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

Abstract

Four radionuclides with electron-capture-based decay schemes have been directly measured by a liquid scintillation coincidence extrapolation technique. The advantage of using (LS) counting in the 4p channel is that self-absorption does not occur, leading to Auger electrons being detected with relatively high efficiency; I-125, Ir-192 and Zn-65 were measured as part of international key comparisons held under the auspices of the International Bureau of Weights and Measures (BIPM). The Ce-139 measurements formed part of a regional comparison organized by the Asia Pacific Metrology Programme (APMP). Since Ce-139 decays simply by electron-capture to the 166 keV excited state of 139La. This state de-excites through the emission of conversion electrons and g-rays. The detection efficiency analysis is based on a counting source being viewed by two phototubes in coincidence, with a single NaI(Tl) crystal detecting the g-rays this technique is a viable technique for the absolute activity measurement of electron-capture-based radionuclides. The efficiency data can generally be fitted with a linear function, particularly in the high-efficiency region, or by a low-order polynomial expression, giving rise to reliable extrapolated activity values. However the application of the method to 125I is more difficult due to the requirement of a proper background correction to the sum peak area. It is surmised that a NaI detector with improved resolution that better separates the sum peak will reduce the discrepancy observed in the more recent activity measurement