Reassessment of fission fragment angular distributions from continuum states in the context of transition-state theory

Fission angular distributions have been studied for years and have been treated as classic examples of trasitions-state theory. Early work involving composite nuclei of relatively low excitation energy E ^∗ (⪅35 MeV) and spin I (⪅25ħ) gave support to theory and delimited interesting properties of the transitions-state nuclei. More recent research on fusion fission and sequential fission after deeply inelastic reactions involves composite nuclei of much higher energies (⪅200 MeV) and spins (⪅100ħ). Extension of the basic ideas developed for low-spin nuclei requires detailed consideration of the role of these high spins and, in particular, the “spin window” for fussion. We have made empirical correlations of cross sections for evaporation residues and fission in order to get a description of this spin window. A systematic reanalysis has been made for fusion fission induced by H, He and heavier ions. Empirical correlations of K ²₀ (K ²₀ = {I_effT }/{h̷²}) are presented along with comparisons of I_eff to moments of inertia for saddle-point nuclei from the rotating liquid drop model. This model gives an excellent guide for the intermidiate spin zone (30⪅ I ⪅65), while strong shell and/or pairing effects are evident for excitations less than ⪅35 MeV. Observations of strong anisotropies for very high-spin systems signal the demise of certain approximation commonly made in the theory, and suggestions are made toward this end.