Large quadrupole deformation in Ne 20 challenges rotor model and modern theory

The spectroscopic quadrupole moment of the first excited state, QS(21+), at 1.634 MeV in Ne20 was determined from sensitive reorientation-effect Coulomb-excitation measurements using a heavy target and safe energies well below the Coulomb barrier. Particle-γ coincidence measurements were collected at iThemba LABS with a digital data-acquisition system using the AFRODITE array coupled to an annular, doubled-sided silicon detector. A precise value of QS(21+)=-0.22(2)eb was determined at backward angles in agreement with the only safe-energy measurement prior to this work, QS(21+)=-0.23(8)eb. This result adopts 1ℏω shell-model calculations of the nuclear dipole polarizability of the 21+ state that contributes to the effective quadrupole interaction and determination of QS(21+). It disagrees, however, with the ideal rotor model for axially symmetric nuclei by almost 3σ. Larger discrepancies are computed by modern state-of-the-art calculations performed in this and prior work, including ab initio shell model with chiral effective interactions and the multireference relativistic energy density functional (MR-EDF) model. The intrinsic nucleon density of the 21+ state in Ne20 calculated with the MR-EDF model illustrates the presence of α clustering, which explains the largest discrepancy with the rotor model found in the nuclear chart and motivates the explicit inclusion of α clustering for full convergence of E2 collective properties.