The structure of28Si above 10 MeV excitation energy I: gamma-decay modes and radiative widths of levels

Abstract

Abstract. The systematics of 7-decay modes and radiative widths of highly excited states in 28Si has been extended by taking 7-ray spectra on 60 resonances of the 27Al(p, 7) reaction in the range 1097keV_< E,_< 4492keV (12643 keV _< Ex _< 15915 keV) and on the 24Mg(e,7) resonances at E~ = 3355, 3431, 4003 keV (Ex = 12860, 12925, 13415 keV). The y-decay modes of levels in the sub-resonance region (Ex = 10-12.5 MeV) were studied with both the 27Al(p, 7) reaction and the 27Al(d, nT) reaction at Ee = 4, 5, 6 MeV. Information on radiative widths of levels was obtained by measuring the resonance strengths S 7 = (2I + 1)FpF~fF for 52 resonances of the 27Al(p, 7) reaction with an emphasis on prospective T = 1 states.

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

“…The T = 1 assignments to the 11 572, 11 779 (U = 5 +) and the 11 901/11977 keV levels rest on the M1 rate of feeding y-rays from higher excited states? The T = 0 character of the feeding states at 12 817, 13 231 and 14633keV is undoubted, because these levels are weak resonances only of the 27Al(p, 7) reaction [2]. Further we know from the comparison with the 28A1 level scheme in Table 5 that there is no room for T = 1 states with U= 5 + and 6 + , respectively, at 12817 and 13231keV excitation energy.…”

“…It is still undecided which of these states is responsible for the observed feeding [-9] of the 1779, 7416 and 10900keV states. a We cite from left to right the excitation energy of the final state, the hypothetical quantum numbers of the initial state, the quantum numbers of the final state and the lower limits of the hypothetical transition rates u From the preceding paper [2] The quantum numbers have been established previously [1] except for the 10 311, 10 945 and 11 196 kcV states. In the latter cases they follow from the results of the subsequent Tables 3, 4 independent of the content of the present table a The recommended Table 5 we establish a T = 1 character of the initial state independent of its spin assignment.…”

“…Hence we shall prefer the alternative I = 4 assignment in a forthcoming comparison of experimental-theoretical properties of states. A I ~ = 0 + assignment can be made to the new level [2] at 8953 keV excitation energy. Bohne et al [16] have studied twoparticle transfer to 28Si with the Z6Mg(z, n) reaction and observed an L = 0 transfer to the 8.95 MeV state in seeming disagreement with the I n= 5 + assignment to the known 8945keV level.…”

“…The hypothetical multipole transition rates, mostly of the M2 character, which are the basis for the exclusion of quantum numbers, are included in Table 2. They have been calculated using the resonance strengths and branching ratios obtained in the preceding paper [2] and the mixing ratios from the 7~2-fits. The rejection of hypothetical M2 transition rates leads to several parity assignments for resonant states of Table 2.…”

“…Roughly 80% out of more than 250 levels have already unique and undoubted assignments of quantum numbers. 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.…”