Large quadrupole deformation in \(^{20}\)Ne challenges rotor model and modern theory: urging for \(\alpha\) clusters in nuclei

The spectroscopic quadrupole moment of the first excited state, \(Q_{_S}(2^{+}_{1})\), at 1.634 MeV in \(^{20}\)Ne was determined from sensitive reorientation-effect Coulomb-excitation measurements using a heavy target and safe energies well below the Coulomb barrier. Particle-\(\gamma\) coincidence measurements were collected at iThemba LABS with a digital data-acquisition system using the {\sc AFRODITE} array coupled to an annular, doubled-sided silicon detector. A precise value of \(Q_{_S}(2^{+}_{1})=-0.22(2)\) eb was determined at backward angles in agreement with the only safe-energy measurement prior to this work, \(Q_{_S}(2^{+}_{1})=-0.23(8)\) eb. This result adopts 1\(\hbar\omega\) shell-model calculations of the nuclear dipole polarizability of the 2\(^+_1\) state that contributes to the effective quadrupole interaction and determination of \(Q_{_S}(2^{+}_{1})\). It disagrees, however, with the ideal rotor model for axially-symmetric nuclei by almost \(3\sigma\). Larger discrepancies are computed by modern state-of-the-art calculations performed in this and prior work, including {\it ab initio} shell model with chiral effective interactions and the multi-reference relativistic energy density functional ({\sc MR-EDF}) model. The intrinsic nucleon density of the 2\(^+_1\) state in \(^{20}\)Ne calculated with the {\sc MR-EDF} model illustrates the presence of \(\alpha\) clustering, which explains the largest discrepancy with the rotor model found in the nuclear chart and motivates the explicit inclusion of \(\alpha\) clustering for full convergence of \(E2\) collective properties.