594 research outputs found
Sub-barrier fusion of drip-line nuclei
We discuss the role of break-up process of a loosely-bound projectile in
subbarrier fusion reactions. Coupled-channels calculations are carried out for
Be + Pb and He + U reactions by discretizing in
energy the particle continuum states. Our calculations show that the coupling
to the break-up channel has two effects, namely the loss of flux and the
dynamical modulation of fusion potential. Their net effects differ depending on
the energy region. At energies above the Coulomb barrier, the former effect
dominates over the latter and cross sections for complete fusion are hindered
compared with the no coupling case. On the other hand, at below the barrier,
the latter effect is much larger than the former and complete fusion cross
sections are enhanced consequently.Comment: 12 pages, 3 eps figures, uses psfig.sty and sprocl.sty. To be
published in the Proceedings of the international workshop on "Fusion
dynamics at the extremes", Dubna, Russia, May 2000 (World Scientific
Publishing Co.
Reaction dynamics for fusion of weakly-bound nuclei
We discuss several open problems of fusion reactions induced by weakly bound
nuclei. For this purpose, we solve a one dimensional three-body Hamiltonian
with the coupled-channels formalism. We show that the continuum-continuum
couplings substantially reduce the total fusion probability at energies above
the barrier compared with the no-breakup case, although the fusion probability
remains enhanced at subbarrier energies. We then discuss a role of transfer
process in fusion of weakly bound nuclei, and point out that removing spurious
Pauli forbidden transfer components from the calculation may be crucial at
energies below the barrier. Calculations based on the three-body classical
trajectory Monte Carlo (CTMC) method are also presented in order to discuss how
to model complete fusion process.Comment: 8 pages, 3 eps figures. Uses ptptex.sty. A talk given at the
International conference FUSION03, November 12 - 15, 2003, Matsushima,
Miyagi, Japan. To be published in Prog. Theo. Phys. Supp
Nuclear fusion as a probe for octupole deformation in Ra
: Nuclear fusion has been shown to be a perfect probe to
study the different nuclear shapes. However, the possibility of testing
octupole deformation of a nucleus with this tool has not been fully explored
yet. The presence of a stactic octupole deformation in nuclei will enhanced a
possible permanent electric dipole moment, leading to a possible demonstration
of parity violation.
: To check whether static octupole deformation or octupole
vibration in fusion give qualitatively different results so that both
situations can be experimentally disentangled.
: Fusion cross sections are computed in the Coupled-Channels
formalism making use of the Ingoing-Wave Boundary Conditions (IWBC) for the
systems O+Ba and O+Ra.
: Barrier distributions of the two considered schemes show
different patterns. For the Ra case, the octupole deformation parameter
is large enough to create a sizeable difference.
: The measurement of barrier distributions can be an
excellent probe to clarify the presence of octupole deformation.Comment: Important changes from previous version, 6 pages, 5 figures, 2
tables, submitted to Phys. Rev.
Electromagnetic selection rules in the triangular alpha-cluster model of 12C
After recapitulating the procedure to find the bands and the states occurring
in the alpha-cluster model of C in which the clusters
are placed at the vertexes of an equilateral triangle, we obtain the selection
rules for electromagnetic transitions. While the alpha cluster structure leads
to the cancellation of E1 transitions, the approximations carried out in
deriving the roto-vibrational hamiltonian lead to the disappearance of M1
transitions. Furthermore, although in general the lowest active modes are E2,
E3, and M2, M3, , the cancellation of M2, M3 and M5
transitions between certain bands also occurs, as a result of the application
of group theoretical techniques drawn from molecular physics. These
implications can be very relevant for the spectroscopic analysis of
-ray spectra of C
Electric and magnetic response to the continuum for A=7 isobars in a dicluster model
Mirror isobars Li and Be are investigated in a dicluster model. The
magnetic dipole moments and the magnetic dipole response to the continuum are
calculated in this framework. The magnetic contribution is found to be small
with respect to electric dipole and quadrupole excitations even at
astrophysical energies, at a variance with the case of deuteron. Energy
weighted molecular sum rules are evaluated and a formula for the molecular
magnetic dipole sum rule is found which matches the numerical calculations.
Cross-sections for photo-dissociation and radiative capture as well as the
S-factor for reactions of astrophysical significance are calculated with good
agreement with known experimental data.Comment: Accepted in EPJ
The electron screening puzzle and nuclear clustering
Accurate measurements of nuclear reactions of astrophysical interest within,
or close to, the Gamow peak, show evidence of an unexpected effect attributed
to the presence of atomic electrons in the target. The experiments need to
include an effective "screening" potential to explain the enhancement of the
cross sections at the lowest measurable energies. Despite various theoretical
studies conducted over the past 20 years and numerous experimental
measurements, a theory has not yet been found that can explain the cause of the
exceedingly high values of the screening potential needed to explain the data.
In this letter we show that instead of an atomic physics solution of the
"electron screening puzzle", the reason for the large screening potential
values is in fact due to clusterization effects in nuclear reactions, in
particular for reaction involving light nuclei.Comment: 6 pages, 2 figures, accepted for publication in Physics Letters
Pairing in the continuum: the quadrupole response of the Borromean nucleus 6He
The ground state and low-lying continuum states of 6He are found within a
shell model scheme, in a basis of two-particle states built out of continuum
p-states of the unbound 5He nucleus, using a simple pairing contact-delta
interaction. This accounts for the Borromean character of the bound ground
state, revealing its composition. We investigate the quadrupole response of the
system and we put our calculations into perspective with the latest
experimental results. The calculated quadrupole strength distribution
reproduces the narrow 2+ resonance, while a second wider peak is found at about
3.9 MeV above the g.s. energy.Comment: 5 pages, 5 figure
Electric multipole response of the halo nucleus He
The role of different continuum components in the weakly-bound nucleus He
is studied by coupling unbound spd-waves of He by means of simple pairing
contact-delta interaction. The results of our previous investigations in a
model space containing only p-waves, showed the collective nature of the ground
state and allowed the calculation of the electric quadrupole transitions. We
extend this simple model by including also sd-continuum neutron states and we
investigate the electric monopole, dipole and octupole response of the system
for transitions to the continuum, discussing the contribution of different
configurations.Comment: 22 pages, 10 figure
Pair-transfer probability in open- and closed-shell Sn isotopes
Approximations made to estimate two-nucleon transfer probabilities in
ground-state to ground-state transitions and physical interpretation of these
probabilities are discussed. Probabilities are often calculated by
approximating both ground states, of the initial nucleus A and of the final
nucleus A\pm 2 by the same quasiparticle vacuum. We analyze two improvements of
this approach. First, the effect of using two different ground states with
average numbers of particles A and A\pm2 is quantified. Second, by using
projection techniques, the role of particle number restoration is analyzed. Our
analysis shows that the improved treatment plays a role close to magicity,
leading to an enhancement of the pair-transfer probability. In mid-shell
regions, part of the error made by approximating the initial and final ground
states by a single vacuum is compensated by projecting onto good particle
number. Surface effects are analyzed by using pairing interactions with a
different volume-to-surface mixing. Finally, a simple expression of the
pair-transfer probability is given in terms of occupation probabilities in the
canonical basis. We show that, in the canonical basis formulation, surface
effects which are visible in the transfer probability are related to the
fragmentation of single-particle occupancies close to the Fermi energy. This
provides a complementary interpretation with respect to the standard
quasiparticle representation where surface effects are generated by the
integrated radial profiles of the contributing wave functions.Comment: 12 pages, 7 figure
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