922 research outputs found
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.
Approximations in Fusion and Breakup reactions induced by Radioactive Beams
Some commonly used approximations for complete fusion and breakup
transmission coefficients in collisions of weakly bound projectiles at near
barrier energies are assessed. We show that they strongly depend on the adopted
classical trajectory and can be significantly improved with proper treatment of
the incident and emergent currents in the WKB approximation.Comment: 15 pages, 7 figure
Theoretical treatments of fusion processes in collisions of weakly bound nuclei
We review the theoretical methods to evaluate fusion cross sections in
collisions of weakly bound nuclei. We piont out that in such collisions the
coupling to the breakup channel leads to the appearance of different fusion
processes. The extentsion of the coupled-channel method to coupling with the
continuum is the most successful treatment for these collisions. However,
evaluating separate cross section for each fusion process remains a very hard
task.Comment: 8 pages, 3 figures. Invited talk (LFC) at the IX International
Conference on Nucleus-Nucleus Collisions, Rio de Janeiro, August 28-
September 1, 2006. Nuclear Physics A, in pres
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
Dynamic effective potential for α-particle bound and quasibound states
We exploit analytic properties of the optical potential for elastic scattering of α particles on nuclei to extract information on the effective interaction that should be used to describe the motion of a cluster of two neutrons and two protons bound to a nuclear system. This prescription solves long-standing ambiguities in the formalisms used for the study of α decay.EU Human Capital and Mobility program ERBCHRX-CT92-007CICYT PB92-066
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
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
Scaling and Interference in the Dissociation of Halo Nuclei
The dissociation of halo nuclei through their collision with light and heavy
targets is considered within the Continuum Discretized Coupled Channels theory.
We study the one-proton halo nucleus B and the one-neutron halo nucleus
Be, as well as the more normal Be. The procedure previously employed
to extract the Coulomb dissociation cross section by subtracting the nuclear
one is critically assessed, and the scaling law usually assumed for the target
mass dependence of the nuclear breakup cross section is also tested. It is
found that the nuclear breakup cross section for these very loosely bound
nuclei does indeed behave as . However, it does not have the
geometrically inspired form of a circular ring which seems to be the case for
normal nuclei such as Be. We find further that we cannot ignore
Coulomb-nuclear interference effects, which may be constructive or destructive
in nature, and so the errors in previously extracted B(E1) using the
subtraction procedure are almost certainly underestimated.Comment: version submitted to PRL + minor text change
Fusion versus Breakup: Observation of Large Fusion Suppression for ^9Be + ^{208}Pb
Complete fusion excitation functions for Be + Pb have been
measured to high precision at near barrier energies. The experimental fusion
barrier distribution extracted from these data allows reliable prediction of
the expected complete fusion cross-sections. However, the measured
cross-sections are only 68% of those predicted. The large cross-sections
observed for incomplete fusion products support the interpretation that this
suppression of fusion is caused by Be breaking up into charged fragments
before reaching the fusion barrier. Implications for the fusion of radioactive
nuclei are discussed.Comment: RevTex, 11 pages, 2 postscript figures, to appear in PR
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