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 ν_Q = | frac(e Q V_{z z}, h) | = 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 (³¹Al) | = 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 (³¹Al) | 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 e_p = 1.1 e provides a much better description of the nuclear properties in the sd-shell than the adopted value e_p = 1.3 e.