A new method has been developed to measure W, the ratio of the energy lost by a proton to the number of ion pairs produced when these particles interact with a gas. In this method, protons are Rutherford scattered from a beam by a thin gold foil into a W cell where total gas ionization is measured. The number of protons scattered into the W cell is measured with a silicon surface barrier detector, and the energy lost to the gas is calculated from stopping power data. Most of the protons pass through the thin gold foil and enter an emission cell where emission spectra are studied with a vacuum ultraviolet scanning monochromator. At 400 torr for pure gases the following values of W (eV per ion pair) were found for 3.6-MeV protons: He, 4S.2±0.9; Ne, 39.3±0.8; Ar, 26.6±0.S; Kr, 23.0±0.S; Xe, 20.S±1.2; N2, 36.6±0.7. No appreciable pressure dependence was observed for the pure gases. The Jesse effect, which is the increase in ionization observed when impurities are added to certain gases, was studied as a function of concentration and pressure for mixtures of C2H., C2H2, and CO2 with Ar. At small concentrations of CJI. or C2H2 the Jesse effect decreases as the total gas pressure increases. These data, along with other facts cited in this paper, tend to confirm an energy pathways model for argon. Based on a comparison of our data with this model, we deduce the values 6.SXlO-1o and 1.1X1Q-9 cm S seC I for the rate constants for producing the Jesse effect from the IPI (1048 A) resonance state of argon by collisions with CJI. and Ct H2, respectively. W values were also studied as a function of pressure and concentration for mixtures of Ne, Ar, Kr, and Xe with He. The dependence of W on pressure and concentration was similar to that of argon. PARKS, HURST, STEWART, AND WEIDNER MONOCHROMATOR

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Letters 20, 575 (1968).~R . V. Krotkov {private communication).

When charged particles interact with the noble gases, energy is radiated in the vacuum ultraviolet (VUV) region of the electromagnetic spectrum. Some recent measurements of the absolute yield of VUV energy per cm of proton track length show that this quantity d&/dx is an appreciable part of the total stopping power dE/dx. Here, we show by direct comparisons and by arguments based on the principle of conservation of energy that the Jesse effect (Penning ionization) in helium, neon, and argon is associated with radiation energy in the VUV region.