Are There Signatures of Harmonic Oscillator Shells Far from Stability? First Spectroscopy of ¹¹⁰Zr

The first measurement of the low-lying states of the neutron-rich ¹¹⁰Zr and ¹¹²Mo was performed via in-beam γ-ray spectroscopy after one proton removal on hydrogen at ∼200 MeV/nucleon. The 2₁⁺ excitation energies were found at 185(11) keV in ¹¹⁰Zr, and 235(7) keV in ¹¹²Mo, while the R₄₂ = E(4₁⁺)/E(2₁⁺) ratios are 3.1(2), close to the rigid rotor value, and 2.7(1), respectively. These results are compared to modern energy density functional based configuration mixing models using Gogny and Skyrme effective interactions. We conclude that first levels of ¹¹⁰Zr exhibit a rotational behavior, in agreement with previous observations of lighter zirconium isotopes as well as with the most advanced Monte Carlo shell model predictions. The data, therefore, do not support a harmonic oscillator shell stabilization scenario at Z = 40 and N = 70. The present data also invalidate predictions for a tetrahedral ground state symmetry in ¹¹⁰Zr. (authors)