345 research outputs found
Systematics of heavy-ion fusion hindrance at extreme sub-barrier energies
The recent discovery of hindrance in heavy-ion induced fusion reactions at
extreme sub-barrier energies represents a challenge for theoretical models.
Previously, it has been shown that in medium-heavy systems, the onset of fusion
hindrance depends strongly on the "stiffness" of the nuclei in the entrance
channel. In this work, we explore its dependence on the total mass and the
-value of the fusing systems and find that the fusion hindrance depends in a
systematic way on the entrance channel properties over a wide range of systems.Comment: Submitted to Phys. Rev. Lett., 5 pages, 3 figure
3-D unrestricted TDHF fusion calculations using the full Skyrme interaction
We present a study of fusion cross sections using a new generation
Time-Dependent Hartree-Fock (TDHF) code which contains no approximations
regarding collision geometry and uses the full Skyrme interaction, including
all of the time-odd terms. In addition, the code uses the Basis-Spline
collocation method for improved numerical accuracy. A comparative study of
fusion cross sections for is made with the older TDHF
results and experiments. We present results using the modern Skyrme forces and
discuss the influence of the new terms present in the interaction.Comment: 7 pages, 10 figure
Upper Limit on the molecular resonance strengths in the C+C fusion reaction
Carbon burning is a crucial process for a number of important astrophysical
scenarios. The lowest measured energy is around E=2.1 MeV, only
partially overlapping with the energy range of astrophysical interest. The
currently adopted reaction rates are based on an extrapolation which is highly
uncertain because of potential resonances existing in the unmeasured energy
range and the complication of the effective nuclear potential. By comparing the
cross sections of the three carbon isotope fusion reactions,
C+C, C+C and C+C, we have
established an upper limit on the molecular resonance strengths in
C+C fusion reaction. The preliminary results are presented
and the impact on nuclear astrophysics is discussed.Comment: 4 pages, 3 figures, FUSION11 conference proceedin
Shell Model Description of Isotope Shifts in Calcium
Isotope shifts in the nuclear charge radius of even and odd calcium isotopes
are calculated within the nuclear shell model. The model space includes all
configurations of nucleons in the
orbits. The shell model describes well the energies of the intruder states in
Sc and Ca, as well as the energies of the low-lying and states in
the even Ca isotopes. The characteristic features of the isotope shifts, the
parabolic dependence on and the prominent odd-even staggering, are well
reproduced by the model. These features are related to the partial breakdown of
the shell closure caused by promotion, due to the neutron-proton
interaction, of the shell protons into the shell.Comment: 4 pages, 4 figures include
Coupled-channels analysis of the O+Pb fusion barrier distribution
Analyses using simplified coupled-channels models have been unable to
describe the shape of the previously measured fusion barrier distribution for
the doubly magic O+Pb system. This problem was investigated by
re-measuring the fission excitation function for O+Pb with
improved accuracy and performing more exact coupled-channels calculations,
avoiding the constant-coupling and first-order coupling approximations often
used in simplified analyses. Couplings to the single- and 2-phonon states of
Pb, correctly taking into account the excitation energy and the phonon
character of these states, particle transfers, and the effects of varying the
diffuseness of the nuclear potential, were all explored. However, in contrast
to other recent analyses of precise fusion data, no satisfactory simultaneous
description of the shape of the experimental barrier distribution and the
fusion cross-sections for O+Pb was obtained.Comment: RevTex, 29 pages, 7 postscript figures, to appear in PR
Chaotic scattering on surfaces and collisional damping of collective modes
The damping of hot giant dipole resonances is investigated. The contribution
of surface scattering is compared with the contribution from interparticle
collisions. A unified response function is presented which includes surface
damping as well as collisional damping. The surface damping enters the response
via the Lyapunov exponent and the collisional damping via the relaxation time.
The former is calculated for different shape deformations of quadrupole and
octupole type. The surface as well as the collisional contribution each
reproduce almost the experimental value, therefore we propose a proper
weighting between both contributions related to their relative occurrence due
to collision frequencies between particles and of particles with the surface.
We find that for low and high temperatures the collisional contribution
dominates whereas the surface damping is dominant around the temperatures
of the centroid energy.Comment: PRC su
Elastic scattering and breakup of 17^F at 10 MeV/nucleon
Angular distributions of fluorine and oxygen produced from 170 MeV 17^F
incident on 208^Pb were measured. The elastic scattering data are in good
agreement with optical model calculations using a double-folding potential and
parameters similar to those obtained from 16^O+208^Pb. A large yield of oxygen
was observed near \theta_lab=36 deg. It is reproduced fairly well by a
calculation of the (17^F,16^O) breakup, which is dominated by one-proton
stripping reactions. The discrepancy between our previous coincidence
measurement and theoretical predictions was resolved by including core
absorption in the present calculation.Comment: 9 pages, 5 figure
Role of dynamical particle-vibration coupling in reconciliation of the puzzle for spherical proton emitters
It has been observed that decay rate for proton emission from
single particle state is systematically quenched compared with the prediction
of a one dimensional potential model although the same model successfully
accounts for measured decay rates from and states. We
reconcile this discrepancy by solving coupled-channels equations, taking into
account couplings between the proton motion and vibrational excitations of a
daughter nucleus. We apply the formalism to proton emitting nuclei
Re to show that there is a certain range of parameter set of the
excitation energy and the dynamical deformation parameter for the quadrupole
phonon excitation which reproduces simultaneously the experimental decay rates
from the 2, 3 and 1 states in these nuclei.Comment: RevTex, 12 pages, 4 eps figure
Role of break-up processes in fusion enhancement of drip-line nuclei at energies below the Coulomb barrier
We carry out realistic coupled-channels calculations for
Be + Pb reaction in order to discuss the effects of break-up
of the projectile nucleus on sub-barrier fusion.
We discretize in energy the particle continuum states, which are associated
with the break-up process, and construct the coupling form factors to these
states on a microscopic basis.
The incoming boundary condition is employed in solving coupled-channels
equations, which enables us to define the flux for complete fusion inside the
Coulomb barrier. It is shown that complete fusion cross sections are
significantly enhanced due to the couplings to the continuum states compared
with the no coupling case at energies below the Coulomb barrier, while they are
hindered at above barrier energies.Comment: RevTex, 3 pages, 5 figure
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