Pb206+nresonances forE=25−600keV:s-,p-, andd-wave doorway states andM1ground-state radiative strength inPb207

High resolution neutron transmission and differential scattering measurements have been performed on ${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}$ and the results for $E=25-600\mathrm{}$ keV are presented. Data analyses were performed utilizing multilevel $R$-matrix codes. Resonance parameters (i.e., $E$, 1, $J$, and ${\Gamma }_n$) were deduced for a large fraction of the 223 resonances observed. In addition to the previously known doorway in the $s$-wave channel, the $p$-wave neutron strength function exhibits significant changes near 40 and 145 keV indicative of doorway states in the $p_{\frac{1}{2}}$ and $p_{\frac{3}{2}}$ channels, respectively. These results are in good agreement with those observed in a study of the ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$+$n$ reaction. It is suggested that these doorway states arise from a ($d_{\frac{5}{2}}$, $3^{-}$) particle-core excitation. The $d$-wave strength function shows indications of a doorway at about $E=425\mathrm{}$ keV. The latter represents doorway resonances in the $d_{\frac{3}{2}}$ and $d_{\frac{5}{2}}$ channels which probably arise from a recoupling of the particle-core excitation that has been used to explain the $s$-wave doorway. A cumulative plot of the number of $s$-wave resonances vs $E$ shows considerable curvature which indicates an increase of about 12% in the level density per 100 keV of excitation. These data were analyzed in terms of a constant temperature model with a nuclear temperature of about 0.9 MeV. The model reproduced the cumulative plot of $d$-wave resonances reasonably well up to $E\approx 500$ keV, but underestimated the number of $p$-wave resonances. This indicates a parity dependence of the level density formula for this nucleus. Many of the resonances which had previously been thought to contribute to the ground-state $M1\mathrm{}$ radiative strength are in fact formed by $s$ and $d$ waves, and hence, decay by $E1$ radiation. We deduce a considerably smaller amount of $M1\mathrm{}$ radiation for $E_x=6738-7340\mathrm{}$ keV in ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$ than had been reported earlier.