High resolution inelastic electron scattering on ⁹⁰Zr at low momentum transfer and strong fragmentation of the magnetic quadrupole strength

High-resolution (FWHM ≈ 30 keV) inelastic electron scattering on ⁹⁰Zr at low momentum transfer (0.20 < q < 0.62 fm⁻¹) has been used to study magnetic transitions at excitation energies E_x = 8–10 MeV. The experimental data were analyzed in the distorted-wave Born approximation (DWBA) with wave functions calculated in the random phase approximation (RPA). Three J^π = 1⁺ states have been identified E_x = 8.233, 9.000 and 9.371 MeV. There is some indication of further very fragmented dipole strength and the upper limit for the total Ml strength in the investigated energy region is ΣB(M1) ↑ ⩽ 2.5 μ_K². It is much smaller than any theoretical prediction. Further-more, a large number of 2⁻ states has been observed, with the center of gravity located at E_x = 9 MeV. These states carry a total strength of ΣB(M2) ↑ = 1000 μ_K² · fm². Their strong fragmentatio is in qualitative agreement with theoretical calculations, but the deduced strength is much smaller than theoretically predicted. In addition the distributions of spacings and radiative widths of the 2⁻ states are consistent with a Wigner and a Porter-Thomas distribution, respectively.