International Equivalence of 60Co: Sir Measurements of BIPM Comparison

The concept of equivalence was formulated to harmonize the different standards developed and maintained by national metrology institutes (NMIs) worldwide. Equivalence of a NMI with other NMIs or designated institute (DIs) is realized through international comparisons. International comparisons ensure the consistency and accuracy of measurements of various radionuclides that are carried out in laboratories worldwide. Organizing key comparisons by CCRI(II) meeting the requirement of NMIs/DIs for various radionuclides is challenging and cumbersome. Consequently, Syst`eme International de R´ef´erence (SIR), a system that enables NMIs to compare measurements standards over a wide spectrum of radionuclides, was approved by Section II of Consultative Committee on Ionising Radiation (CCRI II). Under this SIR intercomparison programme, BARC standardized radioactive solution of ⁶⁰Co by the primary methods using the 4π β-γ coincidence systems which are indigenously developed and maintained at BARC. The degree of equivalence (D_i) of BARC for ⁶⁰Co (BIPM.RI(II)-K1.Co-60) comparison is D_i =  − 13 ± 42 kBq at expanded uncertainty (k = 2). This comparison demonstrates the measurement equivalence for ⁶⁰Co, and inclusion of BARC equivalent activity in KCRV indicates the consistency of the measurements and the standards maintained at BARC. This paper describes the various methods used for standardizing ⁶⁰Co and the results of the comparison.