Properties of neutron-rich nuclei

Spectroscopic studies of decay nuclei have been actively developed owing to a variety of isotope production technologies and advance of highly efficient measuring system. Addition to the conventional thermal neutron induced 235U fission at the KUR-ISOL (Kyoto University Reactor - Isotope Separator On Line), we utilized also the proton (of about 20 MeV) induced 238U fission at TIARA-ISOL in JAERI, which has high mass yields above 160 and around 110. The reflection asymmetric levels having octapole deformation which is called as Parity Doublet, is confirmed for 153Pm by the experiment by at the KUR-ISOL. It is expected that the excitation levels of isotones of N=85 such as 147Sm and 145Nd are in triplet 3/2-, 5/2-, and 7/2- from the shell (2f7/2)3. As the feature of these states, the enhanced E2 transition and the suppressed M1 transition have been observed. Since the same states are expected for 143Ce and 141Ba, we measured the life of excitation levels and the internal conversion factor of gamma transition for 143Ce and 141Ba which were obtained from the decays of 143La (14m) and 141Cs (25s), respectively. From the energy systematics and E2 and M1 transition, it was judged that the excitation levels of isotones of N=85 such as 143Ce and 141Ba are in triplet. By using TIARA-ISOL we identified new six nuclei; 159Pm, 161Sm, 165Gd, 166Tb, 167Tb and 168Tb. The measured beta decay constants of these nuclei are better explained by the gross theory than by the random phase approximation (pn-QRPA). We also measured the decay energies Qβ of the neutron-deficient nuclei 124-127,130La. By the comparison of the experimental values of the half-lives and those calculated values from the decay energies, we reconfirmed the importance of Qβ as the input of the theoretical calculation. The performance test of the high-efficient beta decay energy measuring device under development is also described. About one thousand times high efficiency to the conventional type is expected. (author)