Measurement of electric quadrupole moments of neutron-rich nuclei¹⁴B and¹⁵B

The experimental study of nuclear moments of several neutron-rich nuclei, using the phenomenon of ejectile spin polarization in the projectile fragmentation reaction, is reported. The spin-polarized fragments ¹⁴B and ¹⁵B were produced in the fragmentation of ¹⁸O projectiles on a Nb target at 64.7 MeV/u. They were implanted into a Mg single crystal, and the quadrupole moments Q were determined by the β-NMR method. The results, β18O = 29.84 ± 0.75 mb and (15B)| = 38.01 ± 1.08 mb, are compared with theoretical models. Shell model calculations in a Ofiut model space, using effective charges commonly accepted in this mass region, predict values about 40% larger than the experimental Q(¹⁵B). Agreement is obtained when a much smaller effective charge for neutrons is employed. The method was also applied to the measurement of the magnetic moments of ¹⁷N and ¹⁷B. The result reveals an important contribution of configurations with excess neutrons coupled to form J^π = 2⁺.