STANDARDIZATION OF SB-125 AND EU-154, AND MEASUREMENT OF ABSOLUTE GAMMA-RAY EMISSION PROBABILITIES

A method has been devised to separate Sb-125 from its short-lived daughter Te-125m. This technique was based on the repeated use of columns of anion exchange resin and supplemented by chemical procedures chosen to avoid any steps involving evaporation, which in initial trials gave severe problems due to the volatility of the antimony. Standardisation by 4-pi-beta-gamma coincidence counting was carried out within a few days of the separation, allowing for the daughter which grows in at about 0.25% per day. Beta counting efficiencies, as measured by the ratio of coincidence to gamma count rates, of up to 90% and 95% respectively were obtained for the two gamma-ray energy windows used. The ingrowth was monitored over a period of one month by gamma-ray spectrometry, using the weak 109 keV gamma-ray of Te-125m, to check the original separation. Samples of Eu-154 were produced by neutron irradiation of high purity Eu-153, which was obtained by passing inactive europium through the NPL isotope separator. A solution of the Eu-154 was standardised by coincidence counting, again using two gamma-ray energy windows, to give efficiencies of 96% and 98%. Absolute gamma-ray emission probabilities were measured for both nuclides using a germanium spectrometer calibrated with a range of nuclides of known activities and gamma-ray emissions. Sources were positioned 55 cm from the detector to ensure that real summing effects were negligible.