X-rays from radiative electron capture of highly-charged heavy ions in an electron cooler

The x-ray spectra induced by spontaneous capture of free cooling electrons into bare and hydrogenlike heavy ions have been observed. Au78+, Au79+ and U91+ ions were stored in the storage ring ESR at velocities between 60% and 65% of the velocity of light and the x rays were measured in coincidence with stored particles having captured one electron in the electron cooler. The lines observed can be attributed to the direct transition of cooling electrons into theK shell of the projectile and to the characteristicL →K transitions. The latter are mainly induced by capture into higher shells and subsequent cascade transitions. The measurements are compared to detailed calculations within the framework of presently available theories. Although the relative structure of the measured spectra is in fair agreement with the theoretical prediction, the total charge-changing rate in the electron cooler is about a factor of three smaller than expected.