942 research outputs found
Continuum effects in reactions involving weakly bound nuclei
The relevance of the continuum effects in transfer reactions is discussed in
conection with the determination of observables of astrophysical interest. In
particular, we examine the validity of the Distorted Wave Born Approximation
(DWBA) as a tool to extract the astrophysical \emph{(S_{17}(0))} factor. For
this purpose, we present calculations for the reaction (^{14})N((^{7}
)Be,(^{8})B)(^{13})C comparing the DWBA method with the more sophysticated
CDCC-Born approximationComment: Contribution to International Symposium on Physics of Unstable Nuclei
(ISPUN02) Halong Bay (Vietnam) November 20 to 25, 2002. To be published in
Nucl. Phy.
Effect of continuum couplings in fusion of halo Be on Pb around the Coulomb barrier
The effect of continuum couplings in the fusion of the halo nucleus Be
on Pb around the Coulomb barrier is studied using a three-body model
within a coupled discretised continuum channels (CDCC) formalism. We
investigate in particular the role of continuum-continuum couplings. These are
found to hinder total, complete and incomplete fusion processes. Couplings to
the projectile bound excited state redistribute the complete and
incomplete fusion cross sections, but the total fusion cross section remains
nearly constant. Results show that continuum-continuum couplings enhance the
irreversibility of breakup and reduce the flux that penetrates the Coulomb
barrier. Converged total fusion cross sections agree with the experimental ones
for energies around the Coulomb barrier, but underestimate those for energies
well above the Coulomb barrier.Comment: 15 pages, 7 figures, accepted in Phys. Rev.
Alpha particle production by molecular single-particle effect in reactions of Be just above the Coulomb barrier
The -particle production in the dissociation of Be on
Bi and Zn at energies just above the Coulomb barrier is studied
within the two-center shell model approach. The dissociation of Be on
Bi is caused by a molecular single-particle effect (Landau-Zener
mechanism) before the nuclei reach the Coulomb barrier. Molecular
single-particle effects do not occur at that stage of the collision for
Be+Zn, and this explains the absence of fusion suppression
observed for this system. The polarisation of the energy level of the last
neutron of Be and, therefore the existence of avoided crossings with that
level, depends on the structure of the target.Comment: 5 pages, 4 figure
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
Astrophysical S-factor of the Be(B reaction from Coulomb dissociation of B
The Coulomb dissociation method to obtain the astrophysical S-factor,
, for the Be(B reaction at solar energies is
investigated by analysing the recently measured data on the breakup reaction
PbB,BePb at 46.5 MeV/A beam energy. Breakup cross
sections corresponding to E1, and transitions are calculated with a
theory of Coulomb excitation that includes the effects of the Coulomb recoil as
well as relativistic retardation. The interplay of nuclear and Coulomb
contributions to the breakup process is studied by performing a full quantum
mechanical calculation within the framework of the distorted-wave Born
Approximation. In the kinematical regime of the present experiment, both
nuclear as well as Coulomb-nuclear interference processes affect the pure
Coulomb breakup cross sections very marginally. The cross sections are
strongly dependent on the model used to describe the structure of B. The
value of is deduced with and without and contributions
added to the cross sections and the results are discussed.Comment: 10 pages, with 4 figures included with psfig; Physics Letters B, in
pres
XCDCC: Core Excitation in the Breakup of Exotic Nuclei
The eXtended Continuum Discretized Coupled Channel (XCDCC) method is
developed to treat reactions where core degrees of freedom play a role. The
projectile is treated as a multi-configuration coupled channels system
generated from a valence particle coupled to a deformed core which is allowed
to excite. The coupled channels initial state breaks up into a coupled channels
continuum which is discretized into bins, similarly to the original CDCC
method. Core collective degrees of freedom are also included in the interaction
of the core and the target, so that dynamical effects can occur during the
reaction. We present results for the breakup of C=C+n and
Be=Be+n on Be. Results show that the total cross section
increases with core deformation. More importantly, the relative percentage of
the various components of the initial state are modified during the reaction
process through dynamical effects. This implies that comparing spectroscopic
factors from structure calculations with experimental cross sections requires
more detailed reaction models that go beyond the single particle model.Comment: 14 pages, revtex, submitted to Phys Rev
Ground--state energies and widths of He and Li
We extract energies and widths of the ground states of He and Li from
recent single--level R--matrix fits to the spectra of the H)He and the He)Li reactions. The widths
obtained differ significantly from the formal R--matrix values but they are
close to those measured as full widths at half maxima of the spectra in various
experiments. The energies are somewhat lower than those given by usual
estimates of the peak positions. The extracted values are close to the
S--matrix poles calculated previously from the multi--term analyses of the
N-He elastic scattering data.Comment: 3 pages, no figures, uses revtex.sty, accepted for publication in
PRC, uuencoded postscript and tex-files available at
ftp://is1.kph.tuwien.ac.at/pub/ohu/fwidth.u
A simple and efficient numerical scheme to integrate non-local potentials
As nuclear wave functions have to obey the Pauli principle, potentials issued
from reaction theory or Hartree-Fock formalism using finite-range interactions
contain a non-local part. Written in coordinate space representation, the
Schrodinger equation becomes integro-differential, which is difficult to solve,
contrary to the case of local potentials, where it is an ordinary differential
equation. A simple and powerful method has been proposed several years ago,
with the trivially equivalent potential method, where non-local potential is
replaced by an equivalent local potential, which is state-dependent and has to
be determined iteratively. Its main disadvantage, however, is the appearance of
divergences in potentials if the wave functions have nodes, which is generally
the case. We will show that divergences can be removed by a slight modification
of the trivially equivalent potential method, leading to a very simple, stable
and precise numerical technique to deal with non-local potentials. Examples
will be provided with the calculation of the Hartree-Fock potential and
associated wave functions of 16O using the finite-range N3LO realistic
interaction.Comment: 8 pages, 2 figures, submitted to Eur. Phys. J.
Non-adiabatic corrections to elastic scattering of halo nuclei
We derive the formalism for the leading order corrections to the adiabatic
approximation to the scattering of composite projectiles. Assuming a two-body
projectile of core plus loosely-bound valence particle and a model (the core
recoil model) in which the interaction of the valence particle and the target
can be neglected, we derive the non-adiabatic correction terms both exactly,
using a partial wave analysis, and using the eikonal approximation. Along with
the expected energy dependence of the corrections, there is also a strong
dependence on the valence-to-core mass ratio and on the strength of the
imaginary potential for the core-target interaction, which relates to
absorption of the core in its scattering by the target. The strength and
diffuseness of the core-target potential also determine the size of the
corrections. The first order non-adiabatic corrections were found to be smaller
than qualitative estimates would expect. The large absorption associated with
the core-target interaction in such halo nuclei as Be11 kills off most of the
non-adiabatic corrections. We give an improved estimate for the range of
validity of the adiabatic approximation when the valence-target interaction is
neglected, which includes the effect of core absorption. Some consideration was
given to the validity of the eikonal approximation in our calculations.Comment: 14 pages with 10 figures, REVTeX4, AMS-LaTeX v2.13, submitted to
Phys. Rev.
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