Electromagnetic Transition Strengths in ${\mathrm{Pb}}^{206}$, ${\mathrm{Pb}}^{207}$, and ${\mathrm{Pb}}^{208}$ from Threshold Photoneutron Cross-Section Measurements

The photoneutron cross sections for ${\mathrm{Pb}}^{206}$, ${\mathrm{Pb}}^{207}$, and ${\mathrm{Pb}}^{208}$ have been measured for photon energies up to 40 keV above threshold. Several resonances were resolved for each isotope. Most of the resonances seen in the ${\mathrm{Pb}}^{207}$ and ${\mathrm{Pb}}^{208}$ spectra correspond to resonances detected by neutron-capture or total cross-section measurements on ${\mathrm{Pb}}^{206}$ and ${\mathrm{Pb}}^{207}$, respectively. Values for ${\Gamma }_{\gamma 0}$, the ground-state $\gamma$-ray width, were determined for a number of resonances in each target. By comparison with the neutron-capture experiments, the multipolarities of the $\gamma$-ray transitions were determined, and an average $\gamma$-ray transition strength was obtained. The $M1\mathrm{}$ transition strengths for ${\mathrm{Pb}}^{206}$ and ${\mathrm{Pb}}^{207}$ were found to be markedly enhanced over the expected value. One $E2\mathrm{}$ transition in ${\mathrm{Pb}}^{208}$ was found to have a strength of 0.14 Weisskopf units, and there is a possibility of two other $E2\mathrm{}$ transitions with similar strength.