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Description
Since 1976, the International Bureau of Weights and Measures (BIPM) operates the International Reference System (SIR) which provides international comparisons of radioactivity standards. This system is based on a re-entrant ionization chamber and a specific approach to provide robust comparison values over decades [1]. Although the system has proven its worth for many gamma-emitting radionuclides, it is not optimized for comparing pure beta emitters and some electron-capture nuclides without gamma-ray emissions.
As a result, there has been a long-standing desire to supplement the system to allow comparisons for these types of radionuclides. This extension of the SIR is referred to as the extended SIR (ESIR) and has been under active development since 2018 [2]. It is based on Liquid Scintillation (LS) counting combined with the Triple to Double Coincidence Ratio (TDCR) method [3]. Here, the TDCR approach is used to compensate for potential slight changes of the counting efficiency without using reference sources. Moreover, an extended TDCR methodology can account for a potential asymmetry of the 3 photomultiplier tubes (PMT) involved in the system. The validity of a new approach to compensate for counting efficiency and PMT asymmetry variations has been demonstrated in preliminary studies simulating long-term instabilities via neutral density filters [4].
In this work, we report on a validation of the new ESIR system. To this end, 60Co was selected as it can be measured in both systems, the established SIR and the new ESIR. A total of 13 laboratories took part in the pilot study, CCRI(II)-P1.Co-60, by sending 1 or 2 ampoules with an activity between 280 kBq and 2.16 MBq of 60Co standardized solution. The preparation of the liquid scintillation samples (10 per submitted solution) and the TDCR measurements (10 per LS vial) were carried out following the same procedures that would have been employed in a real comparison exercise. A good agreement between the two systems has been observed for almost all the laboratory submissions. The ESIR has demonstrated its capability to provide robust degrees of equivalence for radionuclides emitting medium/high-energy charged particles, with a high precision (relative standard uncertainty less than 10-3). A few discrepant values have been investigated in more detail, by analysing the results from different standardization methods and additional feedback from the participating laboratories. The study reveals that discrepancies are likely due to the presence of radioactive impurities with low energy emissions that are not detected in the conventional SIR system.
[1] Rytz A 1978 Environment International 1 15-18
[2] Coulon R et al. 2022 Journal of Radioanalytical and Nuclear Chemistry
[3] Broda R 2003 Applied Radiation and Isotopes 58 585–94
[4] Coulon R et al. 2020 Metrologia 57 035009
