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 Fe-55 X-ray initiated avalanches, the degradation of the energy resolution due to electron attachment, and the effective ionization coefficient alphaBAR(= alpha - eta). A set of three prototype straw-tube modules each containing an 8 x 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.