2,557 research outputs found
A new approach to barrier-top fission dynamics
We proposed a calculational framework for describing induced fission that
avoids the Bohr-Wheeler assumption of well-defined fission channels. The
building blocks of our approach are configurations that form a discrete,
orthogonal basis and can be characterized by both energy and shape. The
dynamics is to be determined by interaction matrix elements between the states
rather than by a Hill-Wheeler construction of a collective coordinate. Within
our approach, several simple limits can be seen: diffusion; quantized
conductance; and ordinary decay through channels. The specific proposal for the
discrete basis is to use the quantum numbers of the axially symmetric
Hartree-Fock approximation to generate the configurations. Fission paths would
be determined by hopping from configuration to configuration via the residual
interaction. We show as an example the configurations needed to describe a
fictitious fission decay . We also examine the geometry of the path for fission of U,
measuring distances by the number of jumps needed to go to a new
partition.Comment: Write-up of a talk given at the Workshop "Compound-nuclear reactions
2015" Tokyo, Oct. 19-23, 2015; 11 pages and 11 figures. To be published in
European Journal of Physics, Web of Conference
Pairing gaps in Hartree-Fock Bogoliubov theory with the Gogny D1S interaction
As part of a program to study odd-A nuclei in the Hartree-Fock-Bogoliubov
(HFB) theory, we have developed a new calculational tool to find the HFB minima
of odd-A nuclei based on the gradient method and using interactions of Gogny's
form. The HFB minimization includes both time-even and time-odd fields in the
energy functional, avoiding the commonly used "filling approximation". Here we
apply the method to calculate neutron pairing gaps in some representative
isotope chains of spherical and deformed nuclei, namely the Z=8,50 and 82
spherical chains and the Z=62 and 92 deformed chains. We find that the gradient
method is quite robust, permitting us to carry out systematic surveys involving
many nuclei. We find that the time-odd field does not have large effect on the
pairing gaps calculated with the Gogny D1S interaction. Typically, adding the
T-odd field as a perturbation increases the pairing gap by ~100 keV, but the
re-minimization brings the gap back down. This outcome is very similar to
results reported for the Skyrme family of nuclear energy density functionals.
Comparing the calculated gaps with the experimental ones, we find that the
theoretical errors have both signs implying that the D1S interaction has a
reasonable overall strength. However, we find some systematic deficiencies
comparing spherical and deformed chains and comparing the lighter chains with
the heavier ones. The gaps for heavy spherical nuclei are too high, while those
for deformed nuclei tend to be too low. The calculated gaps of spherical nuclei
show hardly any A-dependence, contrary to the data. Inclusion of the T-odd
component of the interaction does not change these qualitative findings
Unitary Fermi Gas in a Harmonic Trap
We present an {\it ab initio} calculation of small numbers of trapped,
strongly interacting fermions using the Green's Function Monte Carlo method
(GFMC). The ground state energy, density profile and pairing gap are calculated
for particle numbers using the parameter-free "unitary"
interaction. Trial wave functions are taken of the form of correlated pairs in
a harmonic oscillator basis. We find that the lowest energies are obtained with
a minimum explicit pair correlation beyond that needed to exploit the
degeneracy of oscillator states. We find that energies can be well fitted by
the expression where is the
Thomas-Fermi energy of a noninteracting gas in the trap and is a
pairing gap. There is no evidence of a shell correction energy in the
systematics, but the density distributions show pronounced shell effects. We
find the value for the pairing gap. This is smaller
than the value found for the uniform gas at a density corresponding to the
central density of the trapped gas.Comment: 2 figures, 2 table
How harmonic is dipole resonance of metal clusters?
We discuss the degree of anharmonicity of dipole plasmon resonances in metal
clusters. We employ the time-dependent variational principle and show that the
relative shift of the second phonon scales as in energy, being
the number of particles. This scaling property coincides with that for nuclear
giant resonances. Contrary to the previous study based on the boson-expansion
method, the deviation from the harmonic limit is found to be almost negligible
for Na clusters, the result being consistent with the recent experimental
observation.Comment: RevTex, 8 page
Application of the gradient method to Hartree-Fock-Bogoliubov theory
A computer code is presented for solving the equations of
Hartree-Fock-Bogoliubov (HFB) theory by the gradient method, motivated by the
need for efficient and robust codes to calculate the configurations required by
extensions of HFB such as the generator coordinate method. The code is
organized with a separation between the parts that are specific to the details
of the Hamiltonian and the parts that are generic to the gradient method. This
permits total flexibility in choosing the symmetries to be imposed on the HFB
solutions. The code solves for both even and odd particle number ground states,
the choice determined by the input data stream. Application is made to the
nuclei in the -shell using the USDB shell-model Hamiltonian.Comment: 20 pages, 5 figures, 3 table
Bose-condensation through resonance decay
We show that a system described by an equation of state which contains a high
number of degrees of freedom (resonances) can create a considerable amount of
superfluid (condensed) pions through the decay of short-lived resonances, if
baryon number and entropy are large and the dense matter decouples from
chemical equilibrium earlier than from thermal equilibrium. The system cools
down faster in the presence of a condensate, an effect that may partially
compensate the enhancement of the lifetime expected in the case of
quark-gluon-plasma formation.Comment: 12 pages GSI-93-27 PREPRIN
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