153 research outputs found
Adiabatic fission barriers in superheavy nuclei
Using the microscopic-macroscopic model based on the deformed Woods-Saxon
single-particle potential and the Yukawa-plus-exponential macroscopic energy we
calculated static fission barriers for 1305 heavy and superheavy nuclei
, including even - even, odd - even, even - odd and odd -
odd systems. For odd and odd-odd nuclei, adiabatic potential energy surfaces
were calculated by a minimization over configurations with one blocked neutron
or/and proton on a level from the 10-th below to the 10-th above the Fermi
level. The parameters of the model that have been fixed previously by a fit to
masses of even-even heavy nuclei were kept unchanged. A search for saddle
points has been performed by the "Imaginary Water Flow" method on a basic
five-dimensional deformation grid, including triaxiality. Two auxiliary grids
were used for checking the effects of the mass asymmetry and hexadecapole
non-axiallity. The ground states were found by energy minimization over
configurations and deformations. We find that the non-axiallity significantly
changes first and second fission barrier in many nuclei. The effect of the mass
- asymmetry, known to lower the second, very deformed barriers in actinides, in
the heaviest nuclei appears at the less deformed saddles in more than 100
nuclei. It happens for those saddles in which the triaxiallity does not play
any role, what suggests a decoupling between effects of the mass-asymmetry and
triaxiality. We studied also the influence of the pairing interaction strength
on the staggering of for odd- and even-particle numbers. Finally, we
provide a comparison of our results with other theoretical fission barrier
evaluations and with available experimental estimates.Comment: submitted to PR
Candidates for Long Lived High-K Ground States in Superheavy Nuclei
On the basis of systematic calculations for 1364 heavy and superheavy nuclei,
including odd-systems, we have found a few candidates for high-K ground states
in superheavy nuclei. The macroscopic-microscopic model based on the deformed
Woods-Saxon single particle potential which we use offers a reasonable
description of SH systems, including known: nuclear masses,
-values, fission barriers, ground state deformations, super- and
hyper-deformed minima in the heaviest nuclei. %For odd and odd-odd systems,
both ways of including pairing correlations, % blocking and the quasi-particle
method, have been applied. Exceptionally untypical high-K intruder contents of
the g.s. found for some nuclei accompanied by a sizable excitation of the
parent configuration in daughter suggest a dramatic hindrance of the
-decay. Multidimensional hyper-cube configuration - constrained
calculations of the Potential Energy Surfaces (PES's) for one especially
promising candidate, Mt, shows a 6 MeV increase in the
fission barrier above the configuration- unconstrained barrier. There is a
possibility, that one such high-K ground- or low-lying state may be the longest
lived superheavy isotope.Comment: Accepted in PR
Ground State and Saddle Point: masses and deformations for even-even superheavy nuclei with 98 < Z < 126 and 134< N < 192
We determine ground-state and saddle-point shapes and masses of even-even
superheavy nuclei in the range of proton numbers and
neutron numbers . Our study is performed within the
microscopic-macroscopic method. The Strutinsky shell and pairing correction is
calculated for the deformed Woods-Saxon single-particle potential and the
Yukawa-plus-exponential energy is taken as a smooth part. We use parameters of
the model that were fitted previously to this region of nuclei. A
high-dimensional deformation space, including nonaxial and
reflection-asymmetric shapes, is used in the search for saddle points. Both
ground-state and saddle-point shapes are found with the aid of the minimization
procedure, with dynamical programming technique of search for saddle points.
The results are collected in two tables. Calculated ground-state mass-excess,
Q_{\alpha energies, total and macroscopic energies normalized to the
macroscopic energy at the spherical shape, shell corrections (including
pairing) and deformations are given for each nucleus in the table one. The
second table gives the same properties, but at the saddle-point configuration.
The obtained results are discussed and compared with available experimental
data for alpha-decay energies () and ground-state masses.Comment: 35 pages, 9 figures, 2 tables, submitted to ADND
Superdeformed Oblate Superheavy Nuclei?
We study stability of superdeformed oblate (SDO) superheavy
nuclei predicted by systematic macroscopic-microscopic calculations in 12D
deformation space and confirmed by the Hartree-Fock calculations with the
realistic SLy6 force. We include into consideration high- isomers that very
likely form at the SDO shape. Although half-lives s
are calclulated or estimated for even-even spin zero systems, decay hindrances
known for high- isomers suggest that some SDO superheavy nuclei may be
detectable by the present experimental technique.Comment: 4 pages, 5 figure
Sizes and shapes of very heavy nuclei in high-K states
We have investigated shapes and sizes of selected two- and four-quasiparticle
\mbox{high-} states in nobelium and rutherfordium isotopes within the
microscopic-macroscopic model with the deformed Woods-Saxon potential. Excited
nuclear configurations were obtained by blocking single-particle states lying
close to the Fermi level. Their energies and deformations were found by the
four-dimensional energy minimization over shape variables. We have selected the
most promising candidates for \mbox{-isomers} by analyzing the isotopic
dependence of excitation energies, and compared our results to available
experimental data. We calculated differences in quadrupole moments and charge
radii between nuclei in their \mbox{high-} and ground states and found their
quite different pattern for four-quasiparticle states in neighboring No and Rf
isotopes. The leading role of the quadrupole and hexadecapole deformations as
well as the importance of higher rank symmetries are also discussed. The
current development of laser techniques and the resulting ability to measure
discussed effects in the near future is the motivation of our study
8Dim calculations of the third barrier in Th and a conflict between theory and experiment on uranium nuclei
We find the height of the third fission barrier and energy of the
third minimum in Th using the macroscopic - microscopic
model, very well tested in this region of nuclei. For the first time it is done
on an 8-dimensional deformation hypercube. The dipole distortion is included
among the shape variables to assure that no important shapes are missed. The
saddle point is found on a lattice containing more than 50 million points by
the immersion water flow (IWF) method. The shallow third minimum,
MeV, agrees with experimetal data of Blons et al.
