  Electron avalanching in liquid argon is being studied as a function of
voltage, pressure, radiation intensity, and the concentrations of certain
additives, especially xenon. The avalanches produced in an intense electric
field at the tip of a tungsten needle are initiated by ionization from a
moveable americium (241Am) gamma ray source. Photons from xenon excimers are
detected as photomultiplier signals in coincidence with the current pulse from
the needle. In pure liquid argon the avalanche behavior is erratic, but the
addition of even a small amount of xenon (>100ppm) stabilizes the performance.
Similar attempts with neon (30%) as an additive to argon have been
unsuccessful. Tests with higher energy gamma rays (57Co) yield spectra and
other performance characteristics quite similar to those using the 241Am
source. Two types of signal pulses are commonly observed: a set of pulses that
are sensitive to ambient pressure, and a set of somewhat smaller pulses that
are not pressure dependent.
