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

Abstract

Abstract. New assignments of quantum numbers have been obtained for more than 50 levels in 2ssi between 8953 and 15915 keV excitation energy. They are based on the measurement of 7-ray angular distributions or anisotropics on 29 resonances of the 27Al(p,7) reaction and on the 24Mg(c~,7) resonance at E~ = 3355keV. A total of 15 high-spin states with I = 5-7 has been obtained and the spectrum of T = 1 states has been identified up to 15 915 keV excitation energy and a maximum spin I=6.

Nuclear level density: Shell-model approach

“…The model works extremely well for practically all observables of sd-nuclei and not only for the lowest states. For example, both, the experiment [44], and the sd-shell model [42], indicate the existence of ten stationary states with J Π = 0 + up to excitation energy of 15 MeV in 28 Si, therefore providing the same average level density at least at not very high energy; the mean level spacing between those 0 + levels is 0.95 MeV in the experiment and 1.02 MeV in the shell-model calculation. Fig.…”

“…The situation is different in two next examples, the bottom graphs in Fig. 15, 44 Ca and 64 Cr. Here we do not have a regular energy dependence and, therefore a clearly defined parameter σ.…”

“…The whole idea of the Gaussian random walk in the angular momentum space breaks down here because of the isospin limitations. The nucleus 44 Ca in the shell-model description has only four identical f 7/2 neutrons which allow for the isospin T = 2 only and for the interaction in the particle-particle channel with the total isospin T = 1. Therefore many values of the total spin are forbidden.…”

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

“…For E~ > 10 MeV the comparison of experimental data with the shell-model is confined to selected topics so far because of significant gaps of our experimental knowledge. In two preceding papers [10,11] we have closed most of these gaps by assigning quantum numbers to more than 50 levels between 8953 and 15 915 keV excitation energy. This gives us the chance of arriving at a more systematic test of the model at unprecedentedly high excitation energies.…”

“…The experimental level scheme of 288i ; T b Table 1 gives a new evaluation of the 2sSi levels and assigned values of I, ~, T up to 14.53 MeV excitation energy and a selection of still higher excited states which will be of interest here. The evaluation is largely based on the 1990 compilation [12] amended by the new levels [10] and new assignments of quantum numbers [11] obtained in two preceding papers. In Table 1 we omit the dubious [10] states at 11241 and 11 388 keV excitation energy and the proposed [15] 24Mg (cq?)…”