Recently Lawrence Livermore National Laboratory produced bare uranium ions (U92+) in Super-EBIT. This achievement is the first time these ions have been produced without the use of a high-energy particle accelerator.
The bare uranium ions produced by Super-EBIT are essentially at rest, not moving at relativistic velocities like those produced in accelerators. This feature makes them useful for a number of previously impossible studies, such as the interaction of low-energy highly-charged ions with surfaces and precision studies of binding energies. The low energy of these ions also makes them available for experiments in the RETRAP, which requires cold, highly-charged ions.
The ions were produced in Super-EBIT with an electron beam energy of about 198 keV, well above the binding energy of the most tightly bound electron (that of hydrogenic uranium) of about 131 keV. The were detected by means of their emitted radiative recombination. From the intensity of the recombination, it can be infered that there were about 8 bare uranium ions in the trap at any time.
Also the radiative recombination was used to infer the ionization cross section for heliumlike and hydrogenic uranium, as described in a paper on the subject (Phys. Rev. Lett. 72, 4082 (1994)).
Below is a spectrum of the radiative recombination showing the features from heliumlike, hydrogenic, and bare uranium ions.