883 research outputs found
Spin-isospin nuclear response using the existing microscopic Skyrme functionals
Our paper aims at providing an answer to the question whether one can
reliably describe the properties of the most important spin-isospin nuclear
excitations, by using the available non-relativistic Skyrme energy functionals.
Our method, which has been introduced in a previous publication devoted to the
Isobaric Analog states, is the self-consistent Quasiparticle Random Phase
Approximation (QRPA). The inclusion of pairing is instrumental for describing a
number of experimentally measured spherical systems which are characterized by
open shells. We discuss the effect of isoscalar and isovector pairing
correlations. Based on the results for the Gamow-Teller resonance in Zr,
in Pb and in few Sn isotopes, we draw definite conclusions on the
performance of different Skyrme parametrizations, and we suggest improvements
for future fits. We also use the spin-dipole resonance as a benchmark of our
statements.Comment: Submitted to Phys. Rev.
The fully self-consistent quasiparticle random phase approximation and its application to the isobaric analog resonances
A microscopic model aimed at the description of charge-exchange nuclear
excitations along isotopic chains which include open-shell systems, is
developed. It consists of quasiparticle random phase approximation (QRPA) made
on top of Hartree-Fock-Bardeen-Cooper-Schrieffer (HF-BCS). The calculations are
performed by using the Skyrme interaction in the particle-hole channel and a
zero-range, density-dependent pairing force in the particle-particle channel.
At variance with the (many) versions of QRPA which are available in literature,
in our work special emphasis is put on the full self-consistency. Its
importance, as well as the role played by the charge-breaking terms of the
nuclear Hamiltonian, like the Coulomb interaction, the charge symmetry and
charge independence breaking (CSB-CIB) forces and the electromagnetic
spin-orbit, are elucidated by means of numerical calculations of the isobaric
analog resonances (IAR). The theoretical energies of these states along the
chain of the Sn isotopes agree well with the experimental data in the stable
isotopes. Predictions for unstable systems are presented.Comment: 15 pages, 6 figure
Scaling of the giant dipole resonance widths in hot rotating nuclei from the ground state values
The systematics of the giant dipole resonance (GDR) widths in hot and
rotating nuclei are studied in terms of temperature T, angular momentum J and
mass A. The different experimental data in the temperature range of 1 - 2 MeV
have been compared with the thermal shape fluctuation model (TSFM) in the
liquid drop formalism using a modified approach to estimate the average values
of T, J and A in the decay of the compound nucleus. The values of the ground
state GDR widths have been extracted from the TSFM parametrization in the
liquid drop limit for the corrected T, J and A for a given system and compared
with the corresponding available systematics of the experimentally measured
ground state GDR widths for a range of nuclei from A = 45 to 194. Amazingly,
the nature of the theoretically extracted ground state GDR widths matches
remarkably well, though 1.5 times smaller, with the experimentally measured
ground state GDR widths consistently over a wide range of nuclei.Comment: 15 pages, 4 figures, Accepted for publication in Physical Review
Proton decay from the isoscalar giant dipole resonance in Ni
Proton decay from the 3 isoscalar giant dipole resonance (ISGDR)
in Ni has been measured using the () reaction at a
bombarding energy of 386 MeV to investigate its decay properties. We have
extracted the ISGDR strength under the coincidence condition between
inelastically scattered particles at forward angles and decay protons
emitted at backward angles. Branching ratios for proton decay to low-lying
states of Co have been determined, and the results compared to
predictions of recent continuum-RPA calculations. The final-state spectra of
protons decaying to the low-lying states in Co were analyzed for a more
detailed understanding of the structure of the ISGDR. It is found that there
are differences in the structure of the ISGDR as a function of excitation
energy.Comment: Minor changes after review. Accepted for publication in Phys. Rev. C.
19 pages; 7 figure
Self-consistent calculation of nuclear photoabsorption cross section: Finite amplitude method with Skyrme functionals in the three-dimensional real space
The finite amplitude method (FAM), which we have recently proposed (T.
