An ion beam apparatus is employed to study the reactions of uranium ions with N2, D2, and CD4 at laboratory energies up to 335 eV. The endothermic reaction of U+ with nitrogen leads to the product UN+ for which a bond dissociation energy D (UN+) =4.7±0.2 eV is determined, corresponding to ΔHf(UN+) =272±7 kcal/mole. Endothermic reactions of U+ with D2 and CD4 lead to formation of UD+ with D (UD+) =2.9±0.1 eV and ΔHf(UD+) =254±6 kcal/mole. The dependence of experimental reaction cross sections on relative kinetic energy is discussed in terms of simple models for reaction. The proton affinity of uranium is determined, P.A.(U) =238±5 kcal/mole, and this exceptionally high value is compared to other atomic and molecular base strengths.

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Spark-breakdown delay times were measured for N2, H2, Ar, SF6, and CC12F2 in a uniform field gap provided with a small current (-10 ' A) of free electrons by uv illumination of the cathode. Laue plots of the delay times yielded straight lines with slope iP, where i is the photocurrent and P is the breakdown probability. The dependence of the breakdown probability on voltage for N2, H&, and Ar was in good agreement with predictions of the Townsend breakdown mechanism. In SF6 and CC12F2, a transition was observed with increasing pressure from a dependence that agreed with the Townsend theory to a more gradual rise with voltage, characteristic of a streamer mechanism. This transition was ascribed to a decrease in the secondary-ionization coefficient with increasing pressure in SF6 and CC12F2, which resulted in an average electron-avalanche size at the static breakdown voltage that approached the critical value for streamer formation. A unified breakdownprobability theory, for which the Townsend and streamer mechanisms are limiting cases, was developed to account for the data over the full pressure range. The implications of these results for measurement of the static breakdown voltage and the secondary-ionization coefficient are discussed.

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