This is in a sharp contrast with the status of the IIIrd minima in
U: their experimental depth of MeV contradicts all
realistic theoretical predictions. We emphasize the importance of repeating the
experiment on Th, by a technique similar to that used in the uranium
nuclei, for settling the puzzle of the third minima in actinides
Static fission properties of actinide nuclei
Fission barriers heights and excitation energies of superdeformed isomeric
minima are calculated within the microscopic - macroscopic Woods - Saxon model
for 75 actinide nuclei for which the experimental data are known.
State - of - the - art methods were used: minimization over many deformation
parameters for minima and the imaginary water flow on many - deformation energy
grid for saddles, including nonaxial and reflection-asymmetric shapes.
We obtain 0.82 - 0.94 MeV rms deviation between the calculated and
experimental barriers and 0.53 MeV rms error in the excitation of superdeformed
minima (SD).
Experimental vs theory discrepancies seem to be of various nature and not
easy to eliminate, especially if one cares for more than one or two
observables. As an example, we show that by strengthening pairing in odd
systems one can partially improve agreement in barriers, while spoiling it for
masses. We also discuss the "thorium anomaly" and suggest its possible relation
to a different way in which the Ac and Th barriers are derived from
experimental data.Comment: Submitted to PR
Properties of heaviest nuclei with and
We systematically determine ground-state and saddle-point shapes and masses
for 1305 heavy and superheavy nuclei with and , including
odd- and odd-odd systems. From these, we derive static fission barrier
heights, one- and two-nucleon separation energies, and values for
g.s. to g.s transitions. Our study is performed within the
microscopic-macroscopic method with the deformed Woods-Saxon single-particle
potential and the Yukawa-plus-exponential macroscopic energy taken as the
smooth part. We use parameters of the model that were fitted previously to
masses of even-even heavy nuclei. For systems with odd numbers of protons,
neutrons, or both, we use a standard BCS method with blocking. Ground-state
shapes and energies are found by the minimization over seven axially-symmetric
deformations. A search for saddle-points was performed by using the "imaginary
water flow" method in three consecutive stages, using five- (for nonaxial
shapes) and seven-dimensional (for reflection-asymmetric shapes) deformation
spaces. The results are collected in two main tables. Calculated ground-state
mass excess, nucleon separation- and energies, total,
macroscopic(normalized to the macroscopic energy at the spherical shape) and
shell corrections energies, and deformations are given for each nucleus in
\mbox{Table 1}. \mbox{Table 2} contains calculated properties of the
saddle-point configurations and the fission barrier heights. In \mbox{Tables
3-7}, are given calculated ground-state, inner and outer saddle-point and
superdeformed secondary minima characteristics for 75 actinide nuclei, from Ac
to Cf, for which experimental estimates of fission barrier heights are known.
These results are an additional test of our model.Comment: Submitted to ADNDT. arXiv admin note: text overlap with
arXiv:1203.501
Hindered alpha decays of heaviest high-K isomers
To find candidates for long-lived high-K isomers in even-even Z=106-112
superheavy nuclei we study dominant alpha-decay channel of two- and
four-quasi-particle configurations at a low excitation. Energies are calculated
within the microscopic - macroscopic approach with the deformed Woods-Saxon
potential. Configurations are fixed by a standard blocking procedure and their
energy found by a subsequent minimization over deformations. Different
excitation energies of a high-K configuration in parent and daughter nucleus
seem particularly important for a hindrance of the alpha-decay. A strong
hindrance is found for some four-quasi-particle states, particularly and/or states in Ds. Contrary to what was
suggested in experimental papers, it is rather a proton configuration that
leads to this strong hindrance. If not shortened by the electromagnetic decay,
alpha half-lives of 1 s could open new possibilities for studies of
chemical/atomic properties of related elements
Level-density parameters in superheavy nuclei
We systematically study the nuclear level densities of superheavy nuclei,
including odd systems, using the single-particle energies obtained with the
Woods-Saxon potential diagonalization. Minimization over many deformation
parameters for the global minima - ground states and the "imaginary water flow"
technique on many deformation energy grids for the saddle points, including
nonaxial shapes has been applied. The level density parameters are calculated
by fitting the obtained results with the standard Fermi gas expression. The
total potential energy and shell correction dependencies of the level-density
parameter are analyzed and compared at the ground state and saddle point. These
parameters are compared with the results of the phenomenological expression. As
shown, this expression should be modified for the saddle points, especially for
small excitation energy. The ratio of the level-density parameter at the saddle
point to that at the ground state is shown to be crucial for the survival
probability of the heavy nucleus.Comment: submitted to PR
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