Spontaneous fission measurements for neutron-rich Fm and Lr isotopes using Es target

Asai, Masato  ; Tsukada, Kazuaki  ; Hirose, Kentaro  ; Toyoshima, Atsushi*; Tomitsuka, Tomohiro*; Chiera, N. M.*; Ito, Yuta   ; Makii, Hiroyuki   ; Nagame, Yuichiro* ; Nishio, Katsuhisa   ; Orlandi, R.  ; Ratha, K. K.*; Sato, Tetsuya   ; Suzuki, Hayato*; Tokoi, Katsuyuki*; Vermeulen, M. J.*; Yanagihara, Rikuto*; Nishinaka, Ichiro*; Kamada, Hiroki*; Shibata, Michihiro*; Goto, Shinichi*; Hirose, Hikaru*; Hayashi, Hiroaki*; Sakama, Minoru*; Andreyev, A. N.

Neutron-rich fermium isotopes Fm and Fm and lawrencium isotope Lr were produced in the multinucleon transfer reaction on Es target with heavy-ion beam and in the heavy-ion fusion reaction on Cm target with N beam, respectively. Reaction products were mass-separated with an on-line isotope separator (ISOL), and spontaneous fission (SF) of these isotopes were measured. It was found that the mass distribution of the SF of Fm was asymmetric, while that of Fm was sharp symmetric, which confirmed the previous experimental results. On the other hand, the SF of Lr seemed to contain both symmetric and asymmetric mass distributions. On the basis of the present results of the mass and total kinetic energy distributions, we discussed the mechanism of fission for these isotopes.