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B. V. Carlson

Publications and source records attributed to B. V. Carlson.

At least 55 records · Page 3Linked to original sources
arXiv2009

The Statistical Multifragmentation Model with Skyrme Effective Interactions

The Statistical Multifragmentation Model is modified to incorporate the Helmholtz free energies calculated in the finite temperature Thomas-Fermi approximation using Skyrme effective interactions. In this formulation, the density of the fragments at the freeze-out configuration corresponds to the equilibrium value obtained in the Thomas-Fermi approximation at the given temperature. The behavior of the nuclear caloric curve at constant volume is investigated in the micro-canonical ensemble and a plateau is observed for excitation energies between 8 and 10 MeV per nucleon. A kink in the caloric curve is found at the onset of this gas transition, indicating the existence of a small excitation energy region with negative heat capacity. In contrast to previous statistical calculations, this situation takes place even in this case in which the system is constrained to fixed volume. The observed phase transition takes place at approximately constant entropy. The charge distribution and other observables also turn out to be sensitive to the treatment employed in the calculation of the free energies and the fragments' volumes at finite temperature, specially at high excitation energies. The isotopic distribution is also affected by this treatment, which suggests that this prescription may help to obtain information on the nuclear equation of state.

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arXiv2006

Iterated Resolvent Function for the Ladder Bethe-Salpeter Equation

We developed the formal connection of the field theoretical Bethe-Salpeter equation including the ladder approximation with its representation on the light-front for a bosonic model. We use the light-front Green's function for the N-particle system for two-particles plus N-2 intermediate bosons. We derived an infinite set of coupled hierarchy equations or iterated resolvent from which the Green's function can be calculated. These equations allow a consistent truncation of the light-front Fock-space. We show explicitly the represention of the covariant two-body T-matrix and bound state vertex in the light-front.

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arXiv2005

The Semiclassical Coulomb Interaction

The semiclassical Coulomb excitation interaction is at times expressed in the Lorentz gauge in terms of the electromagnetic fields and a contribution from the scalar electric potential. We point out that the potential term can make spurious contributions to excitation cross sections, especially when the the decay of excited states is taken into account. We show that, through an appropriate gauge transformation, the excitation interaction can be expressed in terms of the electromagnetic fields alone.

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arXiv2003

Light-front time picture of few-body systems

The projection of the four-dimensional two-body Bethe-Salpeter equation at the light-front hypersurface is discussed. Elimination of the relative time is performed using a quasi-potential expansion, which is shown to be equivalent to an infinite set of coupled equations for the Green's functions of the different light-front Fock-states.

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arXiv2003

Multiple giant resonances in nuclei: their excitation and decay

The excitation of multiphonon giant resonances with heavy ions is discussed. The conventional theory, based on the use of the virtual photon number method in conjunction with the harmonic model is presented and its shortcomings are discussed. The recently developed model that invoke the Brink-Axel mechanism as an important contribution to the cross-section is discussed and compared to the conventional, harmonic model. The decay properties of these multiple giant resonances are also discussed within the same coherent + fluctuation model in conjunction with the hybrid decay model. It is demonstrated that the Brink-Axel mechanism enhances the direct decay of the states, as data seem to require. Comparison of our model with other recent theoretical works is presented.

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arXiv2003

Light-front time picture of the Bethe-Salpeter equation

We show the light-front representation of the field theoretical Bethe-Salpeter equation (BSE) in the ladder approximation using the quasi potential reduction. We discuss the equivalence of the covariant ladder Bethe-Salpeter equation with an infinite set of coupled equations for the Green's functions of the different light-front Fock-states.

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arXiv2002

$p+^{4,6,8}He elastic scattering at intermediate energies

Using a relativistic nuclear optical potential consisting of a Lorentz scalar, $V_{s}$, and the time-like component of a four-vector potential, $V_{0}$, we calculate elastic scattering differential cross sections and polarizations for $p+^{4}$He at intermediate energies for which experimental data are available. We also calculate the differential cross sections and analyzing powers for $p+^{6,8}$He at intermediate energies and compare with the few available experimental data.

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arXiv2002

Quasi Free 238U (e,e'f)-Cross Section in Macroscopic-Microscopic Approach

We present the result of a theoretical study of inclusive quasi free electrofission of $^{238}$U. The off-shell $ $cross sections for the quasi free reaction stage have been calculated within the Plane Wave Impulse Approximation (PWIA), using a Macroscopic -Microscopic description of the proton and neutron single particle momentum distributions. Electron wave function distortion corrections were included using the effective momentum approximation, and the Final State Interaction (FSI) effects were calculated using an optical potential. The fissility for the proton single hole excited states of the residual nucleus $^{237}$Pa was calculated both without and with contributions of the pre-equilibrium emission of the particles. The fissility for $^{237,238}U$ residual nuclei was calculated within the compound nucleus model. The $(e,e^{\prime}f)-$cross sections thus obtained were compared with available experimental data.

