Half-life measurements of long-lived radionuclides—New data analysis and systematic effects

Half-lives of long-lived radionuclides, for example, 85Kr, 90Sr, 108mAg, 133Ba, 137Cs, 152Eu and 154Eu, were measured at the Physikalisch-Technische Bundesanstalt (PTB) by ionization chamber measuring systems, for some nuclides, during a period of about 30 years. In particular, the relative residuals of fitted raw data from current measurements by a modified Townsend balance (compensation method) showed small periodic yearly fluctuations. The interpretation of these fluctuations is discussed. To solve problems of instrument instabilities (fluctuations), a new method of data analysis is applied using least-squares fits of combinations of the current ratios of the various radionuclide sources with corresponding data points measured each at nearly the same time. The data are corrected for instabilities similarly to when using current ratios with a 226Ra reference source. The new method of data analysis excludes possible systematic effects due to disequilibrium of the 226Ra decay chain. The limits of the method are discussed including the elimination of outliers due to systematic errors. An uncertainty budget is presented with the corresponding uncertainty components. Relative residuals of the fits in the best cases of the order of ±5×10−4 have been achieved. The results comprise data until the end of 2008. They agree within the limits of their relative uncertainties, typically better than 1.5×10−3, with those published earlier by PTB authors.