514 research outputs found
Low energy behavior of astrophysical S factor in radiative captures to loosely bound final states
The low-energy behavior of the astrophysical S-factor for E1 direct radiative
captures a(p,gamma)b leading to loosely bound final states (b=a+p) is
investigated. We derive a first-order integral representation for S(E) and
focus on the properties around zero energy. We show that it is the competition
between various effects, namely the remnant Coulomb barrier, the initial and
final centrifugal barriers and the binding energy, that defines the behavior of
the S(E->0). Contrary to previous findings, we prove that S(E->0) is not
determined by the pole corresponding to the bound state. The derivative S'(0)
increases with the increase of the centrifugal barrier, while it decreases with
the charge of the target. For l_i=l_f+1 the increase of the binding energy of
the final nucleus increases the derivative S'(0) while for l_i=l_f-1 the
opposite effect is found. We make use of our findings to explain the low energy
behavior of the S-factors related to some notorious capture reactions: 7Be(p,
gamma)8B, 14N(p,gamma)15O, 16O}(p,gamma)17F, 20Ne(p, gamma)21Na and 22Mg(p,
gamma)23Al.Comment: 30 pages, TeX (or Latex, etc). Nucl. Phys. A (in press
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
Multipole Expansion for Relativistic Coulomb Excitation
We derive a general expression for the multipole expansion of the
electro-magnetic interaction in relativistic heavy-ion collisions, which can be
employed in higher-order dynamical calculations of Coulomb excitation. The
interaction has diagonal as well as off-diagonal multipole components,
associated with the intrinsic and relative coordinates of projectile and
target. A simple truncation in the off-diagonal components gives excellent
results in first-order perturbation theory for distant collisions and for beam
energies up to 200 MeV/nucleon.Comment: 3 figures, Accepted for publication in Phys. Rev.
Effects of deformation in the three-body structure of 11Li
11Li is studied within a three-body model 9Li+n+n where the core is allowed
to be deformed and/or excite. In particular, we include reorientation couplings
and couplings between the two bound states of 9Li. Contrary to the other
examples studied within this model, we find that core excitation does not
affect the structure of 11Li significantly. Reorientation couplings of the
deformed 9Li can change the ground state of 11Li from a predominantly two
neutron s1/2^2 configuration into a p1/2^2. In addition, we see no evidence for
the existence of significant d-wave strength in its ground state, as opposed to
the prediction by shell model. A comparison with shell model is presented.Comment: 13 pages, 9 figure
Transfer to the continuum and Breakup reactions
Reaction theory is an essential ingredient when performing studies of nuclei
far from stability. One approach for the calculation of breakup reactions of
exotic nuclei into two fragments is to consider inelastic excitations into the
single particle continuum of the projectile. Alternatively one can also
consider the transfer to the continuum of a system composed of the light
fragment and the target. In this work we make a comparative study of the two
approaches, underline the different inputs, and identify the advantages and
disadvantages of each approach. Our test cases consist of the breakup of
Be on a proton target at intermediate energies, and the breakup of B
on Ni at energies around the Coulomb barrier. We find that, in practice
the results obtained in both schemes are in semiquantitative agreement. We
suggest a simple condition that can select between the two approaches.Comment: 10 pages, 12 figures. Replaced by accepted version. To appear in
Nucl. Phys.
One-neutron halo structure by the ratio method
We present a new observable to study halo nuclei. This new observable is a
particular ratio of angular distributions for elastic breakup and scattering.
For one-neutron halo nuclei, it is shown to be independent of the reaction
mechanism and to provide significant information about the structure of the
projectile, including binding energy, partial-wave configuration, and radial
wave function of the halo. This observable offers new capabilities for the
study of nuclear structure far from stability.Comment: 9 pages, 4 figure
A student-centered approach for developing active learning: the construction of physical models as a teaching tool in medical physiology
BACKGROUND: Teaching physiology, a complex and constantly evolving subject, is not a simple task. A considerable body of knowledge about cognitive processes and teaching and learning methods has accumulated over the years, helping teachers to determine the most efficient way to teach, and highlighting student's active participation as a means to improve learning outcomes. In this context, this paper describes and qualitatively analyzes an experience of a student-centered teaching-learning methodology based on the construction of physiological-physical models, focusing on their possible application in the practice of teaching physiology. METHODS: After having Physiology classes and revising the literature, students, divided in small groups, built physiological-physical models predominantly using low-cost materials, for studying different topics in Physiology. Groups were followed by monitors and guided by teachers during the whole process, finally presenting the results in a Symposium on Integrative Physiology. RESULTS: Along the proposed activities, students were capable of efficiently creating physiological-physical models (118 in total) highly representative of different physiological processes. The implementation of the proposal indicated that students successfully achieved active learning and meaningful learning in Physiology while addressing multiple learning styles. CONCLUSION: The proposed method has proved to be an attractive, accessible and relatively simple approach to facilitate the physiology teaching-learning process, while facing difficulties imposed by recent requirements, especially those relating to the use of experimental animals and professional training guidelines. Finally, students' active participation in the production of knowledge may result in a holistic education, and possibly, better professional practices
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
Two-neutron overlap functions for 6He from a microscopic structure model
A fully antisymmetrized microscopic model is developed for light two-neutron
halo nuclei using a hyper-spherical basis to describe halo regions. The
many-body wavefunction is optimized variationally. The model is applied to 6He
bound by semi realistic Minnesota nucleon-nucleon forces. The two-neutron
separation energy and the radius of the halo are reproduced in agreement with
experiment. Antisymmetrization effects between 4He and halo neutrons are found
to be crucial for binding of 6He. We also properly extract two-neutron overlap
functions and find that there is a significant increase of 30%-70% in their
normalization due to microscopic effects as compared to the results of
three-body models.Comment: To be published in Nucl. Phys.
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