Fission and complete-fusion probabilities as a function of angular momentum forYb170compound nuclei excited to 107 MeV

Abstract

Fission and complete-fusion cross sections are presented for four entrance channels leading to $^{170}\mathrm{Yb}$ compound nuclei excited to 107 MeV: $^{11}\mathrm{B}$ + $^{159}\mathrm{Tb}$, $^{12}\mathrm{C}$ + $^{158}\mathrm{Gd}$, $^{16}\mathrm{O}$ + $^{154}\mathrm{Sm}$, and $^{20}\mathrm{Ne}$ + $^{150}\mathrm{Nd}$. The measured fission cross sections are 5.9 \ifmmode\pm\else\textpm\fi{} 0.6, 16. \ifmmode\pm\else\textpm\fi{} 2., 40. \ifmmode\pm\else\textpm\fi{} 4., and 89. \ifmmode\pm\else\textpm\fi{} 9. mb for the $^{11}\mathrm{B}$, $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, and $^{20}\mathrm{Ne}$ entrance channels, respectively. The completefusion cross sections for these same entrance channels are 980 \ifmmode\pm\else\textpm\fi{} 150, 1100 \ifmmode\pm\else\textpm\fi{} 160, 1260 \ifmmode\pm\else\textpm\fi{} 190, and 1450 \ifmmode\pm\else\textpm\fi{} 220 mb. The data are combined using a technique which yields the dependence of fission probability over three relatively narrow ranges of angular momentum. The results of the analysis just described are compared to a calculation based on the Bohr-Wheeler formalism for fission widths and the Weisskopf formalism for neutron and charged-particle widths. The calculations include the effects of multiple chance fission but assume that second and higher chance fission is nonnegligible only if preceded by neutron emission rather than charged particle emission. Agreement between the experimental results and the theoretical calculations is found for a ratio of level density parameters for the compound nucleus and the saddle point equal to 1.2 \ifmmode\pm\else\textpm\fi{} 0.1. Fission-fragment angular distributions for the four entrance channels are presented and compared to calculationsNUCLEAR REACTIONS, COMPLETE FUSION, FISSION $^{159}\mathrm{Tb}$($^{11}\mathrm{B}$, $^{170}\mathrm{Yb}$*), $E=115$ MeV; $^{158}\mathrm{Gd}$($^{12}\mathrm{C}$, $^{170}\mathrm{Yb}$*), $E=126$ MeV; $^{154}\mathrm{Sm}$($^{16}\mathrm{O}$, $^{170}\mathrm{Yb}$*), $E=137$ MeV; $^{150}\mathrm{Nd}$($^{20}\mathrm{Ne}$, $^{170}\mathrm{Yb}$*), $E=144$ MeV; measured $\ensuremath{\sigma}(\ensuremath{\theta})$ for evaporation residues and fission fragments, deduced ${J}_{\mathrm{CRIT}}$, ${\ensuremath{\Gamma}}_{f}$.