90,91Zr photoproton spectra and the core-excitation model

Abstract Photoproton energy spectra have been measured for the 90,91 Zr(γ,P) 89,90 Y reactions at an E γ endpoint energy of 30 MeV, and for the 90,91 Zr,(e, e'p) 89,90 Y reactions at a number of different electron beam energies around E e = 21 MeV. Isotopically enriched target foils of metallic 90 Zr(97.65%) and 91 Zr(89.2%) were used, and the proton spectra measured at 90°. Prominent proton groups are observed in the 91 Zr spectra, particularly around E p ≈ 11 MeV, which closely resemble groups produced in the 90 Zr photoreactions. Moreover, the correlating non-ground state proton groups are being produced in transitions leaving the corresponding 89 Y, 90 Y residual nuclei in excited levels which also correlate in energy. These photoproton groups from 90 Zr have previously been identified as representing T > strength. A qualitative explanation is proposed in terms of the core-excitation model, in which the structure in the 91 Zr proton spectrum is described as representing dipole T > (T = 13 2 ) strength formed by coupling the 2d 5 2 neutron to dipole T > ( T = 6) excitations of a 90 Zr core.