High-resolution photoneutron study of E1 and M1 transitions in 208Pb

The photoneutron cross section of 208Pb 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 900 and 1350. The deduced ground-state radiative widths were calibrated to the well-known 2H(γ,n)1H reaction cross section. The 7.99-MeV resonance, previously believed to be an M1 excitation, is shown to be an E1 resonance. Current progress in the search for the collective M1 resonance in 208Pb is reviewed. In addition, recent theoretical predictions of the properties of the giant M1 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 E1 excitations