Influences from delayed events on the activity determination of 75Se by the coincidence counting method

The activity concentration of a 75Se solution was determined by the 4π(e, X)-γ coincidence counting method under the auspices of an international comparison exercise organized by BIPM. Four different beta-particle and gamma-ray window settings were used to verify the influence of the delayed events that originated from the long-lived metastable level of 304 keV. The electronic threshold level variation was used to change the beta-efficiency, and linear and quadratic extrapolation were performed to 100% efficiency. These extrapolated values were corrected for the delayed events registered in the proportional counter. The results are discussed and compared with other values reported in the literature. The half-life of 75Se was also determined by means of a re-entrant gamma-ray ionization chamber. (orig.)