299 research outputs found
Rate for Laser-Induced Nuclear Dipole Absorption
Using the Brink-Axel hypothesis we derive the rate for nuclear dipole
excitation by a laser pulse carrying photons with average energy
MeV. As expected . The
rate is also proportional to the aperure of the laser pulse. Perhaps
less expected is the fact that , irrespective of the degree of
coherence of the laser pulse. The expression for , derived for a nearly
stationary laser pulse, is valid also for short times and can, thus, be used in
simulations via rate equations of multiple nuclear dipole excitations by a
single pulse. The explicit dependence of on the parameters of the laser
pulse and on nuclear parameters given in the paper should help to optimize
experiments on laser-nucleus reactions.Comment: 12 pages, v2 slightly modified to match the published versio
Statistical correlations of nuclear quadrupole deformations and charge radii
Shape deformations and charge radii, basic properties of atomic nuclei, are
influenced by both the global features of the nuclear force and the nucleonic
shell structure. As functions of proton and neutron number, both quantities
show regular patterns and, for nuclei away from magic numbers, they change very
smoothly from nucleus to nucleus. In this paper, we explain how the local shell
effects are impacting the statistical correlations between quadrupole
deformations and charge radii in well-deformed even-even Er, Yb, and Hf
isotopes. This implies, in turn, that sudden changes in correlations can be
useful indicators of underlying shell effects. Our theoretical analysis is
performed in the framework of self-consistent mean-field theory using
quantified energy density functionals and density-dependent pairing forces. The
statistical analysis is carried out by means of the linear least-square
regression. The local variations of nuclear quadrupole deformations and charge
radii, explained in terms of occupations individual deformed Hartree-Fock
orbits, make and imprint on statistical correlations of computed observables.
While the calculated deformations or charge radii are, in some cases,
correlated with those of their even-even neighbors, the correlations seem to
deteriorate rapidly with particle number. The statistical correlations between
nuclear deformations and charge radii of different nuclei are affected by the
underlying shell structure. Even for well deformed and superfluid nuclei for
which these observables change smoothly, the correlation range usually does not
exceed and , i.e., it is rather short. This result
suggests that the frequently made assumption of reduced statistical errors for
the differences between smoothly-varying observables cannot be generally
justified.Comment: 7 pages, 6 figure
Stochastic TDHF in an exactly solvable model
We apply in a schematic model a theory beyond mean-field, namely Stochastic
Time-Dependent Hartree-Fock (STDHF), which includes dynamical electron-electron
collisions on top of an incoherent ensemble of mean-field states by occasional
2-particle-2-hole () jumps. The model considered here is inspired by a
Lipkin-Meshkov-Glick model of particles distributed into two bands of
energy and coupled by a two-body interaction. Such a model can be exactly
solved (numerically though) for small . It therefore allows a direct
comparison of STDHF and the exact propagation. The systematic impact of the
model parameters as the density of states, the excitation energy and the
bandwidth is presented and discussed. The time evolution of the STDHF compares
fairly well with the exact entropy, as soon as the excitation energy is
sufficiently large to allow transitions. Limitations concerning low
energy excitations and memory effects are also discussed.Comment: 23 pages, 8 figures, accepted in Annals of Physic
Shell structure of superheavy nuclei in self-consistent mean-field models
We study the extrapolation of nuclear shell structure to the region of superheavy nuclei in self-consistent mean-field modelsâthe Skyrme-Hartree-Fock approach and the relativistic mean-field modelâusing a large number of parametrizations which give similar results for stable nuclei but differ in detail. Results obtained with the folded-Yukawa potential which is widely used in macroscopic-macroscopic models are shown for comparison. We focus on differences in the isospin dependence of the spin-orbit interaction and the effective mass between the models and their influence on single-particle spectra. The predictive power of the mean-field models concerning single-particle spectra is discussed for the examples of 208Pb and the spin-orbit splittings of selected neutron and proton levels in 16O, 132Sn, and 208Pb. While all relativistic models give a reasonable description of spin-orbit splittings, all Skyrme interactions show a wrong trend with mass number. The spin-orbit splitting of heavy nuclei might be overestimated by 40%â80%, which exposes a fundamental deficiency of the current nonrelativistic models. In most cases the occurrence of spherical shell closures is found to be nucleon-number dependent. Spherical doubly magic superheavy nuclei are found at 184298114, 172292120, or 184310126 depending on the parametrization. The Z=114 proton shell closure, which is related to a large spin-orbit splitting of proton 2f states, is predicted only by forces which by far overestimate the proton spin-orbit splitting in 208Pb. The Z=120 and N=172 shell closures predicted by the relativistic models and some Skyrme interactions are found to be related to a central depression of the nuclear density distribution. This effect cannot appear in macroscopic-microscopic models or semiclassical approaches like the extended Thomas-Fermi-Strutinski integral approach which have a limited freedom for the density distribution only. In summary, our findings give a strong argument for 172292120 to be the next spherical doubly magic superheavy nucleus
Potential energy surfaces of superheavy nuclei
We investigate the structure of the potential energy surfaces of the superheavy nuclei 158258Fm100, 156264Hs108, 166278112, 184298114, and 172292120 within the framework of self-consistent nuclear models, i.e., the Skyrme-Hartree-Fock approach and the relativistic mean-field model. We compare results obtained with one representative parametrization of each model which is successful in describing superheavy nuclei. We find systematic changes as compared to the potential energy surfaces of heavy nuclei in the uranium region: there is no sufficiently stable fission isomer any more, the importance of triaxial configurations to lower the first barrier fades away, and asymmetric fission paths compete down to rather small deformation. Comparing the two models, it turns out that the relativistic mean-field model gives generally smaller fission barriers
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