Isospin effect on probing nuclear dissipation with fission cross sections

Abstract

Nuclear dissipation retards fission. Using the stochastic Langevin model, we calculate the drop of fission cross section caused by friction over its standard statistical-model value, ${\ensuremath{\sigma}}_{f}^{\mathrm{drop}}$, as a function of the presaddle friction strength for fissioning nuclei $^{195}\mathrm{Bi}, ^{202}\mathrm{Bi}$, and $^{209}\mathrm{Bi}$ as well as for different angular momenta. We find that friction effects on ${\ensuremath{\sigma}}_{f}^{\mathrm{drop}}$ are substantially enhanced with increasing isospin of the Bi system and become greater with decreasing angular momentum. Our findings suggest that in experiments, to better constrain the strength of presaddle dissipation through the measurement of fission excitation functions, it is optimal to yield those compound systems with a high isospin and a low spin. Furthermore, we analyze the data of fission excitation functions of $^{210}\mathrm{Po}$ and $^{209}\mathrm{Bi}$ systems, which are populated in $p+^{209}\mathrm{Bi}$ and $p+^{208}\mathrm{Pb}$ reactions and which have a high isospin and a low spin, and find that Langevin calculations with a presaddle friction strength of (3--5) $\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}$ ${\mathrm{s}}^{\ensuremath{-}1}$ describe these experimental fission data very well.