A Study of the 3·00 MeV Level of 27Al via 24Mg(a, p?)27Al

Abstract

It is at present considered (Thankappan 1966) that the weak-coupling- model is more appropriate to the 27 Al nucleus than the strong-coupling (Nilsson) model, which has been applied with some success to 25Mg, 25Al, and other lighter nuclei in the 2s-1d shell (Gove 1960). Thankappan (1966) has interpreted the low-lying J = ~, ~, ~, ~, and ~ levels as a multiplet formed by the coupling of a IdG/, proton hole to the first excited (2+) state of 28Si. This model is able to fit simultaneously the level positions and many of the ,,-ray transition rates. The proton inelastic scattering cross sections (Crawley and Garvey 1965) are also in better agreement with this model than with the strong-coupling model. The 3·00 MeV level has for some time been regarded as the low-lying J1r = ;+ state predicted by Thankappan's excited-core model, but the experimental evidence is somewhat contradictory. Towle and Gilboy (1962) claimed an unambiguous assignment of J = ~ from inelastic neutron scattering; the ,,-ray correlation results of Lawergren (1964) are consistent with this assignment. Wakatsuki and Kern (1966) used ,,-ray correlation methods to make a firm assignment of J = i, but it is not obvious that the data used to exclude a J = ; assignment were. free of contributions from the 2· 98 MeV level. In addition, those authors made the dubious assumption that the reaction proceeds through an isolated level of the compound nucleus, and the mixing ratio obtained for the 3·00 to 0 transition is inconsistent with a recent lifetime measurement (Broude, Smulders, and Sharpey-Schafer 1967). Sheppard and van dar Leun (1967), using a Ge(Li) ,,-ray spectrometer, have measured gamma.-gamma correlations in the 26Mg(p,'Y'Y)27Al reaction and claim a definite assignment of J =;. Gove et al. (1967a, 1967b) have performed a D.W.B.A. analysis of the angular distributions in the 28Si(d,3He)27Al reaction using a magnetic spectrograph to resolve the 2·98 and 3·00 MeV levels; they conclude that the 3·00 MeV level has J = I or~. This raises the possibility that the 3·00 MeV "level" is, in fact, a doublet, one member of which has J =; and the other, populated in the (d, SHe) reaction, J = I or~.