An equilibrium population of approximately 500 hydrogenlike ${\mathrm{U}}^{91+}$ ions and ten fully stripped ${\mathrm{U}}^{92+}$ ions has been produced and trapped in an electron beam ion trap at 198-keV electron energy. The equilibrium ionization balance, determined from the intensities of radiative recombination x rays, gives values of 1.55 ± 0.27 and 2.82 ± 0.35 b for the electron impact ionization cross sections of hydrogenlike and heliumlike uranium, respectively. These values are somewhat larger than relativistic distorted wave calculations, and much smaller than previous values inferred from stripping of accelerator beams.

• Received 10 January 1994

DOI:https://doi.org/10.1103/PhysRevLett.72.4082