NEW NEUTRON-RICH ISOMERS OBSERVED AMONG FISSION FRAGMENTS OF 345 MEV/U ~(238)U AND EVOLUTION OF NUCLEAR STRUCTURE

摘要

We have observed 18 new isomers: ~(59m)Ti, ~(90m)As,~(92m)Se, ~(93m)Se,~(94m)Br, ~(95m)Br, ~(96m)Br, ~(97m)Rb, ~(108m)Nb, ~(109m)Mo, ~(117m)Ru, ~(119m)Ru, ~(120m)Rh, ~(122m)Rh, ~(121m)Pd, ~(124m)Pd, ~(124m)Ag and ~(126m)Ag among fission fragments produced by in-flight fission of 345 MeV/u ~(238)U, and obtained a wealth of new spectroscopic information not only for these new isomers but also for known isomers such as ~(82m)Ga, ~(92m)Br, ~(98m)Rb, ~(108m)Zr and ~(125m)Ag. The measurement was simultaneously made during the search-for-new-isotope runs using the BigRIPS separator at RIKEN RIBF [1],in which delayed γ-rays were detected at the focal plane. The obtained results have allowed us to propose level schemes for 17 very neutron-rich exotic nuclei including the nature of nuclear isomerism, by which evolution of nuclear structure in this region is discussed. For instance, we propose shape isomerism for ~(97m)Rb,~(95m)Br and ~(98m)Rb based on the systematics of shape coexistence in the region with N-60 and Z=38-42. In case of ~(108m)Zr, we extend the ground-state rotational band up to the 8+ state and discuss the K isomerism based on the recent theoretical prediction [2]. Shape isomerism is suggested for ~(117m)Ru based on the observed transition probability. We also propose shape isomerism for the neighboring isomers such as ~(119m)Ru, ~(120m,122m)Rh based on the theoretical mass systematics [3] that predict the occurrence of large deformation in the region with N-75 and Z=39~45. The existence of ~(59m)Ti,which is proposed to be a spherical E2 isomer, suggests that the N=34 sub-shell gap between vp_(1/2) and vf_(5/2) gets smaller as the neutron number increases due to the attractive monopole interaction between πf_(7/2) and vf_(5/2) [4].