Detection of weak γ-activities of neutron-rich isotopes in element-separated sample

A new detection system was constructed by means of combination of a coincidence and an anticoincidence. The γ-ray energy signals detected by a HPGe detector were in coincidence with energy-loss signals of β rays detected by a 4πΔEβ detector, then the coincidence output were in anticoincidence with the positron-annihilating 511 keV γ signals detected by a large BGO detector. This system may reduces the Compton background for γ-spectrum measurement by about one order of magnitude. With strong suppression to the detection of the decaying γ rays from neutron-deficient isotopes, the detection of the decaying γ rays from neutron-rich isotopes can keep a higher efficiency of (46 +- 3)% compared to single-spectrum measurement. This system was used for determining the half-life of the neutron-rich isotope 208Hg. A new value of 41.5-4.1+5.0 min was obtained, which is in agreement with the earlier value obtained by radiochemical milking method, but the measurement error was substantially improved