2. Many of the results were obtained by using Infinite rather than semi-Infinite geom etries. This was done either to simplify the measurement or to facilitate compari son with Spencer's early calculations. In any case, the semi-infinite geometry is of more practice! importance, 3. In the caae of semi-Infinite geometries, there are very little data for electron energies leas than 1.0 MeV for nonnormal incidence, and for multlalab media, 4. Spatial resolution near the surface of aemi-Infinite geometries Is poor, especially for hlgh-atomic-numbcr media. 0,3 a.4 as a* FRACTION OF A ftCAIf RANGE Figure 1.1, Comparison of Experimental Measurements and Theoretical Predictions of Energy Deposition Profiles in Seml-Infinlte Aluminum by Normally Incident 0. 5-MeV Electrons

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A new collision chamber design has made possible differential measurements of scattering of protons on atomic hydrogen target atoms. The scattering takes place in the interior of a furnace where hydrogen gas is dissociated. Electron capture probability is measured vs proton energy for protons passing nearly through the center of isolated hydrogen atoms such that the incident particle is scattered through an angle of 3°. This shows a resonant structure with maxima at energies of 0.78, 1.57, 3.92, and 20.1 kev. A simple model is presented for looking at these resonances. In the light of this model and previous theoretical work, the location of the high-energy maximum is unexpectedly low, and this suggests that a phase constant is needed to achieve agreement between experiment and theory. The H + on H data presented here are compared with those for the other combinations, H + on H 2 , H + on He, and He+ on He, which also show resonant electron capture.