Nuclear Energy Levels in Lead Isotopes

The total neutron cross section of radiogenic lead was studied as a function of neutron energy in the range from 15 to 750 kev, using energy resolutions of 3 to 10 kev. Several resonances were observed; these were interpreted as caused by energy levels in the compound nucleus ${\mathrm{Pb}}^{207}$. The level spacing found in ${\mathrm{Pb}}^{207}$ was of the order of 50 kev. Maxima previously found in the cross section of ordinary lead and attributed to resonance interactions of neutrons with ${\mathrm{Pb}}^{208}$ were re-investigated with better energy resolution. No resonances were observed when earlier measurements of the cross section of Bi were extended to higher energies.