Electric quadrupole moment of the $^{33}$Al ground state

The ground-state $Q $moment of $^{33}$Al has been measured by applying the $\beta $-ray-detected nuclear magnetic resonance technique to spin-polarized $^{33}$Al fragments produced in the projectile fragmentation reaction. The obtained $Q $moment, $\vert Q_{exp}(^{33}$Al)\textit{$\vert $ }= 132(16) $e$mb, shows a significant deviation from the theoretical value predicted in the shell model calculation with the USD interaction. The deviation will be discussed in context of possible erosion of the $N $= 20 shell gap, by comparing the experimental $Q$ and theoretical predictions from the large scale shell model calculation and the particle-vibration coupling calculation.