2,094 research outputs found
Monopole, quadrupole and pairing: a shell model view
The three main contributions to the nuclear Hamiltonian - monopole,
quadrupole and pairing - are analyzed in a shell model context. The first has
to be treated phenomenologically, while the other two can be reliably extracted
from the realistic interactions. Due to simple scaling properties, the
realistic quadrupole and pairing interactions eliminate the tendency to
collapse of their conventional counterparts, while retaining their basic
simplicity.Comment: 6 pages 4 figures revtex contrib. to Conf. on achievements and
perspectives in nucl. str. Crete July 1999, to appear in Physica Script
Shell-model phenomenology of low-momentum interactions
The first detailed comparison of the low-momentum interaction V_{low k} with
G matrices is presented. We use overlaps to measure quantitatively the
similarity of shell-model matrix elements for different cutoffs and oscillator
frequencies. Over a wide range, all sets of V_{low k} matrix elements can be
approximately obtained from a universal set by a simple scaling. In an
oscillator mean-field approach, V_{low k} reproduces satisfactorily many
features of the single-particle and single-hole spectra on closed-shell nuclei,
in particular through remarkably good splittings between spin-orbit partners on
top of harmonic oscillator closures. The main deficiencies of pure two-nucleon
interactions are associated with binding energies and with the failure to
ensure magicity for the extruder-intruder closures. Here, calculations
including three-nucleon interactions are most needed. V_{low k} makes it
possible to define directly a meaningful unperturbed monopole Hamiltonian, for
which the inclusion of three-nucleon forces is tractable.Comment: 5 pages, 4 figures, minor additions, to appear as Rapid Comm. in
Phys. Rev.
Microscopic mass formulae
By assuming the existence of a pseudopotential smooth enough to do
Hartree-Fock variations and good enough to describe nuclear structure, we
construct mass formulae that rely on general scaling arguments and on a
schematic reading of shell model calculations. Fits to 1751 known binding
energies for N,Z lead to RMS errors of 614 keV with 14 parameters and
388 keV with 28 parameters. The latter is easily reduced to a 20 parameter form
at 423 keV.Comment: 6 pages, REVTEX dialec
Radii in the shell and the "halo" orbit: A game changer
Proton radii of nuclei in the shell depart appreciably from the
asymptotic law, . The departure exhibits systematic
trends fairly well described by a single phenomenological term in the
Duflo-Zuker formulation, which also happens to explain the sudden increase in
slope in the isotope shifts of several chains at neutron number . It was
recently shown that this term is associated with the abnormally large size of
the and orbits in the and shells respectively. Further
to explore the problem, we propose to calculate microscopically radii in the
former. Since the (square) radius is basically a one body operator, its
evolution is dictated by single particle occupancies determined by shell model
calculations. Assuming that the departure from the asymptotic form is entirely
due to the orbit, the expectation value is determined by demanding that its evolution be
such as to describe well nuclear radii. It does, for an orbit that remains very
large (about 1.6 fm bigger than its counterparts) up to then
drops abruptly but remains some 0.6 fm larger than the orbits. An
unexpected behavior bound to challenge our understanding of shell formation.Comment: 4 pages 6(7) figure
Microscopic mass estimations
The quest to build a mass formula which have in it the most relevant
microscopic contributions is analyzed. Inspired in the successful Duflo-Zuker
mass description, the challenges to describe the shell closures in a more
transparent but equally powerful formalism are discussed.Comment: 14 pages, 6 figures, submitted to Journal of Physics G, Focus issue
on Open Problems in Nuclear Structure Theor
Canonical form of Hamiltonian matrices
On the basis of shell model simulations, it is conjectured that the Lanczos
construction at fixed quantum numbers defines---within fluctuations and
behaviour very near the origin---smooth canonical matrices whose forms depend
on the rank of the Hamiltonian, dimensionality of the vector space, and second
and third moments. A framework emerges that amounts to a general Anderson model
capable of dealing with ground state properties and strength functions. The
smooth forms imply binomial level densities. A simplified approach to canonical
thermodynamics is proposed.Comment: 4 pages 6 figure
A Shell Model Description of the Decay Out of the Super-Deformed Band of 36Ar
Large scale shell model calculations in two major oscillator shells (sd and
pf) describe simultaneously the super-deformed excited band of 36Ar and its
low-lying states of dominant sd character. In addition, several two particle
two hole states and a side band of negative parity are also well reproduced. We
explain the appearance of the super-deformed band at such low excitation energy
as a consequence of the very large correlation energy of the configurations
with many particles and many holes (np-nh) relative to the normal filling of
the spherical mean field orbits (0p-0h). We study the mechanism of mixing
between these different configurations, to understand why the super-deformed
band survives and how it finally decays into the low-lying sd-dominated states
via the indirect mixing of the 0p-0h and 4p-4h configurations.Comment: 4 pages 5 figures, revtex4, revised version, minor change
Spherical Shell Model description of rotational motion
Exact diagonalizations with a realistic interaction show that configurations
with four neutrons in a major shell and four protons in another -or the same-
major shell, behave systematically as backbending rotors. The dominance of the
component of the interaction is explained by an approximate form of
SU3 symmetry. It is suggested that these configurations are associated with the
onset of rotational motion in medium and heavy nuclei.Comment: 7 pages, RevTeX 3.0 using psfig, 6 Postscript figures included using
uufile
The spectra of mixed He-He droplets
The diffusion Monte Carlo technique is used to calculate and analyze the
excitation spectrum of He atoms bound to a cluster of He atoms, by
using a previously determined optimum filling of single-fermion orbits with
well defined orbital angular momentum , spin and parity quantum numbers.
The study concentrates on the energies and shapes of the three kinds of states
for which the fermionic part of the wave function is a single Slater
determinant: maximum or maximum states within a given orbit, and fully
polarized clusters. The picture that emerges is that of systems with strong
shell effects whose binding and excitation energies are essentially determined
over configuration at fixed number of particles and spin, i.e., by the monopole
properties of an effective Hamiltonian.Comment: 14 pages, 15 figure
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