Precision Measurement of the <beta>+ Decay Strength of Short-Lived Nuclei Using On-Line Mass Separation

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The present project aims at construction of an ultra-low-background <gamma>-ray detector system, with which to measure weak <gamma>-ray branchings of <beta>-decays of short-lived nuclei for deriving precise values of <beta>-decay matrix elements.In the 1st fiscal year the <gamma>-ray detector system based on gating a Ge detector by a plastic <beta>^+ detector and a BaF_2 system consisting of eight crystals to register two annihilation radiations. The performance of the detector system was tested using a ^<22>Na and other standard sources, showing a background suppression factor <greater than or similar> 10^<-4>.In the 2nd fiscal year the detector system was installed into a tape-transport system connected to an IGISOL (Ion-Guide Isotope Separator On-Line), and the maximum velocity of tape transport was improved both in hardware and software by a factor of <greater than or similar> 1.6. An on-line experiment was made for the decay of ^<57>Cu, a mirror nucleus of p_<3/2-> shell region, and the 1111.6 keV <gamma>-ray was detected for the first time for mass- separated ^<57>Cu (T_<1/2> = 0.200 s).