Fragment Anisotropies in Neutron-, Deuteron-, and Alpha-Particle-Induced Fission

Fragment anisotropies have been measured from fission of several compound nuclei each formed by two different projectile-target combinations: U²³⁴ by n+U²³³ and α+Th²³⁰, U²³⁶ by n+U²³⁵ and α+Th²³², Np²³⁸ by n+Np²³⁷ and d+U²³⁶, Pu²³⁹ by d+Np²³⁷ and α+U²³⁵, and Pu²⁴⁰ by n+Pu²³⁹ and α+U²³⁶. These measurements extended over an energy range of approximately 12- to 25-MeV excitation energy of the compound nucleus. Transmission coefficients were calculated to estimate the mean-square orbital angular momentum 〈l²〉ₐᵥ of the fissioning nuclei required for the theory of fragment angular distributions. At the higher energies attained in the present experiment, uncertainties in 〈l²〉ₐᵥ introduced by Coulomb-barrier penetration effects were small, and it was therefore possible to make comparisons between measurements and theory which indicate that the distortion of the compound nucleus at the saddle-point configuration before fission is independent of the total angular momentum and the identity of the bombarding particle. At the lower energies, where the Coulomb parameter η=Z₁Z₂e²v is roughly 15, this comparison has shown that calculations of transmission coefficients for alpha particles give values of 〈l²〉ₐᵥ in agreement with the data only if careful attention is given to the effects of barrier penetration. At the lowest energies, comparisons of data from U²³⁶ and Pu²⁴⁰ indicate that the effect of target spin on fission anisotropy is small. Additional angular distributions from neutroninduced fission of Th²³², U^233,0ex0ex234,0ex0ex235,0ex0ex236,0ex0ex238, Np²³⁷, and Pu²³⁹ and anisotropies of alpha-particle-induced fission of U^233,0ex0ex235,0ex0ex238 are also reported.