A Low Noise Lifetime Measurement of Electrons Drifting in Liquid Argon

A specially designed cryostat apparatus was built to operate a liquid argon ionization cell with very low acoustic noise. The cryostat was equipped with a system to damp out low frequency vibrations and thus reduce induced acoustic noise on the data transients. A liquid argon ionization cell was tested in the new cryostat by measuring the ionization of the argon by cosmic ray muons. Digital filtering of the individual data transients improved the purity of the data set used to generate ensemble averaged transients for different electric fields across the ionization cell. Transient waveform analysis was used on these averaged transients to extract the mean electron lifetime of the ionization electrons drifting in the liquid argon. A result for the free electron lifetime of 0.897 +/- 0.005 (stat) +0.023 - 0.032 (syst) micro seconds was found. The electron lifetime can be used as to determine the concentration of oxygen equivalent contamination in the liquid argon of this experiment. This value of the electron lifetime corresponds to a concentration of oxygen equivalent contamination of 748 +/- 150 (stat) +186 -125 (syst) ppb, at an applied electric field strength of 500 V/cm.