An interacting boson model for light nuclei.
The Interacting Boson Model (IBM) is extended to include singleparticle
levels corresponding to proton-neutron pairs, which might be
expected to have some importance for light nuclei where protons and
neutrons are filling the same shell. Two such versions are considered in
detail:- the first with an isospin triplet (T = 1) of bosons (IBM-3),
the second (IBM-4) with, introducing an intrinsic spin S,
(TS) = (10)
These, together with the standard model IBM-2 which comprises levels
corresponding to proton pairs and neutron pairs only, are applied in
detail to the even-even nuclei in the first half of the sd shell.
Assuming the same Hamiltonian for all nuclei, IBM-4 alone is found to be
consistent with the observed spectra and binding energies relative to
160 . Matrix elements of the E2 operator are also reasonably well reproduced
by this model.
The introduction of 'neutron-proton' bosons allows odd-odd nuclei
to be described, and detailed IBM-4 calculations for 22Na and 26A~
are presented. Satisfactory agreement with the observed energies, and
matrix elements of the Ml and E2 operators, is obtained on the
introduction of a strong non-central interaction. Analysis of the wavefunctions
suggests an 'extended' SU3 classification scheme which also
reproduces qualitatively the spectra of other sd shell odd-odd nuclei.
A considerable breaking of the zeroth-order approximation when
mapping the shell model spin-orbit force is indicated by the presence of this non-central interaction and supported by a simple study In the p
The boson number N is found to have a strong effect on the spectra
of odd-odd nuclei, moreover it is shown that the most natural reproduction
of the data is obtained by adopting the conventional value.