Search for possible fission modes at high excitation energies in 254Fm

Background: Shell effects have been found to influence both the compound nuclear fission (CNF) and quasifission processes. Besides quasifission processes, which fission modes remain active at excitation energy (E*) as high as 56 MeV should be investigated. Purpose: We investigate the signatures of fission modes in Fm-254 populated by O-16 + U-238 through the mass distribution (MD) and total kinetic energy distribution (TKED). Method: The mass-total kinetic energy distributions (M-TKED) of fission fragments of the reaction O-16 + U-238 have been measured at two laboratory energies E-lab = 89 and 101 MeV. The spontaneous fission (SF) of Fm-254, one-dimensional (1D) fragment MD, and two-dimensional (2D) M-TKEDs of O-16 + U-238 have been described by the multimodal random neck rupture (MM-RNR) model. Results: Channel probabilities and the characteristics of different fission modes are obtained and discussed in detail. The enhancement observed in the mass yield (approximate to 10(-2) %) in the region 60-70 u for the light fragments at E* approximate to 45 MeV goes away at the higher E* approximate to 56 MeV. The heavy fragments of S1 and S2 modes are found to be associated with Z approximate to 53 and Z approximate to 56 shells, respectively. The slope of asymmetric to symmetric fission yields (when plotted against E*) of O-16 + U-238 is found to be similar to that of previously reported O-18 + Pb-208. Conclusions: Analysis of 2D M-TKED data by the MM-RNR model reveals the possible presence of fission modes in O-16 + U-238. The liquid-drop-like broad symmetric SL mode is found to peak at a lower energy than predicted by the Viola systematic, which matches mostly with that of Standard 2 mode. No signature of asymmetric quasifission is observed. The MD widths show a linear dependence with the measured energies.

Automated summary

The characteristics of fission modes in O-16 + U-238 reaction are investigated through the mass distribution and total kinetic energy distribution. The results show that the characteristics of fission modes vary at different excitation energies. The heavy fragments are found to be associated with specific shells. Additionally, the ratio of asymmetric to symmetric fission yields shows a similar trend with increasing excitation energy, which is consistent with previous findings.