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 0₂⁺ states are β-vibrations based on the 0₁⁺ 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 0₂⁺ 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.