Measurement of the half-life of Se-79 with accelerator mass spectrometry

The accelerator mass spectrometry (AMS) is an effective method for the determination of the half-life of long-lived radionuclides. In this paper, we report a method for measurement of the half-life of Se-79. The number of Se-79 atoms was determined from measured Se-79/Se absolute ratios with the AMS system at the China Institute of Atomic Energy and the decay rate of Se-79 was determined by counting the emitted beta-rays with a liquid scintillation spectrometer. The major improvements of our measurements include using the high abundance of an Se-79 sample which was cooled for many years to exclude the interference of short-lived nuclides, the extraction of SeO2- molecular ions, that results in a suppression of the Br-79 background by as much as about five orders of magnitude. Also, an AMS measurement of the absolute ratio of Se-79/Se was developed to avoid systematic errors. The results show that Se-79/Se is (2.35 +/- 0.12) x 10(-7) in the reference sample and the radioactivity of Se-79 is (1.24 +/- 0.05) Bq/g, so the half-life of Se-79 is (2.78 +/- 0.18) x 10(5) a.