2,319 research outputs found

### Core swelling in spherical nuclei: An indication of the saturation of nuclear density

Background: Nuclear radius is one of the most important and basic properties
of atomic nuclei and its evolution is closely related to the saturation of the
nuclear density in the internal region but the systematics of the nuclear radii
for the neutron-rich unstable nuclei is not well known.
Purpose: Motivated by the recent interaction cross section measurement which
indicates the 48Ca core swelling in the neutron-rich Ca isotopes, we explore
the mechanism of the enhancement of the neutron and proton radii for spherical
nuclei.
Methods: Microscopic Hartree-Fock calculations with three sets of Skyrme-type
effective interactions are performed for the neutron-rich Ca, Ni and Sn
isotopes. The total reaction cross sections for the Ca isotopes are evaluated
with the Glauber model to compare them with the recent cross section data.
Results: We obtain good agreement with the measured cross sections and charge
radii. The neutron and proton radii of the various "core" configurations are
extracted from the full Hartree-Fock calculation and discuss the core swelling
mechanism.
Conclusions: The core swelling phenomena occur depending on the properties of
the occupying valence single-neutron states to minimize the energy loss that
comes from the saturation of the densities in the internal region, which
appears to be prominent in light nuclei such as Ca isotopes.Comment: 6 pages, 4 figures, to appear in a Rapid Communication in Phys. Rev.

### Correlated-Gaussian approach to linear-chain states -Case of four $\alpha$-particles-

We show that correlated Gaussians with good angular momentum and parity
provide flexible basis functions for specific elongated shape. As its
application we study linear-chain states of four-alpha particles in
variation-after-projection calculations in which all the matrix elements are
evaluated analytically. We find possible chain states for $J=0^+$, $2^+$, $4^+$
and perhaps $6^+$ with the bandhead energy being about 33 MeV from the ground
state of $^{16}$O. No chain states with $J\geq 8$ are found. The nature of the
rotational sequence of the chain states is clarified in contrast to a
rigid-body rotation. The quadrupole deformation parameters estimated from the
chain states increase from 0.59 to 1.07 for $2^+$ to $6^+$. This work suggests
undeveloped fields for the correlated Gaussians beyond those problems which
have hitherto been solved successfully.Comment: 13 pages, 6 figures, accepted for publication in Phys. Rev.

### Emergence of nuclear clustering in electric-dipole excitations of $^6$Li

Nuclear clustering plays an important role, especially in the dynamics of
light nuclei. The importance of the emergence of the nuclear clustering was
discussed in the recent measurement of the photoabsorption cross sections as it
offered the possibility of the coexistence of various excitation modes which
are closely related to the nuclear clustering. To understand the excitation
mechanism, we study the electric-dipole ($E1$) responses of $^6$Li with a fully
microscopic six-body calculation. The ground-state wave function is accurately
described with a superposition of correlated Gaussian (CG) functions with the
aid of the stochastic variational method. The final-state wave functions are
also expressed by a number of the CG functions including important
configurations to describe the six-body continuum states excited by the $E1$
field. We found that the out-of-phase transitions occur due to the oscillations
of the valence nucleons against the $^4$He cluster at the low energies around
10 MeV indicating ``soft'' giant-dipole-resonance(GDR)-type excitations, which
are very unique in the $^6$Li system but could be found in other nuclear
systems. At the high energies beyond $\sim 30$ MeV typical GDR-type transitions
occur. The $^3$He-$^3$H clustering plays an important role to the GDR phenomena
in the intermediate energy regions around 20 MeV.Comment: 13 pages, 10 figure

### Momentum distribution and correlation of two-nucleon relative motion in $^6$He and $^6$Li

The momentum distribution of relative motion between two nucleons gives
information on the correlation in nuclei. The momentum distribution is
calculated for both $^{6}$He and $^6$Li which are described in a three-body
model of $\alpha$+$N$+$N$. The ground state solution for the three-body
Hamiltonian is obtained accurately using correlated basis functions. The
momentum distribution depends on the potential model for the $N$-$N$
interaction. With use of a realistic potential, the $^6$He momentum
distribution exhibits a dip around 2 fm$^{-1}$ characteristic of $S$-wave
motion. In contrast to this, the $^6$Li momentum distribution is very similar
to that of the deuteron; no dip appears because it is filled with the $D$-wave
component arising from the tensor force.Comment: 14 pages, 9 figure

