Proton scattering on the radioactive nucleus 20O and the 0gs+→21+ transition in the neutron-rich oxygen isotopes

Elastic and inelastic proton scattering to the 21+ state of the single closed shell radioactive nucleus 20O have been measured in inverse kinematics with a beam energy of 30 MeV/u. The matrix element determined for the 0gs+→21+ transition in this work is compared with the corresponding electromagnetic matrix element to determine the ratio of the neutron and proton multipole matrix elements for this transition, Mn/Mp=2.9(4), which is quite different from the value Mn/Mp=N/Z=1.5 that would be expected for a purely isoscalar transition. A comparison of this result to a corresponding result for 18O suggests that the strength of the core polarizing interactions between the valence neutrons and the core protons and neutrons is changing significantly with mass.