Ionization of the Noble Gases by Protons: Jesse Effects as a Function of Pressure

Abstract

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, 45.2±0.9; Ne, 39.3±0.8; Ar, 26.6±0.5; Kr, 23.0±0.5; Xe, 20.5±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 C2H4, C2H2, and CO2 with Ar. At small concentrations of C2H4 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 6.5×10−10 and 1.1×10−9 cm3 sec−1 for the rate constants for producing the Jesse effect from the 1P1 (1048 Å) resonance state of argon by collisions with C2H4 and C2H2, 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.