Transfer of the SIR key comparison reference values from one ionization chamber to another

In the framework of the CIPM MRA, the continuous BIPM.RI(II)-K1 key comparison named SIR (Système International de Référence) is based on an ionization chamber (IC1) in which more than 70 radionuclides were measured producing about 800 comparison results (equivalent activities $A_e$ in kBq) since 1976. A second chamber (IC2), built to the same nominal design specification, is available to serve as a back-up should the first chamber fail; however, there will be small variations in the chamber such as in wall thicknesses and the tilt of the well. Because of these differences, the key comparison reference values (KCRVs) from IC1 cannot be applied directly to IC2, and to date only 55 independent measurements of 30 radionuclides are available for IC2. A methodology to transfer all existing key comparison reference values (KCRVs) to the back-up chamber IC2 was developed with the aim of minimizing the impact on the uncertainty of the degrees of equivalence. The selected methodology is based on a mathematical model utilizing an updated version of the SIRIC code previously developed by the current authors. This code takes as input the measured IC2 data and IC1 KCRVs transformed by a scaling function $S$ and provides as output the modelled IC2 equivalent activities $A_e^{\text{IC2,mod}}$ and the quotient $A_e^{\text{IC2,mod}}$/$A_e^{\text{IC1,mod}}$ of modelled equivalent activities for each chamber. The IC2 KCRVs are calculated using ${\mathrm{KCRV}}^{\text{IC2}}={\mathrm{KCRV}}^{\text{IC1}}{A}_e^{\text{IC2,mod}}/A_e^{\text{IC1,mod}}$. The optimum form of the scaling function $S$ was explored and potential IC2 KCRVs and degrees of equivalence were calculated and are presented.