Document Sample
Nucleon Powered By Docstoc
					Activity 1.1 - Nucleon-nucleon interaction and nuclear structure
L. Coraggio*, A. Covello, A. Gargano, N. Itaco*

A fundamental problem in Nuclear Physics is to describe the properties of nuclei
in terms of the interactions between their constituents. This problem may be
tackled within the framework of the nuclear shell model, which is the main
theoretical tool for nuclear structure calculations in terms of nucleons. This
implies the derivation of the model-space effective interaction Veff from the free
nucleon-nucleon (NN) potential, which is a very difficult task owing to both the
nature of the potential and the complexity of the many-body methods needed for
the actual calculation of the matrix elements of Veff. Within this framework, we
have focused our activitity on two major issues:
i) A study of nuclei far from the valley of stability ("exotic nuclei") in the close
vicinity to doubly magic 100Sn and 132Sn. Recently, new experimental data have
become available for some of these nuclei, which provide a challenging testing
ground for the effective interaction and allow exploring the evolution of the shell
structure when approaching the proton and neutron drip lines. We have
performed realistic shell-model calculations, partly in the context of
collaborations with experimental groups, for nuclei around 100Sn and 132Sn
making use of the high-precision CD-Bonn NN potential . The results obtained
are in very good agreement with the available experimental data, which supports
confidence in our predictions of low-energy states which have no experimental
counterpart. This may stimulate, and be helpful to, future experiments.
ii) In our most recent shell-model calculations we have made use of a new
method to renormalize the bare NN interaction. In this approach, a low-
momentum potential, Vlow-k, is derived that preserves the physics of the original
NN potential up to a certain cutoff momentum Λ. The Vlow-k is a smooth
potential, which is suitable for being used directly in nuclear structure
calculations. We have shown that it provides an advantageous alternative to the
traditional approach based on the use of the Brueckner G matrix.
The Vlow-k potential is currently attracting much attention. In particular, there is
evidence that Vlow-k’s derived from different phase-shift equivalent NN potentials
are nearly identical. We have performed a study of the ground-state properties of
the doubly closed nuclei 4He, 16O, and 40Ca within the framework of the
Goldstone expansion and using as input different phase-shift equivalent
potentials . As a first step, we use the smooth Vlow-k directly in a Hartree-Fock
approach. Then, using the obtained self-consistent field as auxiliary potential, we
calculate the Goldstone expansion including diagrams up to the third order in
Vlow-k. While our results are in good agreement with experiment, it turns out that
they are only slihgtly dependent on the choice of the NN potential, which is in
agreement with the conclusions of other authors.

Program for 2005

- It is our intention on the one hand to continue the fruitful collaboration with
experimental groups engaged in the study of nuclei in the regions of shell
closures off satbility, on the other hand to deepen the understanding of the effects
of our shell-model effective interaction. In particular, we will focus attention on
the two Sb isotopes, 134Sb and 135Sb. The former is an odd-odd nucleus which is
the most appropriate system to study the neutron-proton multiplets in this mass
region, the latter with two neutrons and one proton above 132Sn is the most
exotic nucleus beyond 132Sn for which information exists on excited states. Our
shell-model calculations, which are parameter-free, will be performed by making
use of realistic effective interactions derived from the CD-Bonn nucleon-nucleon
(NN) potential.
 We will study if, and how much, the low-momentum potential Vlow-k is
dependent on the original NN potential and also the relationship between the
"optimum" momentum cutoff Λ and the number of intermediate states included
in the perturbative calculation of the effective interaction.

Shared By: