Measurement On The Half-life Of⁷⁹Se By AMS At CIAE

79Se is a long-life pure (3radioactive nuclide, which has important theoretical research significances in the fields of nuclear waste treatment disposal, nuclear astrophysics, nuclear data and the life sciences. Meanwhile, the measurements of79Se are also of great practical value in the studies of reactor design, isotope production, and neutron activation analysis etc. However, the half-life data of the79Se is still controversial due to the production of high content of79Se samples is quite difficult, two kinds of detectors are often used in measurements, and the influence of79Br nuclide.The accurate determination of the half-life of79Se requires accurate measurements of the Se atomic number and beta radioactivity. Considering the fact that the79Se has no standard samples, it needs to build up an AMS measurement method evolving the79Se/Se absolute ratio. In order to reduce the differences among the Se isotopes, the79Se ions were measured by the detector, and the78Se and80Se were determined by the Faraday-cup in the determination of79Se/Se in absolute ratio. However, simultaneous use of semiconductor detector and Faraday-cup will induce system errors between two measurement systems. In order to avoid this kind of system error, only the semiconductor detector was used in the measurements of79Se,78Se and80Se in this study. In addition, two attenuations were firstly introduced in CIAE-AMS system to reduce the counting rates of78Se and80Se. In the measurements of78Se and80Se, the two attenuations were covered for10s, while the two attenuations were moved away for100s in the measurement of the79Se.The experimental results show that the beams of78Se and80Se can be reduced into the detection range of semiconductor detector by the introduction of two attenuations. The measured78Se/80Se ratio was basically the same with the theory abundance ratio. The absolute determination of Se/Se ratio in the samples was measured to be (2.08±0.10)×10-7combined with the known78Se abundance and measured79Se/78Se, which lay the foundation for the accurate determination of the half-life of79Se.The activity of79Se was measured at A317001type in the CSTIND level measurement station Tri-Carb low-background liquid scintillation spectrum measuring instrument. Since the energy of79Se was approximately equal to that of14C (～150KeV), so the14C was chosen to scale the79Se. The activity of79Se was measured to be1.24Bq/g and the uncertainty was4%. In addition, the samples used in this study have been placed for4years, and therefore the effects of volatilization must be taken into account. Thus, the concentration of Se was measured again for four times and the average value was8764μg/ml.The half-life of79Se was calculated as (2.46+0.16)×105year according to the half-life formula, and the uncertainty was6.5%. The obtained value is consistent with that of Ming He et al, which carried out in2002. Hence, the AMS absolute measurement method is demonstrated to be feasible in the measurement of the79Se with no standard sample, and can provide reference data for the establishment of the79Se half-life.