Proton halo ofB8disclosed by its giant quadrupole moment

The quadrupole moment of the ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ (${\mathit{I}}^{\mathrm{\pi }}$=$2^{+}$, ${\mathit{T}}_{1/2}$=769 msec) nucleus was measured as ‖Q${(}^8$B)‖=68.3±2.1 mb by use of modified β-NMR. This value is twice as large as the prediction of the Cohen-Kurath shell model. It is found by subtracting the contribution of deeply bound neutrons that the protons in ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ carry more than 90% of the observed moment. The anomalous value is accounted for fairly well by the proton halo due to the loosely bound valence configuration. This is the first experimental evidence for the existence of a proton halo covering a neutron core.