EC-decay of highly ionized atoms

At the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany a unique combination of a heavy-ion synchrotron (SIS), a projectile fragment separator (FRS) and an experimental storage ring (ESR) with stochastic and electron cooling of coasting ions allows to produce, separate, store, cool, and study various decay modes of exotic nuclei with well-defined number of bound electrons. Masses and life-times of unstable ions can be measured by using high resolution time-resolved Schottky Mass Spectroscopy (SMS).

Using the sensitivity of SMS to measure the revolution frequency of cooled single ions in the ESR, which is a monotonous function of the specific charge q/M of the ion, the time dependence of the EC-decays of H-like ions was studied in 2008. The rates of the number of the EC-decays of H-like 140Pr and 142Pm ions deviate from the expected exponential decay law. The data were described by adding a modulation term with a modulation period T = 7 s, an amplitude of about 20 %, and a phase which was not well determined. In turn, the analysis of the β+-decay of the H-like 142Pm ions showed no deviation from the exponential decay law [PLB 664 (2008) 162].

Although the study has triggered a vivid discussion about the cause of this modulation (see e.g. http://www.nu.to.infn.it/Neutrino_Oscillations/) there exists no conclusive explanation so far. The observation, that the modulation exists in the two-body EC decay but not in the three-body β+-decay could however point to a weak-interaction origin of the modulation which is observed in connection with the mono-energetic electron-neutrinos from EC decays, but not visible for the continuous neutrino spectrum of the β+-decay branch.

The experiment with 142Pm was repeated in 2010 by using a 245 MHz resonator cavity with much improved sensitivity and time resolution. And again a modulation of the exponential decay was found with a period of 7.11 seconds but this time with a somewhat smaller amplitude of ~12 % [1]. No deviation from the exponential decay law was found for the β+-decay in agreement with the previous results. The data analysis of these 2010 data has been carried out by two independent groups at GSI and SMI. At SMI an automatic analysis method was developed for this purpose.

During the measuring period in 2008, in addition to the ion species 142Pm and 140Pr, also the decay of 122I has been investigated. From this data set only a preliminary analysis exists. The final analysis is subject of a diploma thesis which is carried out at SMI and which started in October 2013 and is planned to be finished until the end of 2014. Following the positive experience with the analysis of the 2010 data, independent analysis at GSI and SMI will be combined to obtain the final result.

[1] K. Kienle et al., PLB 726 (2013) 638].