Fragment-mass distributions $\mathrm{in}{}_{16}O$-induced reactions at energies well above the Coulomb barrier

Background: The excitation energy dependence of the fission fragment mass distributions for heavy ion-induced reactions on preactinides targets well above the Coulomb barrier energies has received limited attention due to the lack of data. An extensive study is required to understand the reaction mechanism at high excitation energy as it bridges our understanding of the mechanism of fission and noncompound nuclear reactions.

Purpose: The purpose was to understand the fusion-fission dynamics well above the Coulomb barrier energies, particularly to address if the theoretical models that are valid near the Coulomb barrier can explain the fission data at high excitation energies.

Methods: In the experiment, a pulsed heavy-ion beam from the cyclotron was utilized, and the resulting binary fragments were detected using two position-sensitive multiwire proportional counters. By analyzing the time-of-flight differences and position information ($\theta ,\varphi$) of the binary fragments, mass distributions were obtained for the reactions ${}_{16}O + {}_{181}\mathrm{Ta}, {}_{197}\mathrm{Au}, {}_{205}\mathrm{Tl}$, and ${}_{208}\mathrm{Pb}$.

Results: The variance of the fission fragment mass distributions exhibits a smooth increase with excitation energy, although values are smaller compared to the predicted values of the semiempirical calculation GEF. The measured variation of the variance of the mass distribution with the fissility showed an exponential increase. The contribution of fast fission have been identified at high energies with mass asymmetry value $\approx 0.22$.

Conclusion: Our systematic measurements over a wide range of excitation energy and target mass indicate that the fission fragment mass distributions are consistent with statistical models up to $\approx 2$ times the Coulomb barrier energies for the pre-actinides target nuclei when bombarded with ${}_{16}O$. The result provides benchmark data to test the new reaction models at high excitation energies.