9,195 research outputs found
Binding-energy independence of reduced spectroscopic strengths derived from (p, 2p) and (p, pn) reactions with nitrogen and oxygen isotopes
A campaign of intermediate energy (300-450 MeV/u) proton-induced nucleon
knockout measurements in inverse kinematics has been recently undertaken at the
R 3 B/LAND setup at GSI. We present a systematic theoretical analysis of these
data with the aim of studying the quenching of the single-particle strengths
and its binding-energy dependence. For that, the measured semi-inclusive (p,
2p) and (p, pn) cross sections are compared with theoretical predictions based
on single-particle cross sections derived from a novel coupled-channels
formalism and shell-model spectroscopic factors. A systematic reduction of
about 20-30% is found, with a very limited dependence on proton-neutron
asymmetry.Comment: 8 pages, 3 figure
Comprehensive analysis of large yields observed in Li induced reactions
Background: Large yields have been reported over the years in
reactions with Li and Li projectiles. Previous theoretical analyses
have shown that the elastic breakup (EBU) mechanism (i.e., projectile breakup
leaving the target in its ground state) is able to account only for a small
fraction of the total inclusive breakup cross sections, pointing
toward the dominance of non-elastic breakup (NEB) mechanisms.
Purpose: We aim to provide a systematic study of the inclusive cross
sections observed in nuclear reactions induced by Li projectiles. In
addition to estimating the total singles cross sections, it is our
goal to evaluate angular and energy distributions of these particles
and compare with experimental data, when available.
Method: We compute separately the EBU and NEB components of the inclusive
breakup cross sections. For the former, we use the continuum-discretized
coupled-channels (CDCC) method, which treats this mechanism to all orders. For
the NEB part, we employ the the model proposed in the eighties by Ichimura,
Austern and Vincent [Phys. Rev. C32, 432 (1982)], within the DWBA
approximation.
Results: Overall, the sum of the computed EBU and NEB cross sections is found
to reproduce very well the measured singles cross sections. In all cases
analyzed, we find that the inclusive breakup cross section is largely dominated
by the NEB component.
Conclusions: The presented method provides a global and systematic
description of inclusive breakup reactions induced by Li projectiles. It
provides also a natural explanation of the previously observed underestimation
of the measured yields by CDCC calculations. The method used here can
be extended to other weakly-bound projectiles, including halo nuclei.Comment: 11 pages, 16 figures, fig. 12 updated; some typos correcte
The puzzle of complete fusion suppression in weakly-bound nuclei: a Trojan Horse effect?
Experimental studies of nuclear collisions involving light weakly-bound
nuclei show a systematic suppression of the complete fusion cross section by
30\% with respect to the expectation for tightly bound nuclei, at
energies above the Coulomb barrier. Although it is widely accepted that the
phenomenon is related to the weak binding of these nuclei, the origin of this
suppression is not fully understood. In here, we present a novel approach that
provides the complete fusion for weakly bound nuclei and relates its
suppression to the competition between the different mechanisms contributing to
the reaction cross section. The method is applied to the Li+Bi
reactions, where we find that the suppression of complete fusion is mostly
caused by the flux associated with non-elastic breakup modes, such as the
partial capture of the projectile (incomplete fusion), whereas the elastic
breakup mode is found to play a minor role. Finally, we demonstrate that the
large yields observed in these reactions can be naturally explained as
a consequence of a {\it Trojan Horse} mechanism.Comment: 6 pages, 4 figures. Accepted for publication in Phys. Rev. Letter
Interplay between valence and core excitation mechanisms in the breakup of halo nuclei
The phenomenon of core excitation in the breakup of a two-body halo nucleus
is investigated. We show that this effect plays a significant role in the
reaction dynamics and, furthermore, its interference with the valence
excitation mechanism has sizable and measurable effects on the breakup angular
distributions. These effects have been studied in the resonant breakup of 11Be
on a carbon target, populating the resonances at 1.78 MeV (5/2+) and 3.41 MeV
(3/2+). The calculations have been performed using a recently extension of the
DWBA method, which takes into account the effect of core excitation in both the
structure of the halo nucleus and in the reaction mechanism. The calculated
angular distributions have been compared with the available data [Fukuda et
al., Phys. Rev. C70,054606]. Although each of these resonances is dominated by
one of the two considered mechanisms, the angular patterns of these resonances
depend in a very delicate way on the interference between them. This is the
first clear evidence of this effect but the phenomenon is likely to occur in
other similar reactions.Comment: 5 pages, 2 figures, (Version to appear in Physical Review Letters
Description of the LiLi transfer reaction using structure overlaps from a full three-body model
