860 research outputs found
Neutron skin of 208Pb, nuclear symmetry energy, and the parity radius experiment
A precise determination of the neutron skin thickness of a heavy nucleus sets
a basic constraint on the nuclear symmetry energy (the neutron skin thickness
is the difference of the neutron and proton rms radii of the nucleus). The
parity radius experiment (PREX) may achieve it by electroweak parity-violating
electron scattering (PVES) on 208Pb. We investigate PVES in nuclear mean field
approach to allow the accurate extraction of the neutron skin thickness of
208Pb from the parity-violating asymmetry probed in the experiment. We
demonstrate a high linear correlation between the parity-violating asymmetry
and the neutron skin thickness in successful mean field forces as the best
means to constrain the neutron skin of 208Pb from PREX, without assumptions on
the neutron density shape. Continuation of the experiment with higher precision
in the parity-violating asymmetry is motivated since the present method can
support it to constrain the density slope of the nuclear symmetry energy to new
accuracy.Comment: 4 pages, 3 figures, some changes in text and references, version to
appear in Phys. Rev. Let
Origin of the neutron skin thickness of 208Pb in nuclear mean-field models
We study whether the neutron skin thickness (NST) of 208Pb originates from
the bulk or from the surface of the nucleon density distributions, according to
the mean-field models of nuclear structure, and find that it depends on the
stiffness of the nuclear symmetry energy. The bulk contribution to NST arises
from an extended sharp radius of neutrons, whereas the surface contribution
arises from different widths of the neutron and proton surfaces. Nuclear models
where the symmetry energy is stiff, as typical relativistic models, predict a
bulk contribution in NST of 208Pb about twice as large as the surface
contribution. In contrast, models with a soft symmetry energy like common
nonrelativistic models predict that NST of 208Pb is divided similarly into bulk
and surface parts. Indeed, if the symmetry energy is supersoft, the surface
contribution becomes dominant. We note that the linear correlation of NST of
208Pb with the density derivative of the nuclear symmetry energy arises from
the bulk part of NST. We also note that most models predict a mixed-type
(between halo and skin) neutron distribution for 208Pb. Although the halo-type
limit is actually found in the models with a supersoft symmetry energy, the
skin-type limit is not supported by any mean-field model. Finally, we compute
parity-violating electron scattering in the conditions of the 208Pb parity
radius experiment (PREX) and obtain a pocket formula for the parity-violating
asymmetry in terms of the parameters that characterize the shape of the 208Pb
nucleon densities.Comment: 11 pages, 4 figures; minor stylistic changes in text, new Ref. [56]
added (new measurement of the neutron skin thickness of 208Pb
Estudi comparatiu dels horitzons orgà nics d'un alzinar i d'una pineda de pi roig a l'alta Garrotxa (Girona)
Analysis of bulk and surface contributions in the neutron skin of nuclei
The neutron skin thickness of nuclei is a sensitive probe of the nuclear
symmetry energy having multiple implications for nuclear and astrophysical
studies. However, precision measurements of this observable are difficult. The
analysis of the experimental data may imply some assumptions about the bulk or
surface nature of the formation of the neutron skin. Here, we study the bulk or
surface character of neutron skins of nuclei following from calculations with
Gogny, Skyrme, and covariant nuclear mean-field interactions. These
interactions are successful in describing nuclear charge radii and binding
energies but predict different values for neutron skins. We perform the study
by fitting two-parameter Fermi distributions to the calculated self-consistent
neutron and proton densities. We note that the equivalent sharp radius is a
more suitable reference quantity than the half-density radius parameter of the
Fermi distributions to discern between the bulk and surface contributions in
neutron skins. We present calculations for nuclei in the stability valley and
for the isotopic chains of Sn and Pb.Comment: 13 pages, 9 figure
Analysis of bulk and surface contributions in the neutron skin of nuclei
The neutron skin thickness of nuclei is a sensitive probe of the nuclear
symmetry energy having multiple implications for nuclear and astrophysical
studies. However, precision measurements of this observable are difficult. The
analysis of the experimental data may imply some assumptions about the bulk or
surface nature of the formation of the neutron skin. Here, we study the bulk or
surface character of neutron skins of nuclei following from calculations with
Gogny, Skyrme, and covariant nuclear mean-field interactions. These
interactions are successful in describing nuclear charge radii and binding
energies but predict different values for neutron skins. We perform the study
by fitting two-parameter Fermi distributions to the calculated self-consistent
neutron and proton densities. We note that the equivalent sharp radius is a
more suitable reference quantity than the half-density radius parameter of the
Fermi distributions to discern between the bulk and surface contributions in
neutron skins. We present calculations for nuclei in the stability valley and
for the isotopic chains of Sn and Pb.Comment: 13 pages, 9 figure
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