Emission angle dependence of fission fragment spin in12C,16O,and19F+232Threactions

Abstract

The average total spins of fission fragments were measured in ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O},$ and ${}^{19}{\mathrm{F}+}^{232}\mathrm{Th}$ reactions at near and above barrier energies for fragment emission angles parallel and perpendicular to the beam direction. The fragment spins for perpendicular emissions are observed to be higher compared to that for forward-backward emissions indicating the importance of the tilting mode ($K$ degree) of spin excitations in the fission process. The observed angle dependence of fragment spin could be explained within the statistical model, only if one assumes that the collective modes (other than the tilting mode) also depend on the emission angle of the fragments. It is shown that in this way, the results on both the fragment angular distributions and fragment spins can be explained consistently using the same $K$ distribution at the fission saddle point.