184 research outputs found
Superfluid Properties of the Inner Crust of Neutron Stars
Superfluid properties of the inner crust matter of neutron stars, formed by
nuclear clusters immersed in a dilute neutron gas, are analysed in a self-
consistent HFB approach. The calculations are performed with two pairing
forces, fixed so as to obtain in infinite nuclear matter the pairing gaps
provided by the Gogny force or by induced interactions. It is shown that the
nuclear clusters can either suppress or enhance the pairing correlations inside
the inner crust matter, depending on the density of the surrounding neutrons.
The profile of the pairing field in the inner crust is rather similar for both
pairing forces, but the values of the pairing gaps are drastically reduced for
the force which simulates the polarisation effects in infinite neutron matter.Comment: 13 pages, 6 figures. Corrected typos and new format. To appear in
Phys. Rev.
Treatment of pairing correlations in nuclei close to drip lines
We discuss the HFB equations in coordinate representation,a suitable method
for handling the full effects of the continuous quasiparticle spectrum. We show
how the continuum HFB equations can be solved with the correct asymptotic
conditions instead of the discretization conditions which are commonly used in
the literature. The continuum HFB method is illustrated with a model where the
mean field and pairing field have simple forms. The relationship with the
continuum Hartree-Fock-BCS (HF-BCS) approximation is also discussed. Realistic
HFB and HF-BCS calculations based on Skyrme interactions are compared for the
case of a neutron-rich nucleus.Comment: 12 pages, 4 Postscript figures, uses KapProc.cls, to appear in
Proceedings of NATO School " Nuclei far from stability and astrophysics",
Predeal, Romania, 200
Compression modes in nuclei: microscopic models with Skyrme interactions
The isoscalar giant monopole resonances (ISGMR) and giant dipole resonances
(ISGDR) in medium-heavy nuclei are investigated in the framework of HF+RPA and
HF-BCS+QRPA with Skyrme effective interactions. It is found that pairing has
little effect on these modes. It is also found that the coupling of the RPA
states to 2p-2h configurations results in about (or less than) 1 MeV shifts of
the resonance energies and at the same time gives the correct total widths. For
the ISGMR, comparison with recent data leads to a value of nuclear matter
compression modulus close to 215 MeV. However, a discrepancy between calculated
and measured energies of the ISGDR in Pb is found and remains an open
problem.Comment: To appear in: ``RIKEN Symposium and Workshop on Selected Topics in
Nuclear Collective Excitations'', proceedings of the meeting, RIKEN, Wako
city (Japan), March 20--24, 199
Galectinas durante la placentación porcina
La placenta porcina es epiteliocorial y no invasiva. Aún quedan por comprender muchos de los mecanismos que participan en la placentación de la cerda, y por qué en esta especie la reabsorción embrionaria/fetal es tan elevada. Las galectinas (Gal) son una familia de proteínas que se expresan en diversos órganos, incluidos la placenta materna y fetal. En numerosas especies se demostró la importancia de las galectinas en la regulación de la inmunidad gestacional, como así también de los procesos de adhesión celular, apoptosis, angiogénesis y migración celular durante la gestación. Sin embargo, no se encontraron en la bibliografía estudios sobre las galectinas en la placenta porcina. Hasta el momento se ha determinado y analizado el comportamiento espaciotemporal por inmunohistoquímica de Gal-1 en la interfase fetomaterna. Se procesaron muestras de endometrio no gestante (n=4) y placentas (n=20) de 17, 30, 60, 70 y 114 días de gestación (dg), de cerdas mestizas de la zona de General Pico, La Pampa. Si bien en epitelio endometrial de cerda no gestante se halló expresión leve a negativo de Gal-1; durante la gestación se observaron modificaciones en la expresión de esta molécula. A los 17dg se halló marca leve en epitelio endometrial y fuerte en trofoblasto. A los 30dg se observó fuerte expresión de Gal-1 tanto en epitelio luminal endometrial como en trofoblasto. A los 60dg se observó inmunotinción moderada a alta en epitelio materno y alta en trofoblasto; y a los 70dg Gal-1 se expresó levemente a negativo en epitelio endometrial y levemente en trofoblasto. No se observó expresión a los 114dg. Hay momentos en la gestación porcina de gran remodelación placentaria, lo que implica establecimiento de un diálogo entre las moléculas de adhesión y el sistema inmunológico para el desarrollo de un ambiente tanto proinflamatorio como antiinflamatorio local. Estudios en la Facultad de Ciencias Veterinarias de la Universidad Nacional de La Pampa, demostraron que en dichos periodos predominan ciertas moléculas de adhesión y sus ligandos, como así también citoquinas inmuno reguladoras. La presencia de Gal-1 tanto a los 17dg, como a los 30 y 60-70dg, nos sugiere que podría estar regulando en tiempo y espacio una correcta placentación. Su ausencia a los 114dg regularía el desprendimiento de las placentas en el momento del parto. Conocer la presencia y concentración de las galectinas en la placenta porcina en distintos momentos de la gestación, aportará información de la participación de estas moléculas en la gestación normal; para luego determinar posibles variaciones a lo largo de la preñez que puedan relacionarse con la muerte embrionaria/fetal. El objetivo de este proyecto es analizar la expresión de Gal-1, Gal-3, Gal-9 y Gal-13, en preparados histológicos de interfase fetomaterna, en suero y en homogenatos de placentas porcinas provenientes de diferentes períodos gestacionales y de cerdas no gestantes
Pairing effects on the collectivity of quadrupole states around 32Mg
The first 2+ states in N=20 isotones including neutron-rich nuclei 32Mg and
30Ne are studied by the Hartree-Fock-Bogoliubov plus quasiparticle random phase
approximation method based on the Green's function approach. The residual
interaction between the quasiparticles is consistently derived from the
hamiltonian density of Skyrme interactions with explicit velocity dependence.
