Fission fragment angular distributions.

A statistical scission model is developed and applied to fragment angular distributions from alpha- and heavy-ion-induced fission. Although the formal equations for fragment angular distributions in the statistical scission and transition-state models are of the same structure, the variances in the distribution of angular momentum projections on the fission direction are established at very different stages of the fission process in the two models. The statistical scission model predicts angular distributions in reasonable agreement with those measured for heavy-ion-induced fission of some reaction systems where the fission barrier has vanished or is very small relative to the nuclear temperature. For a number of these systems, the transition state model is inapplicable. The statistical scission model, however, predicts the variance to have too weak an energy dependence, possibly indicating dynamical effects. Inclusion of pre- and post-scission particle emission and asymmetric fission does not completely remove this discrepancy. The transition state model gives a better representation of fragment angular distributions from alpha-particle-induced fission of several targets (low-spin systems) as a function of fissility parameter. The large effective variances, deduced for systems with large spin and no fission barrier, are inconsistent with the concepts of ''fast'' or ''pre-equilibrium'' fission, occurring on a timemore » scale too short for relaxation of the tilting mode. Some of the present discrepancies between the effective variances deduced from fragment angular distributions and those calculated with the statistical scission model may be due to small contributions of incomplete fusion followed by fission or due to sequential fission.« less