Absolute activity measurement of the electron-capture-based radionuclides 139Ce, 125I, 192Ir and 65Zn by liquid scintillation coincidence counting

Four radionuclides with electron-capture-based decay schemes have been directly measured by a liquid scintillation coincidence extrapolation technique. 125I, 192Ir and 65Zn were measured as part of international key comparisons held under the auspices of the International Bureau of Weights and Measures (BIPM). The 139Ce measurements formed part of a regional comparison organized by the Asia Pacific Metrology Programme (APMP). Since 139Ce decays purely by electron-capture, the basic method is described for this radionuclide. Results and difficulties encountered are discussed and uncertainty budgets are presented.