Etude de quelques états excités de 28Si

Abstract

The decay of 28Si energy levels at 4.98, 6.28, 6.69, 7.42, 7.80, and 7.93 MeV excited in the 27Al(p,γ)28Si reaction has been investigated. Lifetime and branching ratio studies on 0+ and 2+ levels leads to the conclusion that the existence of an excited prolate rotational band cannot be definitely ruled out, so that both prolate and oblate intrinsic states may be present as predicted by Hartree–Fock calculations. From these calculations, two low-lying (Jπ, K) = (3+, 3) levels due to particle–hole type excitations of the oblate and prolate intrinsic states should be present: emphasis was given to measurements on levels at 6.28 and 7.80 MeV as possible candidates. The mixing ratio for the 3+ → 3+ gamma ray transition has been studied from angular distribution measurements. An E2 strength of 17 ± 6 Wu (Wu = Weiskopf unit) is proposed, which is in agreement with a strong deformation of both states, but in disagreement with an important shape difference like prolate versus oblate.