Neutron Total Cross Section ofPb204for5<En<80keV and ofPb208for25<En<84keV

The neutron total cross section of ${}_{}{}^{204}{}_{}{}^{}\mathrm{Pb}$ was studied for neutron energies from 5 to 80 keV, using fast time-of-flight techniques. At least 10 $s$-wave resonances were observed in this interval in addition to a number of probable $p$-wave resonances. The average observed $s$-wave level spacing is 2.7±0.5 keV, and the $s$-wave strength function is (0.55±0.2)×${10}^{-4\mathrm{}}$. The total cross section of ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ was measured from 26 to 84 keV. Resonances were observed at about 70 and 77 keV, both apparently due to $l=1\mathrm{}$ neutrons. The effective nuclear radius $R^{\prime }$ for ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}+n$ was found to be 8.4±0.3 F.