821 research outputs found
Event generator to construct cross sections for the multiphonon excitation of a set of collective vibrational modes
The construction of differential cross sections as a function of excitation
energy for systems with a collection of low- and high-lying intrinsic
vibrational modes has been attempted in the past. A prescription is proposed
that simplifies the implementation of such calculation schemes with a
remarkable reduction in computational time.Comment: 6 pages, 3 figures, to be published in Phys. Rev.
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
Study of Giant Pairing Vibrations with neutron-rich nuclei
We investigate the possible signature of the presence of giant pairing states
at excitation energy of about 10 MeV via two-particle transfer reactions
induced by neutron-rich weakly-bound projectiles. Performing particle-particle
RPA calculations on Pb and BCS+RPA calculations on Sn, we
obtain the pairing strength distribution for two particles addition and removal
modes. Estimates of two-particle transfer cross sections can be obtained in the
framework of the 'macroscopic model'. The weak-binding nature of the projectile
kinematically favours transitions to high-lying states. In the case of (~^6He,
\~^4He) reaction we predict a population of the Giant Pairing Vibration with
cross sections of the order of a millibarn, dominating over the mismatched
transition to the ground state.Comment: Talk presented in occasion of the VII School-Semina r on Heavy Ion
Physics hosted by the Flerov Laboratory (FLNR/JINR) Dubna, Russia from May 27
to June 2, 200
Stochastic Semi-Classical Description of Fusion at Near-Barrier Energies
Fusion reactions of heavy ions are investigated by employing a simple
stochastic semi-classical model which includes the coupling between relative
motion and low frequency collective surface modes of colliding ions similarly
to the quantal coupled-channels description. The quantal effect enters into the
calculation through the initial zero-point fluctuations of the surface
vibrations. Good agreement with the result of coupled-channels calculations as
well as data is obtained for the fusion cross sections of nickel isotopes. The
internal excitations in non-fusing events as well as the fusion time are
investigated.Comment: 8 pages, 8 figures, Published in Phys. Rev.
Spinodal Instabilities in Nuclear Matter in a Stochastic Relativistic Mean-Field Approach
Spinodal instabilities and early growth of baryon density fluctuations in
symmetric nuclear matter are investigated in the basis of stochastic extension
of relativistic mean-field approach in the semi-classical approximation.
Calculations are compared with the results of non-relativistic calculations
based on Skyrme-type effective interactions under similar conditions. A
qualitative difference appears in the unstable response of the system: the
system exhibits most unstable behavior at higher baryon densities around
in the relativistic approach while most unstable
behavior occurs at lower baryon densities around in
the non-relativistic calculationsComment: 18 pages, 7 figure
Fusion barrier distributions in systems with finite excitation energy
Eigen-channel approach to heavy-ion fusion reactions is exact only when the
excitation energy of the intrinsic motion is zero. In order to take into
account effects of finite excitation energy, we introduce an energy dependence
to weight factors in the eigen-channel approximation. Using two channel
problem, we show that the weight factors are slowly changing functions of
incident energy. This suggests that the concept of the fusion barrier
distribution still holds to a good approximation even when the excitation
energy of the intrinsic motion is finite. A transition to the adiabatic
tunneling, where the coupling leads to a static potential renormalization, is
also discussed.Comment: 9 pages, 4 figures, Submitted to Physical Review
On nucleon exchange mechanism in heavy-ion collisions at near-barrier energies
Nucleon drift and diffusion mechanisms in central collisions of asymmetric
heavy-ions at near-barrier energies are investigated in the framework of a
stochastic mean-field approach. Expressions for diffusion and drift
coefficients for nucleon transfer deduced from the stochastic mean-field
approach in the semiclassical approximation have similar forms familiar from
the phenomenological nucleon exchange model. The variance of fragment mass
distribution agrees with the empirical formula . The comparison with the time-dependent Hartree-Fock calculations
shows that, below barrier energies, the drift coefficient in the semiclassical
approximation underestimates the mean number of nucleon transfer obtained in
the quantal framework. Motion of the window in the dinuclear system has a
significant effect on the nucleon transfer in asymmetric collisions.Comment: 10 pages, 10 figures, submitted for publicatio
Fluctuation and dissipation dynamics in fusion reactions from stochastic mean-field approach
By projecting the stochastic mean-field dynamics on a suitable collective
path during the entrance channel of heavy-ion collisions, expressions for
transport coefficients associated with relative distance are extracted. These
transport coefficients, which have similar forms to those familiar from nucleon
exchange model, are evaluated by carrying out TDHF simulations. The
calculations provide an accurate description of the magnitude and form factor
of transport coefficients associated with one-body dissipation and fluctuation
mechanism.Comment: 9 pages, 5 figure
Evidence of Double Phonon Excitations in ^{16}O + ^{208}Pb Reaction
The fusion cross-sections for ^{16}O + ^{208}Pb, measured to high precision,
enable the extraction of the distribution of fusion barriers. This shows a
structure markedly different from the single-barrier which might be expected
for fusion of two doubly-closed shell nuclei. The results of exact coupled
channel calculations performed to understand the observations are presented.
These calculations indicate that coupling to a double octupole phonon excited
state in ^{208}Pb is necessary to explain the experimental barrier
distributions.Comment: 6 pages, 2 figures, To be published in the Proceedings of the FUSION
97 Conference, South Durras, Australia, March 1997 (J. Phys. G
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