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 31 Al ground state is measured to be ν Q = | 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 31 Al has been deduced: | Q ( Al 31 ) | = 134.0 ( 16 ) mb . Comparison of this value with large-scale shell-model calculations in the sd and sdpf valence spaces suggests that the 31 Al ground state is dominated by normal sd-shell configurations with a possible small contribution of intruder states. The obtained value for | Q ( Al 31 ) | 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 .