339 research outputs found
Dipole states in stable and unstable nuclei
A nuclear structure model based on linear response theory (i.e., Random Phase
Approximation) and which includes pairing correlations and anharmonicities
(coupling with collective vibrations), has been implemented in such a way that
it can be applied on the same footing to magic as well as open-shell nuclei. As
applications, we have chosen to study the dipole excitations both in
well-known, stable isotopes like Pb and Sn as well as in the
neutron-rich, unstable Sn nucleus, by addressing in the latter case the
question about the nature of the low-lying strength. Our results suggest that
the model is reliable and predicts in all cases low-lying strength of non
collective nature.Comment: 16 pages, 6 figures; submitted for publicatio
Exotic modes of excitation in atomic nuclei far from stability
We review recent studies of the evolution of collective excitations in atomic
nuclei far from the valley of -stability. Collective degrees of freedom
govern essential aspects of nuclear structure, and for several decades the
study of collective modes such as rotations and vibrations has played a vital
role in our understanding of complex properties of nuclei. The multipole
response of unstable nuclei and the possible occurrence of new exotic modes of
excitation in weakly-bound nuclear systems, present a rapidly growing field of
research, but only few experimental studies of these phenomena have been
reported so far. Valuable data on the evolution of the low-energy dipole
response in unstable neutron-rich nuclei have been gathered in recent
experiments, but the available information is not sufficient to determine the
nature of observed excitations. Even in stable nuclei various modes of giant
collective oscillations had been predicted by theory years before they were
observed, and for that reason it is very important to perform detailed
theoretical studies of the evolution of collective modes of excitation in
nuclei far from stability. We therefore discuss the modern theoretical tools
that have been developed in recent years for the description of collective
excitations in weakly-bound nuclei. The review focuses on the applications of
these models to studies of the evolution of low-energy dipole modes from stable
nuclei to systems near the particle emission threshold, to analyses of various
isoscalar modes, those for which data are already available, as well as those
that could be observed in future experiments, to a description of
charge-exchange modes and their evolution in neutron-rich nuclei, and to
studies of the role of exotic low-energy modes in astrophysical processes.Comment: 123 pages, 59 figures, submitted to Reports on Progress in Physic
Calculation of stellar electron-capture cross sections on nuclei based on microscopic Skyrme functionals
A fully self-consistent microscopic framework for evaluation of nuclear
weak-interaction rates at finite temperature is introduced, based on Skyrme
functionals. The single-nucleon basis and the corresponding thermal occupation
factors of the initial nuclear state are determined in the finite-temperature
Skyrme Hartree-Fock model, and charge-exchange transitions to excited states
are computed using the finite-temperature RPA. Effective interactions are
implemented self-consistently: both the finite-temperature single-nucleon
Hartree-Fock equations and the matrix equations of RPA are based on the same
Skyrme energy density functional. Using a representative set of Skyrme
functionals, the model is applied in the calculation of stellar
electron-capture cross sections for selected nuclei in the iron mass group and
for neutron-rich Ge isotopes.Comment: 31 pages, 13 figures, submitted to Physical Review
Room Temperature Organic Superconductor?
The electron--phonon coupling in fullerene C28 has been calculated from first
principles. The value of the associated coupling constant lambda/N(0) is found
to be a factor three larger than that associated with C60. Assuming similar
values of the density of levels at the Fermi surface N(0) and of the Coulomb
pseudopotential for C28-based solids as those associated with alkali-doped
fullerides A3C60, one obtains Tc(C28) \approx 8 Tc(C60).Comment: 10 pages, 2 figure
Sensitivity of the electric dipole polarizability to the neutron skin thickness in Pb
The static dipole polarizability, , in Pb has been
recently measured with high-resolution via proton inelastic scattering at the
Research Center for Nuclear Physics (RCNP). This observable is thought to be
intimately connected with the neutron skin thickness, , of the
same nucleus and, more fundamentally, it is believed to be associated with the
density dependence of the nuclear symmetry energy. The impact of
on in Pb is investigated and discussed on the basis
of a large and representative set of relativistic and non-relativistic nuclear
energy density functionals (EDF).Comment: Proceedings of NSD12, Opatija, Croatia, 9-13 July 201
Nuclear matter incompressibility coefficient in relativistic and nonrelativistic microscopic models
We systematically analyze the recent claim that nonrelativistic and
relativistic mean field (RMF) based random phase approximation (RPA)
calculations for the centroid energy E_0 of the isoscalar giant monopole
resonance yield for the nuclear matter incompressibility coefficient, K_{nm},
values which differ by about 20%. For an appropriate comparison with the RMF
based RPA calculations, we obtain the parameters for the Skyrme force used in
the nonrelativistic model by adopting the same procedure as employed in the
determination of the NL3 parameter set of an effective Lagrangian used in the
RMF model. Our investigation suggest that the discrepancy between the values of
K_{nm} predicted by the relativistic and nonrelativistic models is
significantly less than 20%.Comment: Revtex file (13 pages), appearing in PRC-Rapid Com
Incompressibility of finite fermionic systems: stable and exotic atomic nuclei
The incompressibility of finite fermionic systems is investigated using
analytical approaches and microscopic models. The incompressibility of a system
is directly linked to the zero-point kinetic energy of constituent fermions,
and this is a universal feature of fermionic systems. In the case of atomic
nuclei, this implies a constant value of the incompressibility in medium-heavy
and heavy nuclei. The evolution of nuclear incompressibility along Sn and Pb
isotopic chains is analyzed using global microscopic models, based on both
non-relativistic and relativistic energy functionals. The result is an almost
constant incompressibility in stable nuclei and systems not far from stability,
and a steep decrease in nuclei with pronounced neutron excess, caused by the
emergence of a soft monopole mode in neutron-rich nuclei.Comment: 7 pages, 5 figure
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
First measurement of muon-neutrino disappearance in NOvA
This paper reports the first measurement using the NOvA detectors of nu(mu) disappearance in a nu(mu) beam. The analysis uses a 14 kton-equivalent exposure of 2.74 x 10(20) protons-on-target from the Fermilab NuMI beam. Assuming the normal neutrino mass hierarchy, we measure Delta m(32)(2) = (2.52(-0.18)(+0.20)) x 10(-3) eV(2) and sin(2) theta(23) in the range 0.38-0.65, both at the 68% confidence level, with two statistically degenerate best-fit points at sin(2) theta(23) = 0.43 and 0.60. Results for the inverted mass hierarchy are also presented
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