emitters may be responsible for the discrepancy. The ionisation chamber would not be sensitive to these low energy emissions, whereas the LS and PC detectors would be. Radiochemical and inductively coupled plasma mass spectrometry (ICP-MS) analyses are currently underway to identify any impurities. These methods and results will be presented.
In addition, a new 60Co solution was produced by neutron activation of 99.995% pure cobalt metal in ANSTO's OPAL reactor. Standardisation of this solution by the four different primary detection systems provided results that agree to within standard uncertainties, providing validation for the newly implemented techniques. Pertinent details on the production, dissolution and standardisation, including detailed uncertainty budgets, will be presented. Standard ampoules of both solutions were submitted to the BIPM; the new solution for participation in the BIPM.RI(II)-K1.Co-60 key comparison, and both solutions for participation in the CCRI(II)-P1.Co-60 pilot study for extension of the SIR to beta-emitters (ESIR). The original solution will most likely contribute information on the impact of difficult to measure radionuclidic impurities on the ESIR.
