Decoupling of valence neutrons from the core in 17B

Proton inelastic scattering study of 17B nucleus has been performed at 43.8 A MeV average energy in inverse kinematics using a liquid hydrogen target. In the Doppler corrected γ ray spectrum, a single peak with an energy of 1089(15) keV appeared corresponding to the first excited state of 17B. Based on the reaction cross section, β2pp′ deformation parameter of the transition is deduced by distorted wave analysis. Comparing it with the electric quadrupole moment of the nucleus, it is shown that the deformation of the neutron distribution is large (βn∼0.6), and the neutron effective charge is much smaller (en<0.12) than usual (∼0.5). The small effective charge is interpreted as a consequence of the decoupling of valence neutrons from the core.