Observation of Non-Exponential Orbital Electron Capture Decays of   Hydrogen-Like $^{140}$Pr and $^{142}$Pm Ions
  We report on time-modulated two-body weak decays observed in the orbital electron capture of hydrogen-like $^{140}$Pr$^{59+}$ and $^{142}$Pm$^{60+}$ ions coasting in an ion storage ring. Using non-destructive single ion, time-resolved Schottky mass spectrometry we found that the expected exponential decay is modulated in time with a modulation period of about 7 seconds for both systems. Tentatively this observation is attributed to the coherent superposition of finite mass eigenstates of the electron neutrinos from the weak decay into a two-body final state. 
Yu.A. Litvinov, F. Bosch, N. Winckler, D. Boutin, H.G. Essel, T.   Faestermann, H. Geissel, S. Hess, P. Kienle, R. Kn\"obel,...