Reexamination of fission in the $A\approx 200$ mass region with excitation energy near 50 MeV

Even though the fission of nuclei in the mass region 200 with excitation energy near 50 MeV has been studied extensively, a unique description of the fission probability and prefission neutron multiplicity (${\nu }_{\mathrm{pre}}$) data remains elusive. In the present work, a reexamination of the relevant data along with a new estimate of ${\nu }_{\mathrm{pre}}$ and fission chance distributions, obtained from the experimental fission excitation functions of neighboring Po isotopes, has been carried out. The ${\nu }_{\mathrm{pre}}$ from the above-mentioned method, sensitive to only the presaddle part, is significantly lower than the value obtained from neutron spectra measurements. Further, ${\nu }_{\mathrm{pre}}$ from the fission chance data is in good agreement with the statistical model predictions, which also accounts for the light-ion induced fission probability data up to low excitation energy ($\sim 30$ MeV). From this observation, it is concluded that the presaddle dynamical effects are not significant over this excitation energy range, and the ${\nu }_{\mathrm{pre}}$ data determined from the neutron spectra might have a significant contribution from the near-scission emission.

Figure 1

A schematic representation of the potential energy as a function of deformation in mass $A\approx 200$. Different reference surfaces frequently used in the statistical model calculations are marked as “a”, “b”, and “c”. Dominant deexcitation modes of a excited compound nucleus in this mass region are also shown by vertical and slanting arrows for neutron emission and fission, respectively.

Figure 2

Statistical model predictions for fission (top panel) and prefission neutron multiplicity (bottom panel) with options “a” (dashed line), “b” (continuous line), and “c” (dot-dashed line) are compared with the experimental data and the prediction of the advanced 4D Langevin code (Schmitt 2014 [1]) for the ${}^{12}\mathrm{C}+{}^{198}\mathrm{Pt}$ system. The prefission neutron multiplicity obtained from the fission chance distribution (see text) for ${}^{212}\mathrm{Po}$ is also shown as an open rectangle.

Figure 3

Statistical model predictions of the excitation energy of the nuclei undergoing fission arising from the decay of compound nuclei with same excitation energy (59 MeV) after one or more neutron emissions for the ${}^{12}\mathrm{C}+{}^{198}\mathrm{Pt}$ system with different options for the potential energy surface (see text).

Figure 4

Relative fission probabilities as a function of chance for ${}^{212}\mathrm{Po}$ at $E^{*}=59$ MeV, extracted from the experimental fission excitation functions of ${}^{3,4}\mathrm{He}+{}^{206,207,208}\mathrm{Pb}$ [22], are compared with the statistical model predictions with different options for potential energy surface (see text).