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~.