Electron attachment, effective ionization coefficient, and electron drift velocity for CF₄ gas mixtures

For tracking in a SSC experiment with wire chambers, CF₄ gas mixtures are very advantageous from the point of view of aging, drift velocity, and spatial resolution. However, they are also known to suffer from considerable electron attachment. This attachment can cause serious inefficiencies in the collection of electrons liberated near the anode wire, and causes broadening of the pulse-height spectrum for all ionization electrons. Two bench-top setups have been used to study electron attachment of CF₄ gas mixtures, and related properties such as the gas gain difference between single-electron avalanches and ⁵⁵Fe X-ray initiated avalanches, the degradation of the energy resolution due to electron attachment, and the effective ionization coefficient α(≡ α - η). A set of three prototype straw-tube modules each containing an 8 × 8 bundle of straws has been constructed and tested at a Fermilab test beam with various CF₄ gas mixtures. Some preliminary results on the drift velocity and drift time for these gas mixtures are reported.