J. D. McDonnell, W. Nazarewicz, J. A. Sheikh
Background: Well-developed third minima, corresponding to strongly elongated and reflection-asymmetric shapes associated with di-molecular configurations, have been predicted in some non-self-consistent models to impact fission pathways of thorium and uranium isotopes. These predictions have guided the interpretation of resonances seen experimentally. On the other hand, self-consistent calculations consistently predict shallow third minima. Purpose: We study the isentropic potential energy surfaces of selected even-even thorium and uranium isotopes at several excitation energies. In order to understand the driving effects behind the presence of third minima, we study the interplay between pairing and shell effects. We also investigate the interpretation of third minima in terms of di-molecular (cluster) configurations. Methods: We use the finite-temperature superfluid nuclear density functional theory. We consider several Skyrme energy density functionals, including traditional functionals such as SkM* and a new functional UN- EDF1 recently optimized for fission studies. Results: The potential energy surfaces for 228,232Th and 232U at several excitation energies are presented. We also present isotopic chains to demonstrate the evolution of the depth of the third minimum with neutron number. Conclusions: We demonstrate that the depth of the third minimum is sensitive to the excitation energy of the nucleus. In particular, the thermal reduction of pairing, and related enhancement of shell effects, at small excitation energies help to develop deeper third minima. At large excitation energies, shell effects are washed out and third minima disappear altogether. According to our calculations, third minima become more pronounced in neutron-deficient isotopes such as 226,228Th and 228,230U.
View original:
http://arxiv.org/abs/1302.1165
No comments:
Post a Comment