Competition between mass-symmetric and asymmetric fission modes in Md produced in the He + Es reaction

Nishio, Katsuhisa   ; Hirose, Kentaro  ; Makii, Hiroyuki   ; Orlandi, R.   ; Kean, K. R.*; Tsukada, Kazuaki  ; Toyoshima, Atsushi*; Asai, Masato  ; Sato, Tetsuya   ; Chiera, N. M.*; Momitsuka, Tomohiro*; Tokoi, Katsuyuki*; Yanagihara, Rikuto*; Suzuki, Hayato*; Nagame, Yuichiro* ; Okada, Kazuki; Andreyev, A. N.; Tanaka, Taiki*; Morimoto, Koji*; Ishizaki, Shoma*; Okubayashi, Mizuki*; Miyamoto, Yuya*; Tanaka, Shoya*; Aritomo, Yoshihiro*; Kubono, Shigeru*; Rykaczewski, K. P.*; Boll, R. A.*; Ezold, J. G.*; Felker, L. K.*; Roberto, J. B.*; 6 of others*

In spontaneous fission (SF) it is found that the mass-asymmetric fission of well-known actinides suddenly changes to the symmetric fission when nucleus has a mass number larger than 257. We have measured for the first time fission-fragment mass and total-kinetic-energy (TKE) distributions of the excited compound nucleus Md, populated in the reaction He+Es, using the rare isotope Es with a short half-life, =276 days. The fission of Md can be interpreted as involving 3 fission modes: an mass-asymmetric mode and two modes of mass-asymmetric fissions, the latter two are manifested by different TKE values. Predominantly symmetric fission was obtained at the excitation energy of 15.0 MeV. By increasing the energy to 18.0 MeV, the asymmetric fission mode dominates. Both such a strong change of fission modes within a small increase of excitation energy and the enhancement of asymmetric split are new phenomena in fission.