The structure of²⁸Si above 10 MeV excitation energy III: Level scheme and shell model interpretation

Abstract

²⁸Si level scheme up to 14.5 MeV excitation energy is reevaluated using information from two preceding papers. It consists of approximately 250 levels which are almost completely characterized according to the quantum numbersI, π, T of the levels. The properties of positive-parity states are compared to the predictions of shell model calculations within the completes-d basis space using the unifieds-d shell Hamiltonian. A spectrum of 48 experimentalT=1 states between 9.3 and 16 MeV is reproduced with a rms deviation of only 150 keV. A calculation of radiative widths and γ-decay modes which uses free-nucleong-factors yields excellent agreement with experiment and confirms that quenching of M1 transitions is only marginal in²⁸Si. The detailed shell model analysis of theT=0 spectrum is extended to the limiting energy whereT=1 wave function admixtures, not contained in the theory, become important experimentally. This happens at 6–8 MeV above the yrast state, depending on the spin value. Altogether it appears that a spectrum of 171 levels below 14.5 MeV, which have positive or unassigned parity, is almost completely accounted for by the model. Apparent intruder states from outside thes-d shell space are observed atE _x =10 945 keV (I ^π=4⁺) and 12 860 keV (I ^π=6⁺) and are interpreted as members of aK ^π=0⁺ rotational band.