Q_β measurements with a newly developed total absorption detector.

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1. We have developed a new detector, namely, a total absorption detector, for β-decay energy (Q_β) determination of nuclei far off stability with the accuracy of 0.3 MeV.It consists of two identical BGO scntillation detectors, which are located in face to face. It can detect not only back-scattering electrons but also γ-ray efficiently, moreover, bremsstrahlung photons caused by high-energy electrons. Owing to its high full-energy-detection efficiency, Q_β can be determined without decay scheme information. It means this detector will become a powerful tool for Q_β determination for weak activity like new isotopes.2. For measurement short-lived unstable nuclei from fission products of ^<235>U with thermal neutron, this detector was set in an on-line mass-separator installed at Kyoto University research reactor (KUR-ISOL). At first, some neutron-rich nuclei whose have well-determined Q_β were measured as an on-line energy calibration of the detector. We obtained some important results t … More hat this detector could be certainly determined Q_β without decay scheme information and conventional analyzing method, that is a square root plot, was effective similar to the plastic scintillation detector. So far, the accuracy of the obtained results was evaluated to be 0.1-0.2 MeV.3. On the basis of this result, we successfully determined the Q_β of ^<151>Ce to be 5.27 (20) MeV for the first time. In addition, we succeeded in measurements of Q_β of ^<153,154>Pr and ^<157>Pm for the first time. Taking into account the yield from the mass-separator, the yield of 100 particles/s is enough to determine Q_β. Some new isotopes e.g. ^<116,167>Tb, we succeeded in search at JAERI-ISOL facility, are expected to determine their Q_β by reducing background radiation by shielding the detector with much amount of borated rubber sheets and Cd sheets.4. In conclusion, we succeeded in development a newly total absorption detector for Q_β determination and deduced some Q_βs of neutron-rich isotopes using an in-line mass-separator within the expected accuracy of 0.3 MeV. Less