Electric quadrupole moment of ¹⁷B and anomalous E2 effective charges for neutron-rich nuclei

The electric quadrupole moment Q for the ground state of ¹⁷B was determined by the β-NMR method applied to spin-polarized projectile fragments. From the measured quadrupole coupling constant, |eqQ/h(¹⁷ in Mg)| = 137.4±4.5 kHz, for ¹⁷B in a Mg single crystal stopper the Q moment was deduced as |Q(¹⁷B)| = 38.6±1.5 mb. As was the case in the previous ¹⁵B result, the obtained Q moment is very close to that for the N = 8 isotope ¹³B, in sharp contrast to the results of conventional shell-model calculations using constant effective charges. Comparison of the shell-model calculations indicates substantial reduction of the E2 effective charges for isotopes with large (N-Z)/A ratios. The results were well accounted for in terms of (N-Z)/A dependent E2 polarization charges as predicted by Bohr and Mottelson and by incorporating the decoupling feature of loosely bound particles.