The 2s atomic level in muonic 140Ce

Abstract We have observed the 2 s → 2 p and the 3 p → 2 s muonic X-ray doublets in 140 Ce. Assuming a two-parameter Fermi distribution we find our data are best fit by allowing a 5.4 ± 0.7 keV shift downward in the 1s 1 2 level in reasonably good agreement with the theoretical calculations for nuclear polarization. The sensitivity of our results to a three-parameter Fermi distribution is explored.