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José de San Martín, Argentina

Mohr P.,Diakonie Klinikum Schwabisch Hall GGmbH | Mohr P.,Institute of Nuclear Research ATOMKI | de Faria P.N.,University of Sao Paulo | Lichtenthaler R.,University of Sao Paulo | And 8 more authors.
Physical Review C - Nuclear Physics | Year: 2010

Cross sections of 120Sn(α,α)120Sn elastic scattering have been extracted from the α-particle-beam contamination of a recent 120Sn(6He,6He)120Sn experiment. Both reactions are analyzed using systematic double-folding potentials in the real part and smoothly varying Woods-Saxon potentials in the imaginary part. The potential extracted from the 120Sn(6He,6He)120Sn data may be used as the basis for the construction of a simple global 6He optical potential. The comparison of the 6He and α data shows that the halo nature of the 6He nucleus leads to a clear signature in the reflexion coefficients ηL: The relevant angular momenta L with ηL ≫ 0 and ηL ≪ 1 are shifted to larger L with a broader distribution. This signature is not present in the α-scattering data and can thus be used as a new criterion for the definition of a halo nucleus. © 2010 The American Physical Society.

Garcia F.,CONICET | Garcia F.,National University of La Plata | Aguilera D.N.,Laboratorio TANDAR | Aguilera D.N.,CONICET | And 3 more authors.
Astronomy and Astrophysics | Year: 2014

Context. In the magneto-centrifugal mechanism for jet formation, accreting neutron stars are assumed to produce relativistic jets only if their surface magnetic field is weak enough (B ∼ 108 G). However, the most common manifestation of neutron stars are pulsars, whose magnetic field distribution peaks at B ∼ 1012 G. If the neutron star magnetic field has at least this strength at birth, it must decay considerably before jets can be launched in binary systems. Aims. We study the magnetic field evolution of a neutron star that accretes matter from the wind of a high-mass stellar companion so that we can constrain the accretion rate and the impurities in the crust, which are necessary conditions for jet formation. Methods. We solved the induction equation for the diffusion and convection of the neutron star magnetic field confined to the crust, assuming spherical accretion in a simpliflied one-dimensional treatment. We incorporated state-of-the-art microphysics, including consistent thermal evolution profiles, and assumed two different neutron star cooling scenarios based on the superfluidity conditions at the core. Results. We find that in this scenario, magnetic field decay at long timescales is governed mainly by the accretion rate, while the impurity content and thermal evolution of the neutron star play a secondary role. For accretion rates ? ≥ 10-10M· yr-1, surface magnetic fields can decay up to four orders of magnitude in ∼10 7 yr, which is the timescale imposed by the evolution of the high-mass stellar companion in these systems. Based on these results, we discuss the possibility of transient jet-launching in strong wind-accreting high-mass binary systems like supergiant fast X-ray transients. © 2014 ESO.

Aguilera D.N.,Ohio University | Aguilera D.N.,Laboratorio TANDAR
Proceedings of the International Astronomical Union | Year: 2010

Accreting neutron stars can produce jets only if they are weakly magnetized (B ∼ 108 G). On the other hand, neutron stars are compact objects born with strong surface magnetic fields (B ∼ 1012 G). In this work we study the conditions for jet formation in a binary system formed by a neutron star and a massive donor star once the magnetic field has decayed due to accretion. We solve the induction equation for the magnetic field diffusion in a realistic neutron star crust and discuss the possibility of jet launching in systems like the recently detected Supergiant Fast X-ray Transients. © International Astronomical Union 2011.

Szybisz L.,Laboratorio TANDAR | Szybisz L.,University of Buenos Aires | Szybisz L.,CONICET | Sartarelli S.A.,National University of General Sarmiento
AIP Advances | Year: 2011

Adsorption on single planar walls and filling of slits with identical planar walls are investigated in the frame of the density functional theory. In this sort of slits the external potential is symmetric with respect to its central plane. Calculations were carried out by applying both the canonical and grand canonical ensembles (CE and GCE, respectively). The behavior is analyzed by varying the strength of the adsorbate-substrate attraction, the temperature T, and the coverage Results obtained for physisorption of Xe on alkaline surfaces are reported in the present work. Prewetting (PW) lines and wetting temperatures, T w, are determined from the analysis of adsorption on single walls. The filling of slits is analyzed for temperatures T T w. It is found that whenever for a given Xe-substrate combination the adsorption on a single wall exhibits a first-order wetting transition then asymmetric profiles that break the left-right symmetry of the external potential appear in the filling of an equivalent slit. These spontaneously symmetry breaking (SSB) solutions occur in a restricted range of with a T-dependent width. In the case of closed slits analyzed in the CE scheme, the obtained asymmetric profiles exhibit lower Helmholtz free energies than the symmetric species and, therefore, could be stabilized in this geometry. For open slits, the GCE scheme yields all the symmetric and SSB states in the corresponding convex regimes of the free energy. It is shown that both the CE and the GCE frames yield three coexistent states, two symmetric and one asymmetric twofold degenerate. Both a PW line and the related SSB effect terminate at the same temperature. For rather strongly attractive surfaces reentrant SSB states are found at a fixed value of T. © Copyright 2011 Author(s).

Abriola D.,International Atomic Energy Agency | Carnelli P.,Laboratorio TANDAR | Carnelli P.,CONICET | Arazi A.,Laboratorio TANDAR | And 20 more authors.
Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms | Year: 2010

Elastic cross sections for the 7Li +27Al system were measured at laboratory energies between 7 and 11 MeV in steps of 0.25 MeV, and angles between 135° and 170° in steps of 5°. Excitation functions for the elastic scattering were measured using an array of eight Si surface-barrier detectors whereas a solid-state telescope was used to estimate and subtract background from other reactions. Contamination from α particles arising from the 7Li breakup process at Elab ≥ 10 MeV makes the use of these energies inadvisable for RBS applications. The present results are compared with previous data obtained at 165° (Elab ≤ 6 MeV), 140° and 170° (Elab ≤ 8 MeV). The experimental data were analyzed in terms of the Optical Model. Two different energy-independent potentials were found. These optical potentials allow an interpolation with physical meaning to other energies and scattering angles. The experimental cross sections will be uploaded to the IBANDL database. © 2010 Elsevier B.V. All rights reserved.

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