Activity determination of 60 Co and the importance of its beta spectrum

21st International Conference on Radionuclide Metrology and its Applications (ICRM), Argentinian Comis Nacl Energia Atomica, Buenos Aires, ARGENTINA, MAY 15-19, 2017. Organizer: Argentinian Comision Nacional Energia Atomica; International audience; The activity concentration of a $^{60}$Co solution was measured by means of two 4$\pi\beta$-$\gamma$ coincidence counting systems using a liquid scintillation counter and a proportional counter (PC) in the beta channel, respectively. Additional liquid scintillation measurements were carried out and CIEMAT/NIST efficiency tracing as well as the triple-to double coincidence ratio (TDCR) methods were applied to analyse the data. The last two methods require computed beta spectra to determine the counting efficiencies. The results of both 4$\pi\beta$-$\gamma$ coincidence counting techniques are in very good agreement and yield a robust reference value. The initial activity concentration determined with liquid scintillation counting was found to be significantly lower than the results from 4$\pi\beta$-$\gamma$ a coincidence counting. In addition, the results from TDCR and CIEMAT/NIST show some inconsistency. The discrepancies were resolved by applying new beta spectrum calculations for the dominant allowed beta transition of $^{60}$Co. The use of calculations which take screening effects as well as the atomic exchange effect into account leads to good agreement between all four methods; the combination of these techniques delivers an important validation of beta spectra.