Direct Q -Value Determination of the β − Decay of 187 Re

The cyclotron frequency ratio of 187 Os29+ to 187Re29+ ions was measured with the Penning-trap mass spectrometer PENTATRAP. The achieved result of R = 1.000 000 013 882 ( 5 ) is to date the most precise such measurement performed on ions. Furthermore, the total binding-energy difference of the 29 missing electrons in Re and Os was calculated by relativistic multiconfiguration methods, yielding the value of Δ E = 53.5 ( 10 ) eV . Finally, using the achieved results, the mass difference between neutral 187Re and 187Os, i.e., the Q value of the β − decay of 187Re, is determined to be 2470.9(13) eV.