Study of transversal nuclear excitations in 90Zr and 140Ce by high-resolution inelastic electron scattering

By high-resolution inelastic electron scattering in the nuclei 90Zr and 140Ce magnetic dipole and quadrupole excitations were searched. For this ten 90Zr spectra in the excitation energy range Esub(x) = 8 to 10 MeV at electron energies E0 = 24 to 67 MeV at scattering angles theta = 1410 to 1650 and six 140Ce spectra in the excitation energy range Esub(x) = 5.5 to 11.5 MeV at electron energies Esub(o) = 30 to 50 MeV at scattering angles theta = 930, 1290, and 1650 were taken up. The analysis of the experimental cross sections was performed by microscopical DWBA calculations using RPA amplitudes which were determined in the framework of a separable residual interaction model. In 90Zr three Jsup(π) = 1+ states at Esub(x) = 8.233, 9.000, and 9.371 MeV were found. There are however indications to further strongly fragmented M1-strength. The upper limit for the whole observed M1-strength in the studied excitation energy range is estimated to μ B(M1) up <= 2.5 μ2sub(N). Furthermore a large number of Jsup(π) = 2- states were found, the center of mass of which lies at Esub(x) approx.= 9 MeV. The analysis of the 140 Ce spectra showed that a hitherto as giant M1-resonance interpreted broad structure at Esub(x) approx.= 8.7 MeV represents an accumulation of Jsup(π) = 2- states with a whole strength of μ B(M2) up = (6000 +- 2500) μsub(N)2 fm2. An estimation of the level density of the Jsup(π) = 2- states led to a mean level distance of about 10 keV for 90 Zr and about 2 keV for 140 Ce. (orig./HSI)