Measurement of mass-symmetric spontaneous fission of Lr

Asai, Masato  ; Kamada, Hiroki*; Shigekawa, Yudai*; Tsukada, Kazuaki  ; Sato, Tetsuya   ; Toyoshima, Atsushi; Mitsukai, Akina; Nagame, Yuichiro ; Nishio, Katsuhisa   ; Hirose, Kentaro  ; Andreyev, A. N.; Osa, Akihiko  ; Nishinaka, Ichiro*; Shibata, Michihiro*; Tomitsuka, Tomohiro; Goto, Shinichi*; Sakama, Minoru*

To clarify the fission mechanism of mass-symmetric spontaneous fissions observed around the neutron-rich Fm region, we have measured kinetic energies and mass distribution of spontaneous-fission fragments in this nuclear region precisely. The experiment was performed at the JAEA tandem accelerator facility using an on-line isotope separator (ISOL). The mass-separated Lr nuclei were implanted into thin foil, and two fragments of their spontaneous fission were coincidently measured with two Si detectors. The spontaneous fission of Lr was found to show a narrow symmetric mass distribution. On the other hand, their total kinetic energy TKE was found to be low, which is apparently different from the high-TKE symmetric fissions observed in the Fm isotopes. This indicates that the fission mechanisms of these two symmetric fissions are different.