A double vacuum, configuration dependent pairing and lack of β-vibrations in transitional nuclei: Band structure of N = 88 to N = 91 Nuclei

We have used the AFRODITE γ-ray spectrometer to measure yrare structures in 152,154,155Gd using (α,xn) reactions. The measurements on 152,154Gd have shown that there is no convincing evidence that the lowest excited 0 2 + states are β-vibrations based on the 0 1 + ground state. Rather these levels may be regarded as a “second vacuum” each with its own γ and octupole vibrations. This is confirmed by the blocking of the coupling of this core 0 2 + state in 154Gd to the [ 505 ] 11 / 2 − single-particle quasi-neutron orbital in 155Gd. The coupling of this orbital to the 2 + γ-vibration is observed since this coupling is not Pauli-blocked.