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

Abstract

Fragment anisotropies have been measured from fission of several compound nuclei each formed by two different projectile-target combinations: ${\mathrm{U}}^{234}$ by $n$+${\mathrm{U}}^{233}$ and $\alpha$+${\mathrm{Th}}^{230}$, ${\mathrm{U}}^{236}$ by $n$+${\mathrm{U}}^{235}$ and $\alpha$+${\mathrm{Th}}^{232}$, ${\mathrm{Np}}^{238}$ by $n$+${\mathrm{Np}}^{237}$ and $d$+${\mathrm{U}}^{236}$, ${\mathrm{Pu}}^{239}$ by $d$+${\mathrm{Np}}^{237}$ and $\alpha$+${\mathrm{U}}^{235}$, and ${\mathrm{Pu}}^{240}$ by $n$+${\mathrm{Pu}}^{239}$ and $\alpha$+${\mathrm{U}}^{236}$. 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 ${〈l^2〉}_{\mathrm{av}}$ of the fissioning nuclei required for the theory of fragment angular distributions. At the higher energies attained in the present experiment, uncertainties in ${〈l^2〉}_{\mathrm{av}}$ 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 $\eta =\frac{Z_1{Z}_2{e}^2}{\hslash v}$ is roughly 15, this comparison has shown that calculations of transmission coefficients for alpha particles give values of ${〈l^2〉}_{\mathrm{av}}$ 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 ${\mathrm{U}}^{236}$ and ${\mathrm{Pu}}^{240}$ indicate that the effect of target spin on fission anisotropy is small. Additional angular distributions from neutroninduced fission of ${\mathrm{Th}}^{232}$, ${\mathrm{U}}^{233,234,235,236,238}$, ${\mathrm{Np}}^{237}$, and ${\mathrm{Pu}}^{239}$ and anisotropies of alpha-particle-induced fission of ${\mathrm{U}}^{233,235,238}$ are also reported.