Evidence for CollectiveM1Strength inPb208between 8 and 10 MeV

Abstract

Very-high-energy-resolution photoneutron time-of-flight measurements in combination with high-resolution measurements of photoneutron polarizations from the reaction ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}(\gamma , n_0\uparrow ){}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$ have enabled us to identify seven probable $1^{+}$ resonances at excitations between 8.2 and 9.5 MeV. These resonances have a total strength $B{\mathrm{}\left(M1\mathrm{}\right)\mathrm{}}_{\uparrow }\gtrsim (8.5\mathrm{}\pm 0.5){{\mu }_0}^2$. This strength plus that previously reported at 7 and at 8 MeV can account for the $M1\mathrm{}$ sum rule in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$.