Microscopic effects in fragment mass distribution in fusion-fission reactions of light projectiles with heavy targets

Microscopic effects in fragment mass distributions in fusion-fission reactions of light projectiles (C, O and F) on deformed thorium and spherical bismuth targets in near and below Coulomb barrier energies are investigated. Precisely measured mass distribution shows a sudden anomalous increase in variances of mass distributions ($\sigma_{m}^2$) near Coulomb barrier energies for all three projectiles with deformed thorium target, in contrast to a smooth variation of $\sigma_{m}^2$ with energy for spherical bismuth target. Macroscopic effects of change in mass flow or prolonged mass equilibration time can not explain the observed variation in $\sigma_{m}^2$ with energy. Microscopic effects due to change in entrance channel shape compactness for projectiles hitting the polar region of prolate thorium target is postulated to reach a almost symmetric saddle without complete fusion for events of anomalous fragment widths. Quantitative estimates of mass widths mixed for the two processes explains the observed variation of $\sigma_{m}^2$ with excitation energy.