Microscopic Structure of Low-Lying 0+ States in Deformed Nuclei

Recent high resolution (p; t) experiments have established the existence of a large number of low-lying excited 0+ levels in the deformed 158Gd [1] and, later on, in some actinide nuclei [2] and 168Er [3]. The number of 0+ states detected in 168Er is particularly large, the levels below 4 MeV amounting to about 22 [4]. The distribution pattern of the two-nucleon transfer transition strengths shows that one single 0+ state is strongly populated in all nuclei, with the exception of 168Er where the strength is fragmented into several small peaks. All theoretical studies performed so far have been con¯ned to 158Gd. The ¯rst was a phenomenological investigation [5], which shows that the extended (sdpf) interacting boson model (IBM) can account for a large fraction of the detected 0+ states. The (sdpf) IBM analysis points out the importance of the octupole degrees of freedom. In order to account for all detected states and to gain a detailed information on their properties, it is necessary to adopt microscopic approaches. The ¯rst calculation of this kind was performed in the framework of the projected shell model (PSM) [6] adopting a restricted space spanned by two and four quasiparticle states. Including the latter state