Standardisation of iodine-123 and participation in the comparison BIPM.RI(II)-K4.I-123

Iodine-123 was standardised by the 4π(e_A,X)-γ live-timed anti-coincidence counting technique, with liquid scintillation (LS) counting applied for the 4π channel. Seven LS sources were counted over 4 days. To trace the decay of impurities ¹²⁵I, ¹²¹Te and ^123mTe, one source was counted for an additional 9 times over 100 days. Fitting the extended dataset to the decay function for the component radionuclides provided impurity correction factors. The recorded data were corrected for background, live time, impurities and decay, and plotted as N_LSN_γN_C vs (N_γN_C-1). Weighted least squares regression and extrapolation to (N_γN_C-1)=0 provided the source activity. Data were fitted with a 3rd order polynomial with the 2nd order coefficient = zero to account for curvature present in the data. A 1st order polynomial fit provided an activity concentration that was 1.02 % higher. This primary standard was developed as part of ANSTO's participation in the BIPM. RI(II)-K4. I-123 key comparison, using the Transfer Instrument of the International Reference System (SIRTI). The result that was submitted for participation in the inter-comparison was 0.85 % higher than the result reported in this manuscript. With the latest re-evaluation, the standard uncertainty increased from 0.36 % to 0.53 %, mostly due to the non-linear extrapolation of the detection efficiency curve.