Precise energy determination of the first excited state in 137Cs for absolute energy calibration in muonic atoms

Abstract The energy of the γ-ray de-exciting the 5 2 1 + level of 137 Cs was determined with high accuracy in an isotope separator on-line experiment. The result is (455.490±0.003) keV as referred to the 198 Au energy standard of (411.794±0) keV. Knowledge of this energy is important for the determination of muonic X-ray energies in Ba as a test of the validity of quantum electrodynamics. In addition, the 4 1 + →2 1 + transition in 138 Cs was determined to be (462.785±0.005) keV.