The Stopping Cross Section of Gases for Protons, 30-600 kev

Abstract

The stopping cross section of ${\mathrm{H}}_{2}$, He, ${\mathrm{O}}_{2}$, air, ${\mathrm{N}}_{2}$, Ne, A, Kr, Xe, ${\mathrm{H}}_{2}$O, N${\mathrm{H}}_{3}$, NO, ${\mathrm{N}}_{2}$O, C${\mathrm{H}}_{4}$, ${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$, ${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$, and ${\mathrm{C}}_{6}$${\mathrm{H}}_{6}$ for protons has been measured over the energy range ${E}_{p}=30\ensuremath{-}600$ kev. An electrostatic analyzer measures the energy of protons incident on a gas cell, and the transmitted beam energy is measured with a magnetic spectrometer. The gas cell is closed off with thin aluminum windows. Comparison of the molecular stopping cross section of the compounds with the values obtained by summing the constituent atomic cross sections shows that Bragg's rule does not hold for any of these compounds below ${E}_{p}=150$ kev; for NO the additive rule does not hold at any energy studied. Above 150 kev the stopping cross section of carbon is obtained by subtracting the hydrogen contribution from the values measured for the hydrocarbons. Average ionization potentials are calculated from these measurements. A range energy relation for protons in air is included. Sources of error are discussed; the probable error of the stopping cross section measurements varies between 2-4 percent.