HIGH-RESOLUTION PHOTO-NEUTRON STUDY OF E1 AND M1 TRANSITIONS IN PB-208

The photoneutron cross section of Pb208 was observed using a very high resolution time-of-flight spectrometer. The cross sections were observed in the photoneutron energy range 16 to 1000 keV and at reaction angles of 90°and 135°. The deduced ground-state radiative widths were calibrated to the well-known H2(γ,n)H1 reaction cross section. The 7.99-MeV resonance, previously believed to be an M 1 excitation, is shown to be an E 1 resonance. Current progress in the search for the collective M 1 resonance in Pb208 is reviewed. In addition, recent theoretical predictions of the properties of the giant M 1 resonance are reviewed. Finally, the present high resolution observations in conjunction with previous photoneutron polarization measurements were employed in order to deduce the s-d-wave admixtures for eight E 1 excitations. NUCLEAR REACTIONS Pb208(γ,n)Pb207, Eexc=7.4-8.4 MeV, observed σ(θ), θ=90°,135°; deduced Γγ0, Jπ. © 1979 The American Physical Society.