Large quadrupole deformation in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mi>Ne</mml:mi> <mml:mprescripts/> <mml:none/> <mml:mn>20</mml:mn> </mml:mmultiscripts> </mml:math> challenges rotor model and modern theory

The spectroscopic quadrupole moment of the first excited state, ${Q}_{{}_{S}}({2}_{1}^{+})$, at 1.634 MeV in $^{20}\mathrm{Ne}$ was determined from sensitive reorientation-effect Coulomb-excitation measurements using a heavy target and safe energies well below the Coulomb barrier. Particle-$\ensuremath{\gamma}$ 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 ${Q}_{{}_{S}}({2}_{1}^{+})=\ensuremath{-}0.22(2)\phantom{\rule{0.16em}{0ex}}e\mathrm{b}$ was determined at backward angles in agreement with the only safe-energy measurement prior to this work, ${Q}_{{}_{S}}({2}_{1}^{+})=\ensuremath{-}0.23(8)\phantom{\rule{0.16em}{0ex}}e\mathrm{b}$. This result adopts $1\ensuremath{\hbar}\ensuremath{\omega}$ 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 $3\ensuremath{\sigma}$. 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 ${2}_{1}^{+}$ state in $^{20}\mathrm{Ne}$ calculated with the MR-EDF model illustrates the presence of $\ensuremath{\alpha}$ clustering, which explains the largest discrepancy with the rotor model found in the nuclear chart and motivates the explicit inclusion of $\ensuremath{\alpha}$ clustering for full convergence of $E2$ collective properties.

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