Atomic mass determination through beta-decay energy measurements for nuclei far off the stability line by using total absorption detector.

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Beta-decay energy(Q_β) has been measured with a total absorption type detector which consists of a large volume HPGe detector and an annular BGO scintillation detector for the anti-coincidence detector.The HPGe detector, which has a center hole in which are introduced the radioactive sources, can be measured with large solid angle, and the BGO scintillation detector can measure the scattering and/or transmitting photons from the HPGe detector. Finally, the highest energy point of the measured spectra, namely end-point, corresponds to the Q_β.This detector was set at the on-line mass separator installed at the Kyoto University Reactor (KUR-ISOL) and tested its performance as a Q_β detector. The nuclei of neutron-rich Rb and Cs isotopes that have well-determined Q_β values (3〜8 MeV) were measured. By using the response functions for mono-energetic electrons and photons simulated by the Monte Carlo Code of EGS4, the end-point of the obtained total absorption spectra were determined. Compared with the end-points to the evaluated values, the systematic uncertainty for the detector was evaluated to be 20 keV. On the basis of the above analysis, the nucleus of ^<153>Pr was measured. The Q_β of this nucleus had been measured with another type total absorption detector which consists in two large-volume BGO scintillators. The present value is in agreement with the previous value with in the uncertainty.We are scheduled to measure the nuclei around A=150 with KUR-ISOL and to compare the results with systematics or theoretically predicted values.