Richard Kadel
(LBL)


Studies of Electron Avalanche Gain In Liquid Argon.


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 or
a Co57 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.
We propose an explanation of why the addition of Xe
stabilizes Ar from the perspective of molecular physics, and describe new tracking detectors based on this R&D effort.

http://arxiv.org/abs/hep-ex/0204033