62 research outputs found
Neutron-Rich Nuclei in Heaven and Earth
An accurately calibrated relativistic parametrization is introduced to
compute the ground state properties of finite nuclei, their linear response,
and the structure of neutron stars. While similar in spirit to the successful
NL3 parameter set, it produces an equation of state that is considerably softer
-- both for symmetric nuclear matter and for the symmetry energy. This
softening appears to be required for an accurate description of several
collective modes having different neutron-to-proton ratios. Among the
predictions of this model are a symmetric nuclear-matter incompressibility of
K=230 MeV and a neutron skin thickness in 208Pb of Rn-Rp=0.21 fm. Further, the
impact of such a softening on the properties of neutron stars is as follows:
the model predicts a limiting neutron star mass of Mmax=1.72 Msun, a radius of
R=12.66 km for a ``canonical'' M=1.4 Msun neutron star, and no (nucleon) direct
Urca cooling in neutrons stars with masses below M=1.3 Msun.Comment: 4 pages, 3 tables, and no figure
Insensitivity of the elastic proton-nucleus reaction to the neutron radius of 208Pb
The sensitivity--or rather insensitivity--of the elastic proton-nucleus
reaction to the neutron radius of 208Pb is investigated using a
non-relativistic impulse-approximation approach. The energy region (Tlab=500
MeV and Tlab=800 MeV) is selected so that the impulse approximation may be
safely assumed. Therefore, only free nucleon-nucleon scattering data are used
as input for the optical potential. Further, the optical potential includes
proton and neutron ground-state densities that are generated from
accurately-calibrated models. Even so, these models yield a wide range of
values (from 0.13 fm to 0.28 fm) for the poorly known neutron skin thickness in
208Pb. An excellent description of the experimental cross section is obtained
with all neutron densities. We have invoked analytic insights developed within
the eikonal approximation to understand the insensitivity of the differential
cross section to the various neutron densities. As the diffractive oscillations
of the cross sections are controlled by the matter radius of the nucleus, the
large spread in the neutron skin among the various models gets diluted into a
mere 1.5% difference in the matter radius. This renders ineffective the elastic
reaction as a precision tool for the measurement of neutron radii.Comment: 17 pages with 5 figure
Parity Violation, the Neutron Radius of Lead, and Neutron Stars
The neutron radius of a heavy nucleus is a fundamental nuclear-structure
observable that remains elusive. Progress in this arena has been limited by the
exclusive use of hadronic probes that are hindered by large and controversial
uncertainties in the reaction mechanism. The Parity Radius Experiment at the
Jefferson Laboratory offers an attractive electro-weak alternative to the
hadronic program and promises to measure the neutron radius of 208Pb accurately
and model independently via parity-violating electron scattering. In this
contribution we examine the far-reaching implications that such a determination
will have in areas as diverse as nuclear structure, atomic parity violation,
and astrophysics.Comment: 5 pages, 5 figures, proceedings to the PAVI06 conferenc
Correction to Relativistic Mean Field binding energy and scheme
The differences between the experimental and Relativistic Mean Field binding
energies have been calculated for a large number of even-even nuclei from A=50
to 220. Excluding certain mass regions, the differences, after suitable
corrections for particular isotope chains, are found to be proportional to the
Casten factor , chosen as a measure of n-p interaction strength in a
nucleus. Results for even- odd- nuclei are also seen to follow the same
relation, if the odd-even mass difference is taken into account following the
semiempirical formula. This indicates that the n-p interaction is the major
contributor to the difference between the calculated and the experimental
binding energies
Correlation between muonic levels and nuclear structure in muonic atoms
A method that deals with the nucleons and the muon unitedly is employed to
investigate the muonic lead, with which the correlation between the muon and
nucleus can be studied distinctly. A "kink" appears in the muonic isotope shift
at a neutron magic number where the nuclear shell structure plays a key role.
