Evidence for Doorway States inPb207,Pb208, andFe57from Threshold Photoneutron Cross-Section Measurements

The proposed doorway state in ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$ seen by neutron total cross-section measurements on ${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}$ also has been observed in the ($\gamma ,n$) reaction. Both the envelope of the strength and the correlation between fine structure in the two channels support the doorway-state interpretation. The width for $\gamma$-ray decay of the doorway state to the ground state ${\Gamma }_{\gamma 0}^D$ is found to be 36.5 ± 3.5 eV. Other possible doorway states in the $\gamma$-ray channel in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ at 500 keV, ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$ at 125 keV, and ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ at 50 and 250 keV above threshold are discussed. An attempt is made to produce a correlation between neutron and $\gamma$-ray widths; no significant correlation was observed for these nuclei.