High-resolution photoneutron study ofE1andM1transitions inPb208

Abstract

The photoneutron cross section of ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ 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 ${}_{}{}^2{}_{}{}^{}\mathrm{H}$($\gamma$,$n$)${}_{}{}^1{}_{}{}^{}\mathrm{H}$ 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 ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ 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.