An interacting boson model for light nuclei

• Main Author: Halse, P.
• Published: University of Sussex 1984
• Subjects: 539.7; Nuclear physics & particle accelerators
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.305358

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, and (01). (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 shell. 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.