Effective ionization coefficients, electron drift velocities, and limiting breakdown fields for gas mixtures of possible interest to particle detectors

We have measured the gas-density, N, normalized effective ionization coefficient, α /N, and the electron drift velocity, w, as a function of the density-reduced electric field, E/N, and obtained the limiting, (E/N)_lim, value of E/N for the unitary gases Ar, CO₂, and CF₄, the binary gas mixtures CO₂:Ar (20:80), C02:CH4 (20:80), and CF4:Ar (20:80), and the ternary gas mixtures C02:CF4:Ar (10:10:80) and H₂O:CF₄:Ar (2:18:80). Addition of the strongly electron thermalizing gas CO₂ or H₂O to the binary mixture CF₄: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 α/N for E/N > 50 x 10^-17 V cm². The increase in aplha//N, even though the electron energies are lower in the ternary mixture, is due to the Penning ionization of CO₂ (or H₂O) 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.