### Green's function method for strength function in three-body continuum

Practical methods to compute dipole strengths for a three-body system by
using a discretized continuum are analyzed. New techniques involving Green's
function are developed, either by correcting the tail of the approximate wave
function in a direct calculation of the strength function or by using a
solution of a driven Schroedinger equation in a summed expression of the
strength. They are compared with the complex scaling method and the Lorentz
integral transform, also making use of a discretized continuum. Numerical tests
are performed with a hyperscalar three-body potential in the
hyperspherical-harmonics formalism. They show that the Lorentz integral
transform method is less practical than the other methods because of a
difficult inverse transform. These other methods provide in general comparable
accuracies.Comment: 22 pages, 8 figures, to appear in Progress of Theoretical Physic

### Electric dipole response of $^6$He: Halo-neutron and core excitations

Electric dipole ($E1$) response of $^{6}$He is studied with a fully
microscopic six-body calculation. The wave functions for the ground and excited
states are expressed as a superposition of explicitly correlated Gaussians
(CG). Final state interactions of three-body decay channels are explicitly
taken into account. The ground state properties and the low-energy $E1$
strength are obtained consistently with observations. Two main peaks as well as
several small peaks are found in the $E1$ strength function. The peak at the
high-energy region indicates a typical macroscopic picture of the giant dipole
resonance, the out-of-phase proton-neutron motion. The transition densities of
the lower-lying peaks exhibit in-phase proton-neutron motion in the internal
region, out-of-phase motion near the surface region, and spatially extended
neutron oscillation, indicating a soft-dipole mode (SDM) and its vibrationally
excited mode.Comment: 12 pages, 12 figures, to appear in Phys. Rev.

### Probing neutron-skin thickness with total reaction cross sections

We analyze total reaction cross sections, $\sigma_R$, for exploring their
sensitivity to the neutron-skin thickness of nuclei. We cover 91 nuclei of O,
Ne, Mg, Si, S, Ca, and Ni isotopes. The cross sections are calculated in the
Glauber theory using the density distributions obtained with the
Skyrme-Hartree-Fock method in 3-dimensional coordinate space. Defining a
reaction radius, $a_R=\sqrt{\sigma_R/\pi}$, to characterize the nuclear size
and target (proton or $^{12}$C) dependence, we find an empirical formula for
expressing $a_R$ with the point matter radius and the skin thickness, and
assess two practical ways of determining the skin thickness from proton-nucleus
$\sigma_R$ values measured at different energies or from $\sigma_R$ values
measured for different targets.Comment: 6 pages, 5 figures, to appear in Phys. Rev.

### Nuclear surface diffuseness revealed in nucleon-nucleus diffraction

Nuclear surface provides useful information on nuclear radius, nuclear
structure as well as properties of nuclear matter. We discuss the relationship
between the nuclear surface diffuseness and elastic scattering differential
cross section at the first diffraction peak of high-energy nucleon-nucleus
scattering as an efficient tool in order to extract the nuclear surface
information from limited experimental data involving short-lived unstable
nuclei. The high-energy reaction is described by a reliable microscopic
reaction theory, the Glauber model. Extending the idea of the black sphere
model, we find one-to-one correspondence between the nuclear bulk structure
information and proton elastic scattering diffraction peak. This implies that
we can extract both the nuclear radius and diffuseness simultaneously, using
the position of the first diffraction peak and its magnitude of the elastic
scattering differential cross section. We confirm the reliability of this
approach by using realistic density distributions obtained by a mean-field
model.Comment: 12 pages, 12 figures, to appear in Phys. Rev.

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