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\ifmmode^\circ\else\textdegree\fi{} and 135\ifmmode^\circ\else\textdegree\fi{}. The deduced ground-state radiative widths were calibrated to the well-known $^{2}\mathrm{H}$($\ensuremath{\gamma}$,$n$)$^{1}\mathrm{H}$ 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 $s\ensuremath{-}d$-wave admixtures for eight $E1$ excitations.NUCLEAR REACTIONS $^{208}\mathrm{Pb}$($\ensuremath{\gamma}$,$n$)$^{207}\mathrm{Pb}$, ${E}_{\mathrm{exc}}=7.4\ensuremath{-}8.4$ MeV, observed $\ensuremath{\sigma}(\ensuremath{\theta})$, $\ensuremath{\theta}=90\ifmmode^\circ\else\textdegree\fi{}, 135\ifmmode^\circ\else\textdegree\fi{}$; deduced ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}0}$, ${J}^{\ensuremath{\pi}}$.