Search

We have studied the mass and energy distributions of fission fragments from the two reactions ${}^{12}\mathrm{C}{+}^{204}\mathrm{Pb}$ and ${}^{48}\mathrm{Ca}{+}^{168}\mathrm{Er}$ that lead to the same compound nucleus ${}^{216}{\mathrm{Ra}}^{*}.$ Despite the fact that the excitation energy was around 40 MeV in both cases, the contribution from asymmetric fission in the first reaction is only around 1.5% but is about 30% in the second. This marked increase in the yield of asymmetric products is connected to the quasifission process, in which important shell effects become evident. The mass-energy distributions are interpreted in terms of an independent decay mode competing with the normal fusion-fission process and possibly leading to a significant suppression of the fusion cross section itself.

Two components in the ${M}_{\ensuremath{\gamma}}(M)$ distribution were established in detailed measurements of mean $\ensuremath{\gamma}$-ray multiplicities from fission fragments of ${}^{226}\mathrm{Th}$. For the first time in the ${M}_{\ensuremath{\gamma}}(M)$ dependencies we were able to distinguish two components associated with primary and the final (after the neutron evaporation) fission fragments, and show that at the scission point ${M}_{\ensuremath{\gamma}}$ is extremely sensitive to symmetric and asymmetric modes of fission. Theoretical calculations of the pre-scission shapes of the fissioning nuclei confirm our conclusions.

Results of fission fragment mass-energy distributions of the compound ${}^{226}\mathrm{Th}$ nucleus formed in the sub-barrier fusion reaction ${}^{18}\mathrm{O}{+}^{208}\mathrm{Pb}$ at an energy of ${}^{18}\mathrm{O}$ ions ${E}_{\mathrm{lab}}=78$ MeV are reported. The reaction has been studied twice using two different accelerators, and both sets of experimental data agree quite well. Performed analysis of the experimental data with the use of a new multicomponent method has shown that alongside the well-known modes, i.e., the symmetric (S) and two asymmetric modes standard-one and standard-two, a high-energy mode standard-three has manifested itself. The last named mode appears due to the influence of the close-to-sphere neutron shell with $N\ensuremath{\approx}50$ in the light fission fragment group. Theoretical calculations of the prescission shapes of the fissioning nuclei ${}^{224,226}\mathrm{Th}$ confirm this conclusion.

Mass-energy distributions of the ${}^{220}\mathrm{Ra}$ fission fragments produced in the ${}^{12}\mathrm{C}{+}^{208}\mathrm{Pb}$ reaction at projectile energies higher and deep below the Coulomb barrier have been measured using a two-arm time-of-flight spectrometer CORSET. It has been shown that an asymmetric component appears at the sides of the prevailing symmetric fission with a cooling of the compound nucleus. The component consists of two standard modes with average masses of $\ensuremath{\simeq}132$ and $\ensuremath{\simeq}139.$ A three-component structure has also been observed in the total kinetic energy distributions in the range of asymmetric fission fragment masses.