Half-life measurements of Europium radionuclides and the long-term stability of detectors

Consideration of the long-term stability of detectors has proved to be very important in half-life measurements. For example, studies of a 226Ra reference source with a pressurised 4πγ ionization chamber for stability checks showed an average decrease in efficiency of 0.005% per year. Taking this effect into account, half-lives of T1/2(152Eu)=4936.6(20) d and T1/2(154Eu)=3138.1(16) d were determined. Using the half-life value of 152Eu as a reference, the time-dependent decrease of the efficiency of semiconductor detectors at different photon energies was determined for periods of the order of twenty years. Values decreased by an average of less than 0.01% per year for a high-purity Ge detector, by about 0.03% per year for a Ge(Li) detector and by about 0.07% per year for a Si(Li) detector. Half-life values of T1/2(154Eu)=3146(11) d and T1/2(155Eu)=1739(8) d were determined. The uncertainty of the 155Eu half-life value was reduced by a factor of three in comparison to an earlier PTB measurement to which this correction had not been applied.