THE STRUCTURE OF SI-28 ABOVE 10 MEV EXCITATION-ENERGY .3. LEVEL SCHEME AND SHELL-MODEL INTERPRETATION

The Si-28 level scheme up to 14.5 MeV excitation energy is reevaluated using information from two preceding papers. It consists of approxima tely 250 levels which are almost completely characterized according to the quantum numbers I, pi, T Of the levels. The properties of positiv e-parity states are compared to the predictions of shell model calcula tions within the complete s-d basis space using the unified s-d shell Hamiltonian. A spectrum of 48 experimental T=1 states between 9.3 and 16 MeV is reproduced with a rms deviation of only 150 keV. A calculati on of radiative widths and gamma-decay modes which uses free-nucleon g -factors yields excellent agreement with experiment and confirms that quenching of M1 transitions is only marginal in Si-28. The detailed sh ell model analysis of the T=0 spectrum is extended to the limiting ene rgy where T=1 wave function admixtures, not contained in the theory, b ecome important experimentally. This happens at 6-8 MeV above the yras t state, depending on the spin value. Altogether it appears that a spe ctrum of 171 levels below 14.5 MeV, which have positive or unassigned parity, is almost completely accounted for by the model. Apparent intr uder states from outside the s-d shell space are observed at E(x)=10 9 45 keV (I-pi=4(+)) and 12 860 keV (I-pi=6(+)) and are interpreted as m embers of a K-pi=0(+) rotational band.