Quasi-SU(3) truncation scheme for odd-even sd-shell nuclei

The quasi-SU(3) symmetry, as found in shell model calculations, refers to the dominance of the single particle plus quadrupole-quadrupole terms in the Hamiltonian used to describe well deformed nuclei, and to the subspace relevant in its diagonalization. It provides a very efficient basis truncation scheme. It is shown that a small number of SU(3) coupled irreps, those with the largest C_2 values within the direct product of the proton and neutron SU(3) irreps with spin 0 and 1 (for even number of particles), and spin 1/2 and 3/2 for (for odd number of nucleons), are enough to describe the low energy spectra and B(E2) transition strengths of 21Ne, 23Na and 25Mg. A simple but realistic Hamiltonian is employed. Results compare favorably both with experimental data and with full shell model calculations. Limitations and possible improvements of the schematic Hamiltonian are discussed.