Fragment Angular Anisotropies for Magic and Non-Magic Nuclei*

Fission fragment angular distributions have been studied for $^{239} {Pu} (n, f)$ reaction at several neutron energies. Theoretical calculations have been utilized to determine the variance $K_{0}^{2}$ at each bombarding energy from the observed angular anisotropies. The values of $K_{0}^{2}$ show sharp rise from a value of 5-6 to about 13 for higher neutron energies. From the position of one of the breaks in the $K_{0}^{2}$ value, the magnitude of the pairing gap $2\Delta$ in the highly deformed transition nucleus $^{240} Pu$ is estimated to be 2.4 MeV. Fission fragment angular anisotropies from changed particle fission of $^{208} Pb$, $^{209} Bi$, $ ^{235}U$ and $^{238}U$ at laboratory energy of 43.0 MeV have also been analyzed using the traditional transition state model. In all these calculations, optical model transmission coefficients with spin-orbit interaction have been used. In the case of magic nuclei, $^{208}Pb$ and $^{209} Bi$ it is found that the deduced values of $K_{0}^{2}$ are very small as compared to their corresponding values for $^{235}U$ and $^{238}U$ nuclei. This interpreted as due to shell structure, since significant shell and pairing effects appear for the magic nuclei $^{208} Pb$ and $^{209} Bi$.