Shape Transitional Nuclei: What can we learn from the Yrare States? or Hello the Double Vacuum; Goodbye β‐vibrations!

The results of our measurements on the yrare states up to spin 20ℏ in 152,154,155Gd, using (α,xn) reactions and the AFRODITE γ‐ray spectrometer, are presented. We find that in 155Gd the decay scheme is divided into levels feeding the [505]11/2− band, that is extruded by the prolate deformation from the h11/2 orbital, and levels feeding the i13/2[651]3/2+ intruder orbital and the h9/2[521]3/2− orbital. The decay scheme of 154Gd is very complex. We find no evidence for the existence of β‐vibrational levels below 1.5 MeV. We discover that the level scheme can be best understood as a set of collective states built on the ground state configuration |01+> plus a “congruent” set of collective states based on the |02+> state at 681 keV. The data suggest that this second vacuum has reduced pairing. Our data do not support IBA and phonon interpretations of these transitional nuclei.