222 research outputs found
Microscopic shell-model description of the exotic nucleus ^{16}C
The structure of the neutron-rich carbon nucleus ^{16}C is described by
introducing a new microscopic shell model of no-core type. The model space is
composed of the 0s, 0p, 1s0d, and 1p0f shells. The effective interaction is
microscopically derived from the CD-Bonn potential and the Coulomb force
through a unitary transformation theory. Calculated low-lying energy levels of
^{16}C agree well with the experiment. The B(E2;2_{1}^{+} \to 0_{1}^{+}) value
is calculated with the bare charges. The anomalously hindered B(E2) value for
^{16}C, measured recently, is well reproduced.Comment: 14 pages, 4 figures, considerable results and discussion are added,
but the main conclusion is unchanged, accepted for publication in Phys. Lett.
Factorization of shell-model ground-states
We present a new method that accurately approximates the shell-model
ground-state by products of suitable states. The optimal factors are determined
by a variational principle and result from the solution of rather
low-dimensional eigenvalue problems. The power of this method is demonstrated
by computations of ground-states and low-lying excitations in sd-shell and
pf-shell nuclei.Comment: 5+epsilon pages, 5 eps-figures. Main additions: wave-function
overlaps, angular momentum expectation values, application to Ni56. To be
published as Rapid Communication in PR
Properties of C in the {\it ab initio} nuclear shell-model
We obtain properties of C in the {\it ab initio} no-core nuclear
shell-model. The effective Hamiltonians are derived microscopically from the
realistic CD-Bonn and the Argonne V8' nucleon-nucleon (NN) potentials as a
function of the finite harmonic oscillator basis space. Binding energies,
excitation spectra and electromagnetic properties are presented for model
spaces up to . The favorable comparison with available data is a
consequence of the underlying NN interaction rather than a phenomenological
fit.Comment: 9 pages, 2 figure
Nuclear Structure Calculations with Coupled Cluster Methods from Quantum Chemistry
We present several coupled-cluster calculations of ground and excited states
of 4He and 16O employing methods from quantum chemistry. A comparison of
coupled cluster results with the results of exact diagonalization of the
hamiltonian in the same model space and other truncated shell-model
calculations shows that the quantum chemistry inspired coupled cluster
approximations provide an excellent description of ground and excited states of
nuclei, with much less computational effort than traditional large-scale
shell-model approaches. Unless truncations are made, for nuclei like 16O,
full-fledged shell-model calculations with four or more major shells are not
possible. However, these and even larger systems can be studied with the
coupled cluster methods due to the polynomial rather than factorial scaling
inherent in standard shell-model studies. This makes the coupled cluster
approaches, developed in quantum chemistry, viable methods for describing
weakly bound systems of interest for future nuclear facilities.Comment: 10 pages, Elsevier latex style, Invited contribution to INPC04
proceedings, to appear in Nuclear Physics
Comparison of techniques for computing shell-model effective operators
Different techniques for calculating effective operators within the framework
of the shell model using the same effective interaction and the same excitation
spaces are presented. Starting with the large-basis no-core approach, we
compare the time-honored perturbation-expansion approach and a model-space
truncation approach. Results for the electric quadrupole and magnetic dipole
operators are presented for Li. The convergence trends and dependence of
the effective operators on differing excitation spaces and Pauli Q-operators is
studied. In addition, the dependence of the electric-quadrupole effective
charge on the harmonic-oscillator frequency and the mass number, for A=5,6, is
investigated in the model-space truncation approach.Comment: 18 pages. REVTEX. 4 PostScript figure
Few-nucleon systems in translationally invariant harmonic oscillator basis
We present a translationally invariant formulation of the no-core shell model
approach for few-nucleon systems. We discuss a general method of
antisymmetrization of the harmonic-oscillator basis depending on Jacobi
coordinates. The use of a translationally invariant basis allows us to employ
larger model spaces than in traditional shell-model calculations. Moreover, in
addition to two-body effective interactions, three- or higher-body effective
interactions as well as real three-body interactions can be utilized. In the
