Anomalous increase in width of fission fragment mass distribution in ${}^{19}\mathrm{F}+{}^{232}\mathrm{Th}$

Fission fragment mass distribution has been studied for the system ${}^{19}\mathrm{F}+{}^{232}\mathrm{Th}$ over an energy range of $105.4\text{MeV}$ to $84.2\text{MeV}$ in laboratory frame. For energies, above the Coulomb barrier, the variance of the mass distributions ${\sigma }_m^2$ varies linearly with temperature of the fused system, signifying statistical fusion-fission reaction. However, as energy decreases through the Coulomb barrier, a rapid increase in ${\sigma }_m^2$ is observed for the first time which may be a signal of onset of orientation dependent quasifission reaction.

Figure 1

The (a) time and (b) energy loss correlations of simultaneously detected fission fragments at $E_{\text{cm}}=85.3\text{MeV}$.

Figure 2

Distributions of complimentary fission fragments in $\left(\theta ,\varphi \right)$ at $E_{\text{cm}}=85.3\text{MeV}$. Rectangle ABCD indicates the gate used to select the fusion-fission events for mass determination. Rectangles ABEF and ABGH indicate the gate used to add $\mathrm{50\%}$ and $100\%$ of TF events, respectively.

Figure 3

Variance of mass distributions at different projectile energies (c.m.) as a function of admixture of transfer fission (TF) events. The dotted lines are guide to the eye.

Figure 4

Mass distributions at different projectile energies (c.m.). The Gaussian fits are shown by solid lines.

Figure 5

(a) Mass variance ${\sigma }_m^2$ and (b) anisotropy $A$, as a function of $E_{\text{cm}}$. In (b) the solid line represent the SSPM calculation with correction for prescission neutron correction [13]. Coulomb barrier is indicated by an arrow.