Recent data on the differential angular distribution for the transfer
reaction Li(p,d)Li at MeV in inverse kinematics are
analysed within the DWBA reaction framework, using the overlap functions
calculated within a three-body model of Li. The weight of the different
Li configurations in the system's ground state is obtained from the
structure calculations unambiguously. The effect of the Li spin in the
calculated observables is also investigated. We find that, although all the
considered models succeed in reproducing the shape of the data, the magnitude
is very sensitive to the content of wave in the Li
ground-state wave function. Among the considered models, the best agreement
with the data is obtained when the Li ground state contains a 31\%
of wave in the -Li subsystem. Although this model takes into
account explicitly the splitting of the and resonances due to the
coupling of the wave to the spin of the core, a similar
degree of agreement can be achieved with a model in which the Li spin is
ignored, provided that it contains a similar p-wave content.Comment: 8 pages, 3 figures. Final versio
Linking structure and dynamics in reactions with Borromean nuclei: the LiLi case
One-neutron removal reactions induced by two-neutron Borromean
nuclei are studied within a Transfer-to-the-Continuum (TC) reaction framework,
which incorporates the three-body character of the incident nucleus. The
relative energy distribution of the residual unbound two-body subsystem, which
is assumed to retain information on the structure of the original three-body
projectile, is computed by evaluating the transition amplitude for different
neutron-core final states in the continuum. These transition amplitudes depend
on the overlaps between the original three-body ground-state wave function and
the two-body continuum states populated in the reaction, thus ensuring a
consistent description of the incident and final nuclei. By comparing different
Li three-body models, it is found that the LiLi
relative energy spectrum is very sensitive to the position of the and
states in Li and to the partial wave content of these
configurations within the Li ground-state wave function. The possible
presence of a low-lying resonance is discussed. The coupling of the
single particle configurations with the non-zero spin of the Li core,
which produces a spin-spin splitting of the states, is also studied. Among the
considered models, the best agreement with the available data is obtained with
a Li model that incorporates the actual spin of the core and contains
31\% of -wave content in the -Li subsystem, in accord
with our previous findings for the Li(p,d)Li transfer reaction,
and a near-threshold virtual state.Comment: 7 pages, 4 figures, submitted to PL
Investigating the 10Li continuum through 9Li(d,p)10Li reactions
The continuum structure of the unbound system Li, inferred from the
LiLi transfer reaction, is reexamined. Experimental data for
this reaction, measured at two different energies, are analyzed with the same
reaction framework and structure models. It is shown that the seemingly
different features observed in the measured excitation energy spectra can be
understood as due to the different incident energy and angular range covered by
the two experiments. The present results support the persistence of the
parity inversion beyond the neutron dripline as well as the splitting of the
well-known low-lying -wave resonance. Furthermore, they provide indirect
evidence that most of the single-particle strength, including possible
resonances, lies at relatively high excitations energies.Comment: accepted for publication in Physics Letters
Beyond quantum microcanonical statistics
Descriptions of molecular systems usually refer to two distinct theoretical
frameworks. On the one hand the quantum pure state, i.e. the wavefunction, of
an isolated system which is determined to calculate molecular properties and to
consider the time evolution according to the unitary Schr\"odinger equation. On
the other hand a mixed state, i.e. a statistical density matrix, is the
standard formalism to account for thermal equilibrium, as postulated in the
microcanonical quantum statistics. In the present paper an alternative
treatment relying on a statistical analysis of the possible wavefunctions of an
isolated system is presented. In analogy with the classical ergodic theory, the
time evolution of the wavefunction determines the probability distribution in
the phase space pertaining to an isolated system. However, this alone cannot
account for a well defined thermodynamical description of the system in the
macroscopic limit, unless a suitable probability distribution for the quantum
constants of motion is introduced. We present a workable formalism assuring the
emergence of typical values of thermodynamic functions, such as the internal
energy and the entropy, in the large size limit of the system. This allows the
identification of macroscopic properties independently of the specific
realization of the quantum state. A description of material systems in
agreement with equilibrium thermodynamics is then derived without constraints
on the physical constituents and interactions of the system. Furthermore, the
canonical statistics is recovered in all generality for the reduced density
matrix of a subsystem
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