Ionization cross sections of gases for protons at kinetic energies between 20 MeV and 385 GeV, and applications to vacuum gauges in superconducting accelerators.

Abstract

Measurements have been made of the ionization cross sections of air, hydrogen, and argon by use of the KEK 500-MeV booster, the KEK 12-GeV main ring, and the Fermilab main ring. Within the beam duct of each of those accelerators, we placed a gas ionization monitor and recorded the current in the monitor as a function of the time elapsed since the beam injection for each pulse. This time is uniquely related to the instantaneous kinetic energy of protons. Because gas pressure in the monitor was kept sufficiently low (about ${10}^{\mathrm{\ensuremath{-}}5}$ Torr), the current is attributable to single ionizing collisions of protons with molecules and is therefore proportional to the specific primary ionization, or the ionization cross section (rather than the total ionization). The dependence of the cross section on proton kinetic energy, measured for air, hydrogen, and argon, agrees closely with the prediction of the Bethe theory, and is represented by a straight line on the Fano plot. The data were tested for consistency with other measurements for electrons and protons at lower kinetic energies. Implications of the work for the design of accelerator vacuum components and of particle detectors are included. For instance, the technique used in the present measurements can be readily applied to the determination of the pressure distribution in a beam duct at liquid-helium temperature of an accelerator using superconducting magnets.