Measurement of fission excitation functions in11B,16O,19F+232Threactions in the extreme sub-barrier energy region

Abstract

Fission cross sections have been measured for the systems of ${}^{11}\mathrm{B},$ ${}^{16}\mathrm{O},$ ${}^{19}{\mathrm{F}+}^{232}\mathrm{Th}$ in the sub-barrier to extreme-sub-barrier energy (ESBE) region. Two position sensitive avalanche detectors were employed to measure the fission events due to fusion-fission (FF) and transfer fission (TF) processes using the fission fragment folding angle technique. The cross sections for fusion-fission reactions could be well accounted by the coupled channel calculations over a large energy range except in the ESBE region. The transfer induced fission cross sections are observed to be large at extreme sub-barrier energies. The ratio of transfer fission cross section to that of total fission is observed to increase rapidly as the energy is decreased in the sub-barrier region for all the systems. Also in the ESBE region a number of low momentum transfer fission (LMTF) events having folding angle close to $180\ifmmode^\circ\else\textdegree\fi{}$ are observed with a rather weak dependence on the bombarding energy. These events were investigated by several control experiments. Considering the uncertainties involved in the control experiments, it appears difficult to rule out if they originate completely from background neutron induced reactions or from any new process such as Coulomb fission as conjectured in some earlier studies. The results on different components of the fission cross sections have been discussed.