Angular distributions in heavy-ion-induced fission

Fission fragment angular distributions have been measured in reactions of ${}_{}{}^{16}{}_{}{}^{}$O${+\mathrm{}}^{208}$Pb, ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$, ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{248}{}_{}{}^{}\mathrm{Cm}$; ${}_{}{}^{19}{}_{}{}^{}$F${+\mathrm{}}^{208}$Pb; ${}_{}{}^{24}{}_{}{}^{}$Mg${+\mathrm{}}^{208}$Pb; ${}_{}{}^{28}{}_{}{}^{}$Si${+\mathrm{}}^{208}$Pb; ${}_{}{}^{32}{}_{}{}^{}$S${+\mathrm{}}^{197}$Au; and ${}_{}{}^{32}{}_{}{}^{}$S${+\mathrm{}}^{208}$Pb at several bombarding energies. The data are analyzed within the standard theory and it is found that the angular anisotropies for reactions with ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ and heavier projectiles are significantly larger than expected theoretically. Comparative studies of reactions in which different target-projectile combinations are leading to similar fissioning systems show that the large angular anisotropies are associated with the charge (and mass) of the projectile. This excludes large angular momenta and temperatures as the cause of these discrepancies. It is concluded that the onset of the quasifission reaction is responsible for the large observed anisotropies.