Nakatsukasa, T. Inakura, and K. Yabana, Phys. Rev. C 76, 024318 (2007)),
simplifies significantly the fully self-consistent RPA calculation. Employing
the FAM, we are conducting systematic, fully self-consistent response
calculations for a wide mass region. This paper is intended to present a
computational scheme to be used in the systematic investigation and to show the
performance of the FAM for a realistic Skyrme energy functional. We implemented
the method in the mixed representation in which the forward and backward RPA
amplitudes are represented by indices of single-particle orbitals for occupied
states and the spatial grid points for unoccupied states. We solve the linear
response equation for a given frequency. The equation is a linear algebraic
problem with a sparse non-hermitian matrix, which is solved with an iterative
method. We show results of the dipole response for selected spherical and
deformed nuclei. The peak energies of the giant dipole resonance agree well
with measurements for heavy nuclei, while they are systematically
underestimated for light nuclei. We also discuss the width of the giant dipole
resonance in the fully self-consistent RPA calculation.Comment: 11 pages, 10 figure
Electric dipole polarizability and the neutron skin
The recent high-resolution measurement of the electric dipole (E1)
polarizability (alphad) in 208Pb [Phys. Rev. Lett. 107, 062502 (2011)] provides
a unique constraint on the neutron-skin thickness of this nucleus. The
neutron-skin thickness (rskin) of 208Pb is a quantity of critical importance
for our understanding of a variety of nuclear and astrophysical phenomena. To
assess the model dependence of the correlation between alphad and rskin, we
carry out systematic calculations for 208Pb, 132Sn, and 48Ca based on the
nuclear density functional theory (DFT) using both non-relativistic and
relativistic energy density functionals (EDFs). Our analysis indicates that
whereas individual models exhibit a linear dependence between alphad and rskin,
this correlation is not universal when one combines predictions from a host of
different models. By averaging over these model predictions, we provide
estimates with associated systematic errors for rskin and alphad for the nuclei
under consideration. We conclude that precise measurements of rskin in both
48Ca and 208Pb---combined with the recent measurement of alphad---should
significantly constrain the isovector sector of the nuclear energy density
functional.Comment: Manuscript contains 5 pages, 2 figures, and 1 table. Submitted to
Physical Review Letter
Isoscalar dipole strength in ^{208}_{82}Pb_{126}: the spurious mode and the strength in the continuum
Isoscalar dipole (compression) mode is studied first using schematic
harmonic-oscillator model and, then, the self-consistent Hartree-Fock (HF) and
random phase approximation (RPA) solved in coordinate space. Taking ^{208}Pb
and the SkM* interaction as a numerical example, the spurious component and the
strength in the continuum are carefully examined using the sum rules. It is
pointed out that in the continuum calculation one has to use an extremely fine
radial mesh in HF and RPA in order to separate, with good accuracy, the
spurious component from intrinsic excitations.Comment: 19 pages, 2 figure
Overtones of Isoscalar Giant Resonances in medium-heavy and heavy nuclei
A semi-microscopic approach based on both the
continum-random-phase-approximation (CRPA) method and a phenomenological
treatment of the spreading effect is extended and applied to describe the main
properties (particle-hole strength distribution, energy-dependent transition
density, partial direct-nucleon-decay branching ratios) of the isoscalar giant
dipole, second monopole, and second quadrupole resonances. Abilities of the
approach are checked by description of gross properties of the main-tone
resonances. Calculation results obtained for the resonances in a few singly-
and doubly-closed-shell nuclei are compared with available experimental data.Comment: 12 pages, 14 figures, submitted to Phys. Rev.
Isotopic dependence of the giant monopole resonance in the even-A ^{112-124}Sn isotopes and the asymmetry term in nuclear incompressibility
The strength distributions of the giant monopole resonance (GMR) have been
measured in the even-A Sn isotopes (A=112--124) with inelastic scattering of
400-MeV particles in the angular range
--. We find that the experimentally-observed GMR energies
of the Sn isotopes are lower than the values predicted by theoretical
calculations that reproduce the GMR energies in Pb and Zr very
well. From the GMR data, a value of MeV is obtained
for the asymmetry-term in the nuclear incompressibility.Comment: Submitted to Physical Review Letters. 10 pages; 4 figure
Antimicrobial resistance and pathogenicity of Escherichia coli isolated from common dairy products in the Lebanon
In a recent study, bacteria have been isolated from popular Lebanese dairy products, which had been collected in the Beqaa Valley, in north-eastern Lebanon. The foods investigated were two cheeses (shankleesh and baladi) and a dried fermented mixture of yogurt and wheat grains (kishk). Bacterial colonies on McConkey and sorbitol-McConkey agar that showed the morphology of Escherichia coli were biochemically tested and then classified, using PCR-based assays, into the various strains of pathogenic and non-pathogenic E. coli. Some of the confirmed E. coli isolates were proven to be pathogenic, including two identified as E. coli O157:H7. When the pathogenic isolates were tested for their susceptibility to 10 different antibiotics (all commonly used, by clinicians and veterinarians, for the treatment of infections with Gram-negative bacteria), each tested isolate was found to be highly resistant to at least one antibiotic. It therefore appears that, in Lebanon, some popular dairy products pose a public-health hazard, acting as vehicles for the transmission of drug-resistant pathogens. © The Liverpool School of Tropical Medicine 2009
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