The Deformed shell gap at =100 and new isomers in Cf

Orlandi, R.  ; 牧井 宏之   ; 西尾 勝久   ; 廣瀬 健太郎  ; 浅井 雅人  ; 塚田 和明  ; 佐藤 哲也   ; 伊藤 由太   ; 洲嵜 ふみ  

Orlandi, R.; Makii, Hiroyuki; Nishio, Katsuhisa; Hirose, Kentaro; Asai, Masato; Tsukada, Kazuaki; Sato, Tetsuya; Ito, Yuta; Suzaki, Fumi

Theoretical models predict the existence of an "Island of Stability" (IoS) of long-lived superheavy elements around proton number Z=114-126 and neutron number N=184. Neutron-rich actinides neighboring the deformed shell gaps at Z=100 and N=152 provide indirect information on the IoS: the nuclear structure of these lighter nuclei is in fact influenced by sub-states of nucleon orbits associated with the IoS shell gaps. The nuclear structure of neutron-rich actinide Cf (Z=98, N=150) was investigated using -ray spectroscopy at the JAEA Tokai Tandem accelerator, where this nucleus was produced via the O+Cf multi-nucleon transfer reaction. New -ray transitions were observed using a composite array of Ge detectors and LaBr(Ce) scintillators, leading to an expanded level scheme and a deeper understanding of its structure. Two isomeric states with lifetimes in the nanosecond range, measured by the scintillators, were found among the low-lying excited states of this isotope. One of the two measured lifetimes characterizes the previously known band-head of the 2 octupole-vibrational band at 592 keV. The other isomer decays via a low-energy E1 transition to a longer-lived state, for which we propose a (5) spin-parity assignment, based on the coupled 7/2[633] and 3/2[521] proton orbitals. The evidence for this latter K isomer and its implications for the Z=100 deformed shell gap will be discussed.