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. (c) 2008 Published by Elsevier B.V.