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

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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 (σₘ²) near Coulomb barrier energies for all three projectiles with deformed thorium target, in contrast to a smooth variation of σₘ² 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 σₘ² 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 σₘ² with excitation energy.