Fission of heavy uranium and thorium isotopes: source of new phenomena and dynamics

In view of the energy generation potential of 233U, 235U and 232Th, we have chosen to study the fission properties of the exotic neutron-rich isotopes 250U, 256U, 260U, 240Th, 250Th and 256Th in addition to the known 236U and 232Th in the valley of stability. Our study has been carried out in the frame work of microscopic relativistic mean field theory and semi-phenomenological analysis of data on fission half-life and fission barrier. It is shown that excess of neutron in these isotopes leads to increase in the fission barrier width giving rise to dramatic enhancement in the stability against spontaneous fission. In the typical case of 250U the half-life estimated is ∼ 1024 years. It shows that these isotopes will undergo a new mode of fission termed multifragmentation fission where a number of prompt scission neutron are expected to be simultaneously released along with the two fragments. These are extra neutrons in addition to the usual neutron multiplicity emitted by the two fragments later in the flight. It will have serious implication in the energy generation process in the r - process nucleosynthesis. (author)