The B(E2) transition probabilities and the excitation energies of the first 2+
states are well described within a single framework. We conclude that pairing
effects account largely for the anomalously large B(E2) value and the very low
excitation energy in 32Mg.Comment: 14 pages, 9 figure
Estudio mineralógico de algunas calcedonias argentinas
El trabajo se realizó teniendo en cuenta la reacción de este mineral con el cemento portland, la cuál produce efectos perjudiciales en los hormigones. Se comprobó que en ciertos casos la reacción no se produce. Se investigan las causas de tal comportamiento. El material utilizado para el estudio proviene de los ríos Uruguay y Paraná, ya que son las principales fuentes de abastecimiento de agregados del Gran Buenos Aires.
Las "calcedonias" estudiadas están compuestas por un agregado fino de cuarzo, calcedonia y ópalo. La proporción relativa de dichos minerales condiciona la reactividad de los mismos.This paper deals with the reaction between chalcedony and Portland cement. This reaction damages the structure of the reinforced concrete, when aggregates of this kind are used for that purpose.
In certain cases the concerned reaction do not take place. For that reason a detailed study was carried out to determine the behavior of the materials. The aggregates used for this, study come from Paraná and Uruguay rivers; the sediments of this area supply enough material from the Gran Buenos Aires market.
The so call "chalcedony" studies are mainly composed of fine quartz, chalcedony and opal. The relative amounts of these minerals are thought to affect the behavior of the aggregates when they are mixed with portland cement
Half-lives of rp-process waiting point nuclei
We give results of microscopic calculations for the half-lives of various
proton-rich nuclei in the mass region A=60-90, which are involved in the
astrophysical rp-process, and which are needed as input parameters of numerical
simulations in Nuclear Astrophysics. The microscopic formalism consists of a
deformed QRPA approach that involves a selfconsistent quasiparticle deformed
Skyrme Hartree-Fock basis and residual spin-isospin separable forces in both
the particle-hole and particle-particle channels. The strength of the
particle-hole residual interaction is chosen to be consistent with the Skyrme
effective force and mean field basis, while that of the particle-particle is
globally fixed to 0.07 MeV after a judicious choice from comparison to
experimental half-lives. We study and discuss the sensitivity of the half-lives
to deformation and residual interactions.Comment: 8 pages, 4 figures, to be published in Eur. Phys. J.
Dirac-Brueckner Hartree-Fock Approach: from Infinite Matter to Effective Lagrangians for Finite Systems
One of the open problems in nuclear structure is how to predict properties of
finite nuclei from the knowledge of a bare nucleon-nucleon interaction of the
meson-exchange type. We point out that a promising starting point consists in
Dirac-Brueckner-Hartree-Fock (DBHF) calculations us- ing realistic
nucleon-nucleon interactions like the Bonn potentials, which are able to
reproduce satisfactorily the properties of symmetric nuclear matter without the
need for 3-body forces, as is necessary in non-relativistic BHF calculations.
However, the DBHF formalism is still too com- plicated to be used directly for
finite nuclei. We argue that a possible route is to define effective
Lagrangians with density-dependent nucleon-meson coupling vertices, which can
be used in the Relativistic Hartree (or Relativistic Mean Field (RMF)) or
preferrably in the Relativistic Hartree- Fock (RHF) approach. The
density-dependence is matched to the nuclear matter DBHF results. We review the
present status of nuclear matter DBHF calculations and discuss the various
schemes to construct the self-energy, which lead to differences in the
predictions. We also discuss how effective Lagrangians have been constructed
and are used in actual calculations. We point out that completely consistent
calculations in this scheme still have to be performed.Comment: 16 pages, to be published in Journal of Physics G: Nuclear and
Particle Physics, special issue
Two-neutron transfer in nuclei close to the dripline
We investigate the two-neutron transfer modes induced by (t,p) reactions in
neutron-rich oxygen isotopes. The nuclear response to the pair transfer is
calculated in the framework of continuum-Quasiparticle Random Phase
Approximation (cQRPA). The cQRPA allows a consistent determination of the
residual interaction and an exact treatment of the continuum coupling. The
(t,p) cross sections are calculated within the DWBA approach and the form
factors are evaluated by different methods : macroscopically, following the
Bayman and Kallio method, and fully microscopically. The largest cross section
corresponds to a high-lying collective mode built entirely upon continuum
quasiparticle states.Comment: 12 pages, 7 figure
Resonant continuum in the Hartree-Fock+BCS approximation
A method for incorporating the effect of the resonant continuum into
Hartree-Fock+BCS equations is proposed. The method is applied for the case of
a neutron-rich nucleus calculated with a Skyrme-type force plus a zero-range
pairing interaction and the results are compared with
Hartree-Fock-Bogoliubov calculations. It is shown that the widths of resonant
states have an important effect on the pairing properties of nuclei close to
the drip line.Comment: 9 pages, 2 figures, comparison with HFB adde
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