Fission barriers and asymmetric ground states in the relativistic mean-field theory

Abstract

The symmetric and asymmetric fission path for 240 Pu, 232 Th, and 226 Ra is investigated within the relativistic mean-field model. Standard parametrizations which are well fitted to nuclear ground state properties are found to deliver reasonable qualitative and quantitative features of fission, comparable to similar nonrelativstic calculations. Furthermore, stable octupole deformations in the ground states of Radium isotopes are investigated. They are found in a series of isotopes, qualitatively in agreement with nonrelativistic models. But the quantitative details differ amongst the models and between the various relativsitic parametrizations. I. INTRODUCTIONNuclear fission has been the challenge to develop microscopic theories of nuclear collective motion. Only a subtle interplay of collective deformations and changing microscopic 1 shell structure can explain the quantitative details like tunneling times and fragment mass distributions [1]. One of the most decisive features to pin down the contributions from the shell effects is the occurence of multiple-humped barriers [2]. The macroscopic-microscopic method, based on a phenomenologically fitted shell model plus Strutinsky shell corrections [3], allowed quite early extensive investigations of the various landscapes of collective Potential Energy Surfaces (PES), see e. g. [1,[4][5][6][7]. Since the early seventies, fully selfconsistent mean-field models have been available for nuclei. These are the nonrelativistic Hartree-Fock models with the Skyrme force [8,9] or the Gogny force [10]. At about the same time competitive relativistic mean-field models for nuclear structure were proposed [11,12]. Although much more elaborate, selfconsistent mean-field calculations of the PES for nuclear fission appeared rather soon for the Skyrme-Hartree-Fock models [13] and later for the Gogny force [14], it was only recently that PES for fission have been calculated with the relativistic mean-field model [15]. These calculations have shown that well adjusted parametrizations for the relativistic mean-field model, see e. g. [16,17], deliver reasonable fission barriers comparable to those of nonrelativistic calculations. A variation of the parametrization, in particular with respect to the effective mass, has shown that the requirement to reproduce reasonable fission barriers excludes many variants and only the standard best-fit forces with the typically very low effective nucleon mass remain. It is the aim of this paper to extend the investigations of [15] to asymmetric fission and to study a larger variety of actinides.We confine the consderations to the two parametrizations NL1 from [16] and PL-40 from [18] which have proven to provide reasonable fission barriers [15]. In addition, we consider the parametrization NL-SH which is claimed to be better adjusted with respect to isovector properties [19]. As a second objective of this paper, we exploit the asymmetric degree of freedom in our calculations to investigate the possibility of stable octupole deformations in the ground s...

Nuclear energy density optimization: Large deformations

“…[11][12][13][14]), and relativistic (see, e.g., Refs. [5,15,16]) energy density functionals (EDFs). The fundamental assumption of the nuclear DFT is that one can describe accurately a broad range of phenomena in nuclei, including excited states and large-amplitude collective motion, by enriching the density dependence of the functional while staying at the single-reference Hartree-Fock-Bogoliubov (HFB) level.…”

Fission barriers from multidimensionally-constrained covariant density functional theories

“…It has been known from the macroscopicmicroscopic model calculations that the inner fission barrier is lowered when the triaxial deformation is allowed, so is the outer one by the reflection asymmetric distortion [30,31]. Such lowering effects were also revealed in the non-relativistic [35] and relativistic [38,39] density functional calculations, respectively. Recently, it is shown in Ref.…”

Large-amplitudeQn-Qpcollectivity in the neutron-rich oxygen isotopeSeveryukhin1, Bender2, Flocard3 et al. 2007Phys. Rev. C3020View full textAdd to dashboardCite

“…We achieve this by multiplying the constraining operator with a Fermi function as proposed in Ref. [36]. We first define an equidensity surface on which the nucleon density ρ is equal to one tenth of the maximal density ρ max .…”

“…The main role is played by the cutoff distance δr c which we have set to 5.5 fm, slightly larger than the recommended value of Ref. [36], while the width parameter was set to a = 0.4 fm.…”