Search for missing predicted high-spin states in28Si

Abstract

New shell model calculations have predicted several high-spin (I π=5+ and 6+) levels in28Si near 10 MeV excitation energy which are missing from or ambiguous in existing experimental studies. Angular distributions, linear polarizations and Doppler-shifts ofγ-rays have been measured for theγ-decay of theE p=1,911 and 2,073 KeV resonances of the27Al(p, γ) reaction in an attempt to discover these missing states or confirm the discrepancies between experiment and theory. The excitation energies and spin-parities of the resonances were determined as 13,424.4±0.2 keV,I π=5+ and 13,582.3±0.5 keV,I π=6+. States populated in theγ-decay of these resonances were assigned spins and parities as follows: 11,777 keV,I π=5+; 11,331 keV,I π=6+; 10,417 keV,I π=5+; 9,417 keV,I π=4+ and 8,945 keV,I π=5+. On the basis ofγ-ray transition rates T=1 is assigned to the 13,424 keV level and T=0 to the 10,417 and 11,777 keV levels. With the new data excellent agreement is achieved between the experimental spectrum of28Si and the new shell model predictions. These data provide evidence for aK π=3+ rotational band comprised by the 6,276, 6,889, 8,945 and 11,331 keV levels. This band emerges also from the shell model wave functions as do theK π=0+ bands based on the ground state and the 6,691 keV state.