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

The electric quadrupole moment Q for the ground state of B-17 was determined by the beta-NMR method applied to spin-polarized projectile fragments. From the measured quadrupole coupling constant, \eqQ/h(B-17 inMg)\ = 137.4+/-4.5 kHz, for B-17 in a Mg single crystal stopper the Q moment was deduced as \Q(B-17)\ = 38.6+/-1.5 mb. As was the case in the previous B-15 result, the obtained Q moment is very close to that for the N = 8 isotope B-13, 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.