This behavior may have very important implications for the experimentally
probing the shell structure of the nuclei far away from the -stable
line. We investigate the variations of the nuclear structure due to the
interaction with the muon in the muonic atom and find that the nuclear
structure remains basically unaltered. Therefore, the muon is a clean and
reliable probe for studying the nuclear structure. In addition, a correction
that the muon-induced slight change in the proton density distribution in turn
shifts the muonic levels is investigated. This correction to muonic level is as
important as the Lamb shift and high order vacuum polarization correction, but
is larger than anomalous magnetic moment and electron shielding correction.Comment: 2 figure
Cluster formation in asymmetric nuclear matter: semi-classical and quantal approaches
The nuclear-matter liquid-gas phase transition induces instabilities against
finite-size density fluctuations. This has implications for both
heavy-ion-collision and compact-star physics. In this paper, we study the
clusterization properties of nuclear matter in a scenario of spinodal
decomposition, comparing three different approaches: the quantal RPA, its
semi-classical limit (Vlasov method), and a hydrodynamical framework. The
predictions related to clusterization are qualitatively in good agreement
varying the approach and the nuclear interaction. Nevertheless, it is shown
that i) the quantum effects reduce the instability zone, and disfavor
short-wavelength fluctuations; ii) large differences appear bewteen the two
semi-classical approaches, which correspond respectively to a collisionless
(Vlasov) and local equilibrium description (hydrodynamics); iii) the
isospin-distillation effect is stronger in the local equilibrium framework; iv)
important variations between the predicted time-scales of cluster formation
appear near the borders of the instability region.Comment: 27 pages, 11 figures, Submitted to Nuclear Physics A, Nuclear Physics
A In press (2008
Equation of state of isospin-asymmetric nuclear matter in relativistic mean-field models with chiral limits
Using in-medium hadron properties according to the Brown-Rho scaling due to
the chiral symmetry restoration at high densities and considering naturalness
of the coupling constants, we have newly constructed several relativistic
mean-field Lagrangians with chiral limits. The model parameters are adjusted
such that the symmetric part of the resulting equation of state at supra-normal
densities is consistent with that required by the collective flow data from
high energy heavy-ion reactions, while the resulting density dependence of the
symmetry energy at sub-saturation densities agrees with that extracted from the
recent isospin diffusion data from intermediate energy heavy-ion reactions. The
resulting equations of state have the special feature of being soft at
intermediate densities but stiff at high densities naturally. With these
constrained equations of state, it is found that the radius of a 1.4
canonical neutron star is in the range of 11.9 kmR13.1 km, and the
maximum neutron star mass is around 2.0 close to the recent
observations.Comment: 14 pages, 3 figure
Further explorations of Skyrme-Hartree-Fock-Bogoliubov mass formulas. IX: Constraint of pairing force to neutron-matter gap
In this latest of our series of Skyrme-HFB mass models, HFB-16, we introduce
the new feature of requiring that the contact pairing force reproduce at each
density the pairing gap of neutron matter as determined in microscopic
calculations with realistic nucleon-nucleon forces. We retain the earlier
constraints on the Skyrme force of reproducing the energy-density curve of
neutron matter, and of having an isoscalar effective mass of in
symmetric infinite nuclear matter at the saturation density; we also keep the
recently adopted device of dropping Coulomb exchange. Furthermore, the
correction term for the spurious energy of collective motion has a form that is
known to favour fission barriers that are in good agreement with experiment.
Despite the extra constraints on the effective force, we have achieved a better
fit to the mass data than any other mean field model, the rms error on the 2149
measured masses of nuclei with and 8 having been reduced to 0.632
MeV; the improvement is particularly striking for the most neutron-rich nuclei.
Moreover, it turns out that even with no flexibility at all remaining for the
pairing force, the spectral pairing gaps that we find suggest that level
densities in good agreement with experiment should be obtained. This new force
is thus particularly well-suited for astrophysical applications, such as
stellar nucleosynthesis and neutron-star crusts.Comment: 38 pages, 9 figures accepted for publication in Nuclear Physics
Constraining the Radii of Neutron Stars with Terrestrial Nuclear Laboratory Data
Neutron star radii are primarily determined by the pressure of isospin
asymmetric matter which is proportional to the slope of the nuclear symmetry
energy. Available terrestrial laboratory data on the isospin diffusion in
heavy-ion reactions at intermediate energies constrain the slope of the
symmetry energy. Using this constraint, we show that the radius (radiation
radius) of a 1.4 solar mass neutron star is between 11.5 (14.4) and 13.6 (16.3)
km.Comment: 11 pages, 3 figures; version to be published in Phys. Lett.
Optimization of relativistic mean field model for finite nuclei to neutron star matter
We have optimized the parameters of extended relativistic mean-field model
using a selected set of global observables which includes binding energies and
charge radii for nuclei along several isotopic and isotonic chains and the
iso-scalar giant monopole resonance energies for the Zr and Pb
nuclei. The model parameters are further constrained by the available
informations on the energy per neutron for the dilute neutron matter and bounds
on the equations of state of the symmetric and asymmetric nuclear matter at
supra-nuclear densities. Two new parameter sets BSP and IUFSU* are obtained,
later one being the variant of recently proposed IUFSU parameter set. The BSP
parametrization uses the contributions from the quartic order cross-coupling
between and mesons to model the high density behaviour of the
equation of state instead of the meson self-coupling as in the case of
IUFSU* or IUFSU. Our parameter sets yield appreciable improvements in the
binding energy systematics and the equation of state for the dilute neutron
matter. The importance of the quartic order cross coupling term
of the extended RMF model, as often ignored, is realized.Comment: 22 pages, 11 figures, Nucl. Phys. A (in press
- …