Orbital Electron Capture Rate and Time Modulation of H- and He-like Ions in a Storage-Ring

Abstract

We have studied in a heavy ion storage ring the orbital electron capture decays of H- and He-like ¹⁴⁰Pr and ¹⁴²Pm ions and found that the H-like ions with one electron in the K-shell decay 1.49(8) and 1.44(6) times faster, than the corresponding He-like ions with two electrons in the K-shell. This result is explained by spin statistics due to the hyperfine structure of the H-like ions.

The measured time dependence of the electron capture rate of H-like ¹⁴⁰Pr⁵⁸⁺, ¹⁴²Pm⁶⁰⁺, and ¹²²I⁵²⁺ ions is not constant but time modulated with periods of T=7.06(8) s, T=7.10(22) s, and T=6.04(6) s, respectively, in the laboratory system of the ions moving with 71% of speed of light (Lorentz factor γ=1.43) and with relative amplitudes of a=0.18(3), 0.23(4), and 0.22(2), respectively. Such modulation periods correspond to small energy differences for a quantum beat type phenomenon possibly related to the energy differences of superimposed mass-eigenstates of electron neutrinos with a squared mass difference of 2.19(2)×10⁻⁴ eV². It is 2.9 times larger than reported by the KamLAND antineutrino oscillation experiment. The simultaneously measured β ⁺ branch of ¹⁴²Pm shows no modulation with an amplitude limit a<0.03, as is expected for the broad neutrino spectrum in the case of three-body decays. Mass corrections of the neutrinos by virtual lepton-W boson loops in the high Coulomb field of the daughter nuclei are discussed.