Statistical theory of nuclear fission: α-particle emission

Experimental information on angular correlation of the long-range α-particle emitted in ternary fission with the main fission fragments has recently become available. The results may be compared with theoretical predictions and, as it turned out, provide a very sensitive test of the theory of the fission mechanism. The moving apart of the two fragments and the α-particle under the action of the Coulomb forces between them is a classical three-body problem; it can be solved numerically by a high-speed computer if the initial conditions, i.e. the initial positions and velocities of the three particles at the scission point, are given. The initial conditions are so numerous and the dependence of the angular correlation on these conditions is so sensitive that it is impossible to reconstruct the initial conditions from the final experimental results. The statistical theory of nuclear fission dealing with the mechanism of fission leads to a complete specification of the initial conditions of the present problem and the computer calculation of the angular correlation based on them may be compared with the experimental results in a critical test of the theory. The calculated results are in good agreement with the experiments. In asymmetric fission in which the heavy fragment involves the 82-neutron shell, the α-particle is emitted veering towards the light fragment as expected from shell-effect considerations of nuclear deformability. On the other hand, in far asymmetric fission in which the light fragment involves the 50-neutron shell, the simple shell-effect argument predicts the α-particle veering towards the heavy fragment initially, but detailed dynamical calculation shows that this initial movement leads the α-particle into a region of retarding field and the α-particle is eventually pushed back and emitted veering towards the light fragment as experimentally observed. This reversal occurs only when the initial kinetic energy of the : α-particle takes a value as small as that predicted by statistical theory (about 0.5 MeV). All dynamical theories are likely to lead to a higher value of this energy and it is/therefore, more difficult to account for the angular correlation observed. (author)