The structure of low-lying 1− states in 90,94Zr from (α,α′γ) and (p,p′γ) reactions

Abstract The low-lying dipole strength in the 90,94Zr nuclei was investigated via ( p , p ′ γ ) at 80 MeV and ( α , α ′ γ ) at 130 MeV. The experiments, made at RCNP, used the magnetic spectrometer Grand Raiden for the scattered particles and the array CAGRA with HPGe detectors for the γ-decay. For 94Zr these are the first data for both reactions and for 90Zr these are the first data with ( p , p ′ γ ) and the first ones at high resolution for ( α , α ′ γ ). The comparison of the present results for the two nuclei with existing ( γ , γ ′ ) data shows that both nuclear probes produce an excitation pattern different than that of the electromagnetic probes. DWBA calculations were made using form factors deduced from transition densities, based on RPA calculations, characterized by a strong neutron component at the nuclear surface. A combined analysis of the two reactions was performed for the first time to investigate the isoscalar character of the 1− states in 90,94Zr. The ( p , p ′ γ ) cross section was calculated using values for the isoscalar electric dipole energy-weighted sum rule (E1 ISEWSR) obtained from the ( α , α ′ γ ) data. The isoscalar strength for 90Zr was found to exhaust 20 ± 2.5% of the EWSR in the energy range up to 12 MeV. In case of 94Zr, a strength of 9 ± 1.1% of the EWSR was found in the range up to 8.5 MeV. Although an overall general description was obtained in the studied energy intervals, not all proton cross sections were well reproduced using the isoscalar strength from ( α , α ′ γ ). This might suggest mixing of isoscalar and isovector components and that this mixing and the degree of collectivity are not the same for all the 1− states below the particle binding energy.