present study we apply the formalism to solve three and four nucleon systems
interacting by the CD-Bonn nucleon-nucleon potential. Results of ground-state
as well as excited-state energies, rms radii and magnetic moments are
discussed. In addition, we compare charge form factor results obtained using
the CD-Bonn and Argonne V8' NN potentials.Comment: 25 pages. RevTex. 13 Postscript figure
Realistic shell-model calculations: current status and open problems
The main steps involved in realistic shell-model calculations employing
two-body low-momentum interactions are briefly reviewed. The practical value of
this approach is exemplified by the results of recent calculations and some
remaining open questions and directions for future research are discussed.Comment: 12 pages, 2 figures, contribution to J. Phys G, Special Issue, Focus
Section: Open Problems in Nuclear Structur
Systematics of 2+ states in C isotopes from the ab initio no-core shell model
We study low-lying states of even carbon isotopes in the range A = 10 - 20
within the large- scale no-core shell model (NCSM). Using several accurate
nucleon-nucleon (NN) as well as NN plus three-nucleon (NNN) interactions, we
calculate excitation energies of the lowest 2+ state, the electromagnetic B(E2;
2+1 -> 0+1) transition rates, the 2+1 quadrupole moments as well as se- lected
electromagnetic transitions among other states. Recent experimental campaigns
to measure 2+-state lifetimes indicate an interesting evolution of nuclear
structure that pose a challenge to reproduce theoretically from first
principles. Our calculations do not include any effective charges or other
fitting parameters. However, calculated results extrapolated to infinite model
spaces are also presented. The model-dependence of those results is discussed.
Overall, we find a good agree- ment with the experimentally observed trends,
although our extrapolated B(E2; 2+1 -> 0+1) value for 16C is lower compared to
the most recent measurements. Relative transition strengths from higher excited
states are investigated and the influence of NNN forces is discussed. In
particular for 16C we find a remarkable sensitivity of the transition rates
from higher excited states to the details of the nuclear interactions.Comment: 22 pages, 8 figures, preprint version. Accepted for publication in
Journal of Physics G: Nuclear and Particle Physic
Converged ab initio calculations of heavy nuclei
We propose a novel storage scheme for three-nucleon (3N) interaction matrix
elements relevant for the normal-ordered two-body approximation used
extensively in ab initio calculations of atomic nuclei. This scheme reduces the
required memory by approximately two orders of magnitude, which allows the
generation of 3N interaction matrix elements with the standard truncation of
, well beyond the previous limit of 18. We demonstrate that this
is sufficient to obtain ground-state energies in Sn converged to within
a few MeV with respect to the truncation. In addition, we study the
asymptotic convergence behavior and perform extrapolations to the un-truncated
limit. Finally, we investigate the impact of truncations made when evolving
free-space 3N interactions with the similarity renormalization group. We find
that the contribution of blocks with angular momentum is
dominated by a basis-truncation artifact which vanishes in the large-space
limit, so these computationally expensive components can be neglected. For the
two sets of nuclear interactions employed in this work, the resulting binding
energy of Sn agrees with the experimental value within theoretical
uncertainties. This work enables converged ab initio calculations of heavy
nuclei.Comment: 13 pages, 10 figure
Large-basis shell-model calculation of 10C->10B Fermi matrix element
We use a shell-model calculation with a two-body effective
interaction derived microscopically from the Reid93 potential to calculate the
isospin-mixing correction for the 10C->10B superallowed Fermi transition. The
effective interaction takes into account the Coulomb potential as well as the
charge dependence of T=1 partial waves. Our results suggest the isospin- mixing
correction , which is compatible with previous
calculations. The correction obtained in those calculations, performed in a
space, was dominated by deviation from unity of the radial
overlap between the converted proton and the corresponding neutron. In the
present calculation this effect is accommodated by the large model space. The
obtained correction is about a factor of four too small to obtain
unitarity of the Cabibbo-Kobayashi-Maskawa matrix with the present experimental
data.Comment: 14 pages. REVTEX. 3 PostScript figure
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