The quadrupole moment of the 8 B (/* = 2 + , Ti/2=769 msec) nucleus was measured as |(?( 8 B)| = 68.3 ±2.1 mb by use of modified /2-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 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.PACS numbers: 21.10. Ky, 21.10.Ft, 27.20.+n Nuclear properties that depend upon isospin will be enhanced and clearly observed in high isospin states, i.e., in those nuclei located near the proton and neutron drip lines in the mass chart. One of the peculiar observables of those nuclei is the radius of the nuclear matter distribution. For example, from the measurements of interaction cross sections by use of a high-energy nuclear beam of unstable neutron-rich n Li, a neutron halo covering the 9 Li core was found by Tanihata et al. [1]. Regarding the proton halo, however, no signals have been reported yet. It is quite natural to expect such a halo for protons because of the charge symmetry of the nuclear force. However, the Coulomb force among the protons besides the nuclear force may prevent the growth of the halo and push the proton inside the Coulomb barrier. As a result, the proton halo may be less pronounced than the neutron halo even if it is formed. From an experimental point of view, it is difficult to detect the thin halo by such crosssection measurements because the method is mainly designed to observe matter distributions, and therefore the effect due to the proton halo could be only a small fraction of the cross section. For an investigation of the proton halo, on the other hand, quadrupole moments are the most suitable probe, since they reflect dominantly the radial and angular distributions of the valence protons in the case of spherical nuclei. Of specific interest is the quadrupole moment of the 8 B (/ ;r = 2 4 \ T\/2 == 169 msec) nucleus because it is located near the proton drip line. It is expected that the proton distribution swells out radially in this nucleus because the separation energy of one proton is very small, 0.14 MeV, while, on the contrary, that of the neutron is large, 13 MeV. This means that the valence protons are very loosely bound at the shallow ridge of the nuclear potential, while the neutrons are very tightly bound./3-NMR detections [2] are the most promising method to measure the quadrupole moments of 8 B since it is a short-lived f3 emitter. As is well known, however, the detection of the quadrupole effects in the /3-NMR of such unstable nuclei with a nuclear lifetime of about 1 sec is usually very difficult, and time consuming, because of the complexity of the spectral shape and small NMR effect due to its higher spin (7=2 for 8 B), and because of the poor knowledge of the electric field gradients. Therefore, ne...
