Entrance-channel dependence of fission-fragment anisotropies: A direct experimental signature of fission before equilibration

Fission-fragment angular distributions in reactions of ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ${+}^{232}$Th and ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$, and ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ ${+}^{237}$Np have been measured. While the measured anisotropies in B- and C-induced fission are found to be in agreement with the predictions of the standard Halpern-Strutinsky theory, they are anomalously large in the case of O- and F-induced fission. Such a discontinuous behavior in angular anisotropy with respect to the entrance-channel mass-to-charge asymmetry provides an experimental verification of the predictions of the preequilibrium fission model proposed earlier to explain the anomalous fragment angular distributions.