Precision measurement of the electric quadrupole moment of 31Al and determination of the effective proton charge in the sd-shell

The electric quadrupole coupling constant of the 31Al ground state is measured to be νQ=|eQVzzh|=2196(21)kHz using two different β-NMR (Nuclear Magnetic Resonance) techniques. For the first time, a direct comparison is made between the continuous rf technique and the adiabatic fast passage method. The obtained coupling constants of both methods are in excellent agreement with each other and a precise value for the quadrupole moment of 31Al has been deduced: |Q(Al31)|=134.0(16)mb. Comparison of this value with large-scale shell-model calculations in the sd and sdpf valence spaces suggests that the 31Al ground state is dominated by normal sd-shell configurations with a possible small contribution of intruder states. The obtained value for |Q(Al31)| and a compilation of measured quadrupole moments of odd-Z even-N isotopes in comparison with shell-model calculations shows that the proton effective charge ep=1.1e provides a much better description of the nuclear properties in the sd-shell than the adopted value ep=1.3e.