The effect of the coupling between the neutron multiplicity and the fission competition on the evaporation residue cross sections of the $^{48}$Ca$+^{244}$Pu reaction

Abstract

The survival probability $W_{\rm sur}$ of an excited compound nucleus against fission is a key factor determining the evaporation-residue cross section in the synthesis of superheavy nuclei. The $W_{\rm sur}$ for $^{292}$Fl formed in the $^{48}$Ca+$^{244}$Pu reaction was investigated, with particular attention to the coupling between neutron multiplicity and fission competition during the de-excitation cascade. The entrance-channel capture and fusion cross sections are obtained within the dinuclear system model, while the neutron--fission competition is treated using the Vandenbosch--Huizenga formalism with the Kramers and Strutinsky corrections and a fission barrier damped with temperature and angular momentum. The survival probability is calculated using two treatments of the evaporation--fission cascade. In the conventional factorized approach, the realization probability of a given $xn$ channel is treated independently of the neutron--fission competition, with the branching ratios evaluated along a mean excitation-energy trajectory. In the coupled approach, the full excitation-energy population is propagated through the cascade, so that neutron multiplicity and competition with fission are determined simultaneously at the energy actually reached by each nucleus. The comparison shows that the factorized treatment underestimates the $3n$ survival probability by factors of 17 and 24 at $E^*=39$ and 47 MeV, respectively, while its effect on the $4n$ channel is considerably smaller. The resulting $3n$ and $4n$ evaporation-residue excitation functions reproduce eight of the fourteen measured cross sections within a factor of two. A comparison of two prescriptions for the cold fission barrier further shows that differences of less than 1 MeV can change $W_{\rm sur}$ by more than an order of magnitude.