We channeled 405-MeV/nucleon uranium ions in Si single crystals to determine the electron impact ionization cross section for berylliumlike-hydrogenlike uranium by 222-keV electrons. Our cross sections are 3.9, 11.0, 16.0, and 31.0 b ( + 100%, -50%), respectively, for ionizing Is, Is 2 , 2s, and 2s 2 electrons. Our Is and Is 2 results disagree with present theory. Our 2s and 2s 2 results are not accurate enough to distinguish between theories.PACS numbers: 34.80. Kw, 29.25.Fb, 29.70.Gn, 61.80.Mk In this Letter we report a novel application of channeling: the measurement of electron impact ionization cross sections for few-electron (relativistic) uranium ions. Until now there has been no way to make such measurements, which require, in addition to the very high charge state ions, a dense electron gas of known thickness.We obtain very high charge state uranium ions from the Lawrence Berkeley Laboratory's Bevalac and a dense electron gas of known thickness by channeling 1,2 the uranium ions through a Si single crystal. In the single crystal the atoms are arranged in a periodic structure with "channels" along which there are no nuclei. Ions traveling in these channels make only large-impactparameter collisions with the distant Si nuclei and thus do not acquire enough energy to ionize their tightly bound electrons.For each combination of ion and crystal, there is a maximum transverse energy beyond which the ion will be deflected out of the channel. For few-electron uranium at 405 MeV/nucleon, this transverse energy is reached when the angle between the ions and the crystal axis is about 0.01°. In our measurements, the necessary small transverse energy is achieved by our collimating the uranium ions with circular apertures of 0.30 and 0.15 cm in diameter separated by 10.6 m (C2 and C3, respec-tively, in Fig. 1). This defines a maximum beam divergence half-angle of 0.21 mrad (0.012°) and decreases the beam intensity by about a factor of 5000, yielding an average count rate of one channeled ion per second.In our experiment we use 405-MeV/nucleon uranium. Seen in the rest frame of the uranium, the electrons in the crystal have an energy of 222 keV. (The binding energy of U 91+ is «133 keV.) We measure ionization cross sections of incident charge states from hydrogenlike U 91+ through berylliumlike U 88+ . These charge states (and bare U 92+ ) are prepared by our stripping U 40+ ions at the exit of the Bevalac and magnetically separating the resulting charge states (Fig. 1). The ions lose roughly 2-MeV/nucleon energy in the stripper. The ions are then collimated and channeled along the (110) axis of a 0.11-or a 0.37-mm-thick Si single crystal. We use thick crystals because the charge changing cross sections are very small at relativistic energies. With thick crystals we can ignore the effects of small layers of dirt, oxides, and disoriented atoms on the crystal surface.After the ions exit the Si crystal, the resultant ion charge states are again magnetically analyzed (M4 in Fig. 1), and then detected by a position-sensi...
