Search for missing predicted high-spin states in28Si

Abstract

New shell model calculations have predicted several high-spin (P = 5 + and 6 +) levels in 28Si near 10 MeV excitation energy which are missing from or ambiguous in existing experimental studies. Angular distributions, linear polarizations and Doppler-shifts of ),-rays have been measured for the 7-decay of the Ep = 1,911 and 2,073 KeV resonances of the 27Al(p, 7) 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, P=5 § and 13,582.3_+0.5 keV, P=6 +. States populated in the 7-decay of these resonances were assigned spins and parities as follows: 11,777keV, F=5+; ll,331keV, P=6+; 10,417keV, P=5+; 9,417 keV, P=4 + and 8,945 keV, P=5 +. On the basis of 7-ray transition rates T=I 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 of 28Si and the new shell model predictions. These data provide evidence for a K~=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 the/U=0 + bands based on the ground state and the 6,691 keV state.

The structure of28Si above 10 MeV excitation energy II: Assignments of quantum numbers

“…The doublet character of the 8.95 MeV resolves this problem. We notice that the present work confirms the tentative I n = 2 + assignment to the 9796 keV level [17] and our previous I n = 6 + assignments [4] to the 11 101, 11 333 and 11 510keV states. Also the assignment of negative parity to the l1435keV, I = 4 state which has been proposed in [19] is now established.…”

“…The transitions from the levels with the higher T = 0 percentages are the stronger ones, thus proving that the T = 0 wave function component is responsible for the decay to the 11 779 keV level. In a previous paper [4] we misinterpreted the 13425--* 11 779 keV ,/-ray transition as a T = 1 ~ T = 0 instead of a T = 0 ~ T = 1 decay. Table 5 contains a few changes in spin assignments/restrictions as compared to the literature.…”

“…In the case of the 13 710keV level, the ambiguities in the spin assignment are resolved by the observation that the y-decay modes of this state and of the 13 583keV, U = 6 + state of [4] are nearly identical. We attribute this to isospin-mixing of two closely spaced U = 6 + states with T =0 and T = 1, respectively.…”

“…The observation of the 12 301keV level as a weak resonance in the 24Mg(~,7 ) reaction implies the presence of a small T = 0 component in its wave function which is not embarassing. 4 The study of two-particle transfer in the 26Mg(z, n) reaction [16] has yielded a U = 0 +, 1 + state at 12.27MeV excitation energy, which could be identical with the 12 301 keV level but also with a 12266keV level from the 27Al(d, n) reaction [25], whose quantum numbers are as yet uninvestigated.…”

“…The preceding paper [2] has shown that the remaining states are accessible, almost completely, via the 27Al(p,)0 and/or 24Mg(~,7 ) reactions. The success of shell model calculations for s d shell nuclei [3], and especially those for the A = 28 system [4,5], have on the other hand generated a demand for complete spectroscopic information at high excitation energies. In the following we report on measurements of ~ angular distributions or anisotropies on 29 resonances of the 27Al(p,7) reaction and one 24Mg(c~,y) resonance.…”