Muonic x rays in ${\mathrm{Pb}}^{206}$ were measured with high precision and in addition two of the weak transitions involving the $2s$ state were identified and measured for the first time. The new constraints make it difficult to fit the data with Dirac energies calculated for a finite nucleus and corrected for radiative effects. Consistency can, however, be achieved by depressing the calculated $1s$ energy by 6.8×2.3 keV, an effect tentatively interpreted as due to polarization of the nucleus by the muon.

• Received 2 December 1968

DOI:https://doi.org/10.1103/PhysRevLett.22.221