X rays from radiative electron capture of free cooling electrons for precise Lamb-shift measurements at high Z: Au78+

Abstract Spontaneous capture of free cooling electrons by bare high- Z ions leads to the emission of X rays which have been observed for the first time. The distinctive features of this X-ray source for precision spectroscopy of high- Z few-electron systems are demonstrated. They enabled a determination of the 1s Lamb shift of hydrogen-like Au 78+ (212±15 eV ) to an accuracy of 7%. This is more precise than any other measurement previously reported for a very heavy ion.