Effective Ionization Coefficients, Electron Drift Velocities, and Limiting Breakdown Fields for Gas Mixtures of Possible Interest to Particle Detectors

Abstract

We have measured the gas-density, N, normalized effective ionization coefficient, /spl alpha/ /N, and the electron drift velocity, w, as a function of the density-reduced electric field, E/N, and obtained the limiting, (E/N)/sub lim/, value of E/N for the unitary gases Ar, CO/sub 2/, and CF/sub 4/, the binary gas mixtures CO/sub 2/:Ar (20:80), C02:CH4 (20:80), and CF4:Ar (20:80), and the ternary gas mixtures C02:CF4:Ar (10:10:80) and H/sub 2/O:CF/sub 4/:Ar (2:18:80). Addition of the strongly electron thermalizing gas CO/sub 2/ or H/sub 2/O to the binary mixture CF/sub 4/:Ar (i) "cools" the mixture (i.e., lowers the electron energies), (ii) has only a small effect on the magnitude of w(E/N) in the EN range employed in the particle detectors, and (iii) increases /spl alpha//N for E/N > 50 x 10/sup -17/ V cm/sup 2/. The increase in /spl aplha//N, even though the electron energies are lower in the ternary mixture, is due to the Penning ionization of CO/sub 2/ (or H/sub 2/O) in collisions with excited Ar atoms. The ternary mixtures -- being fast, cool, and efficient -- have potential for advanced gas-filled particle detectors such as those for the SCC muon chambers.