Quadrupole moments of B isotopes and anomalous E2 effective charges for neutron-rich nuclei

The electric quadrupole moments Q of the B isotopes have been measured recently up to the mass number A = 17. The observed Q moment for 17B as well as that for 15B are very close to that for the N = 8 isotope 13B, contrary to the shell-model expectation with constant effective charges that Q increases rapidly with increasing number of neutrons added to the 13B core. Analysis of these results reveals substantial reductions of the E2 effective charges for nuclei with large (N-Z)/A ratios, and cautions that the commonly accepted notion of constant effective charges may not hold in nuclei far from the β-stability line. We discuss the variation of effective charges in relation to the change in properties of the isoscalar and isovector giant resonances in neutron rich nuclei. (author)