Statistical model of heavy-ion fusion-fission reactions

Abstract

Cross-section and neutron-emission data from heavy-ion fusion-fission reactions are consistent with the fission of fully equilibrated systems with fission lifetime estimates obtained via a Kramers-modified statistical model which takes into account the collective motion of the system about the ground state, the temperature dependence of the location and height of fission transition points, and the orientation degree of freedom. If the "standard" techniques for calculating fission lifetimes are used, then the calculated excitation-energy dependence of fission lifetimes is incorrect. We see no evidence to suggest that the nuclear viscosity has a temperature dependence. The strong increase in the nuclear viscosity above a temperature of ~1.3 MeV deduced by others is an artifact generated by an inadequate fission model. DOI: PACS number(s) : 24.75.+i, 24.60.Ky, 25.70. ER fus ER fus f J J J J J ⎛ − + = J J J J T

Dynamical effects in fission investigated at high excitation energy

“…1 shows the sensitivity of the partial fission cross sections and the width of the charge s. This result is in good agreement with others previously obtained using fusion-fission reactions [37][38][39]49]. Moreover, the fact that we are able to describe the data over a broad range of Z 1 +Z 2 values with the same dissipation parameter would indicate that, contrary to some previous results, this parameter does not depend on temperature.…”

“…Those investigations have been mostly carried out using fusionfission reactions [17,[32][33][34][35][36][37][38]. However, those works provided rather unclear conclusions concerning the absolute value of the dissipation parameter and its possible dependences on temperature and deformation.…”

“…Unfortunately, those measurements do not only depend on the presaddle transient time but also on the saddle-to-scission time. Fission probabilities [33,37,39,46,47] or the complementary cross sections of evaporation residues [36,41,48] seem to be better suited to constraint presaddle effects. However, fission or evaporation residue cross sections depend on factors other than dissipation such as the fission barriers or the level densities [37,49,50].…”

“…Fission probabilities [33,37,39,46,47] or the complementary cross sections of evaporation residues [36,41,48] seem to be better suited to constraint presaddle effects. However, fission or evaporation residue cross sections depend on factors other than dissipation such as the fission barriers or the level densities [37,49,50]. Indeed, some authors claim that cross sections of fission reactions induced at relatively high energies can be described using purely statistical models, provided that level densities at saddle and ground configuration are adjusted to specific values [51,52].…”

Dissipation strength of the tilting degree of freedom in fusion-fission reactions

“…A reasonable choice of collective degrees of freedom for modeling shape evolution and considering particle evaporation allow modeling the complex interplay between static and dynamical effects in fission and succeeding in explaining a wide range of experimental data [3,4]. The significance of orientation degree of freedom (K coordinate), which is the projection of the total angular momentum onto the symmetry axis of fissioning nucleus, was demonstrated and an overdamped Langevin equation for K coordinate were introduced [5,6].…”