Beta decay of highly-ionized atoms plays a significant role in stellar nucleosynthesis at temperatures of about 30 keV (s-process) where most nuclei are in a high atomic charge state. The facility at GSI, Darmstadt, providing both unstable highly-charged nuclides and an ion storage-cooler ring (ESR) to preserve their high charge state over a long time (hours) was and still is the only place addressing this field which is interesting for nuclear physics as well as for astrophysics.
During the last decade, the focus was on the investigation of two-body beta decays, i.e. bound-state beta decay and orbital electron capture (EC), where monochromatic (anti)neutrinos in the electron-flavour eigenstate are created. In course of the first measurements of the EC decay probability of few-electron ions it turned out that hydrogen-like 140Pr58+ and 142Pm60+ nuclides decay by about 50% faster than the helium-like ions, and even faster than the corresponding neutral atoms.
This result, although somewhat surprising, can be fully understood in the framework of standard nuclear physics. A few years ago, a new technique, single-ion decay spectroscopy has been developed at the ESR. Here, the number of stored ions is reduced to less than four and the "fate" of each single stored ion is observed continuously and time-resolved. On top of the expected exponentially decreasing EC decay probability, for both hydrogen-like 140Pr and 142Pm ions, periodic modulations were found with a period of about 7s and relative amplitude of 0.2. Tentatively, we argued that these oscillations could be due - as a special kind of "quantum beats"- to the coherent superposition of (at least) two mass eigenstates of the generated electron-neutrino which is a flavour eigenstate, but neither an energy- nor momentum eigenstate.
This very controversially discussed hypothesis predicts that similar modulations should also appear in other two-body beta decays with a period being proportional to the mass of the parent ion. To corroborate or disprove this hypothesis, some months ago an experiment with hydrogen-like 122I ions has been conducted, where a modulation period of about 6s is expected, supposed this "neutrino hypothesis" holds true. First results will be reported.