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arXiv2002

Quasi free electro fission of 238U

We present the result of a theoretical study of the quasi free electrofission of $^{238}$U. The exclusive differential cross sections for the quasi free scattering reaction stage have been calculated in PWIA, using a Macroscopic-Microscopic approach for the description of the proton bound states. The nuclear shape was parametrized in terms of cassinian ovaloids. The equilibrium deformation parameters have been calculated by minimizing the total nuclear energy. In the calculation the axially deformed Woods-Saxon single particle potential was used. The obtained single particle momentum distributions were averaged over the nuclear symmetry axis direction. The occupation numbers were calculated in the BCS approach. The fissility for the single hole excited states of the residual nucleus $^{237}$% Pa was calculated on the statistical theory grounds both without taking into account the pre-equilibrium emission of the particle, and with preequilibrium emission in the framework of the exciton model.

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arXiv2002

A Relativistic Separable Potential to Describe Pairing in Nuclear Matter

Using the Dirac-Hartree-Fock-Bogoliubov approximation to study nuclear pairing, we have found the short-range correlations of the Dirac $^1$S$_0$ pairing fields to be essentially identical to those of the two-nucleon virtual state at all values of the baryon density. We make use of this fact to develop a relativistic separable potential that correctly describes the pairing fields.

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arXiv2002

Excitation of Triple Giant Resonances in Heavy-Ion Reactions

We calculate the cross-section for the excitation and subsequent decay of triple giant resonances(TGDR) in several nuclei excited with heavy ions.The recently developed coherent plus incoherent theory for excitation in conjunction with the hybrid decay model of Dias-Hussein-Adhikari are used for the purpose.It is emphasized that the direct decay of TGDR is expected to deviate appreciably from the harmonic limit especially at low bombarding energies,owing to the incoherent contribution.

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arXiv2002

Anharmonicities of giant dipole excitations

The role of anharmonic effects on the excitation of the double giant dipole resonance is investigated in a simple macroscopic model.Perturbation theory is used to find energies and wave functions of the anharmonic ascillator.The cross sections for the electromagnetic excitation of the one- and two-phonon giant dipole resonances in energetic heavy-ion collisions are then evaluated through a semiclassical coupled-channel calculation.It is argued that the variations of the strength of the anharmonic potential should be combined with appropriate changes in the oscillator frequency,in order to keep the giant dipole resonance energy consistent with the experimental value.When this is taken into account,the effects of anharmonicities on the double giant dipole resonance excitation probabilities are small and cannot account for the well-known discrepancy between theory and experiment.

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arXiv2002

Toward a global description of the nucleus-nucleus interaction

Extensive systematization of theoretical and experimental nuclear densities and of optical potential strengths exctracted from heavy-ion elastic scattering data analyses at low and intermediate energies are presented.The energy-dependence of the nuclear potential is accounted for within a model based on the nonlocal nature of the interaction.The systematics indicate that the heavy-ion nuclear potential can be described in a simple global way through a double-folding shape,which basically depends only on the density of nucleons of the partners in the collision.The poissibility of extracting information about the nucleon-nucleon interaction from the heavy-ion potential is investigated.

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arXiv2000

A Dirac-Hartree-Bogoliubov approximation for finite nuclei

We develop a complete Dirac-Hartree-Fock-Bogoliubov approximation to the ground state wave function and energy of finite nuclei. We apply it to spin-zero proton-proton and neutron-neutron pairing within the Dirac-Hartree-Bogoliubov approximation (we neglect the Fock term), using a zero-range approximation to the relativistic pairing tensor. We study the effects of the pairing on the properties of the even-even nuclei of the isotopic chains of Ca, Ni and Sn (spherical) and Kr and Sr (deformed), as well as the $N$=28 isotonic chain, and compare our results with experimental data and with other recent calculations.

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arXiv1999

Systematic Study of Triaxial Deformation in the Relativistic Mean Field Theory

We use the relativistic mean field (RMF) theory to systematically study the change of deformation of even-even nuclei in the proton-rich Xe region. We investigate the appearance of triaxial deformation in 25 nuclei in the region covering Z=50-58 and N=64-72 by performing constrained, triaxially symmetric RMF calculations of their energy surfaces. We include pairing correlations using the BCS formalism. We find that the Sn isotopes are spherical and the Te isotopes are very gamma unstable with shallow minima around gamma = 60 degrees. Adding more protons, the Xe, Ba and Ce isotopes have prolate deformations with their sizes increasing with proton number. The neutron number dependence is found to be small. We compare the calculated results with the available experimented data on the binding energy and the radii.

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arXiv1999

Light-Front Bethe-Salpeter Equation

A three-dimensional reduction of the two-particle Bethe-Salpeter equation is proposed. The proposed reduction is in the framework of light-front dynamics. It yields auxiliary quantities for the transition matrix and the bound state. The arising effective interaction can be perturbatively expanded according to the number of particles exchanged at a given light-front time. An example suggests that the convergence of the expansion is rapid. This result is particular for light-front dynamics. The covariant results of the Bethe-Salpeter equation can be recovered from the corresponding auxiliary three-dimensional ones. The technical procedure is developed for a two-boson case; the idea for an extension to fermions is given. The technical procedure appears quite practicable, possibly allowing one to go beyond the ladder approximation for the solution of the Bethe-Salpeter equation. The relation between the three-dimensional light-front reduction of the field-theoretic Bethe-Salpeter equation and a corresponding quantum-mechanical description is discussed.

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