Lawrence Berkeley National Laboratory

Electron avalanching in liquid argon is being studied as a function of voltage, pressure, radiation intensity, and the concentrations of additives, especially xenon. The avalanches are produced in the intense electric field at the tip of a tungsten needle by ionization from a moveable gamma source. Photons from excited xenon atoms produce photomultiplier signals in coincidence with the current pulse from the needle. Although avalanches are observed in pure argon, the addition of even a small amount of xenon greatly stabilizes the performance. Similar attempts with (30%) neon have been unsuccessful. Typically the avalanche current (pulse height) spectrum is narrow and the width is relatively independent of voltage, pressure, and radiation intensity. Some conditions produce a second set of pulses that are larger, but very sensitive to pressure. We plan to test a practical wire chamber prototype, using microwires produced lithographically.