Measurement of electron mobility in liquid and gaseous argon at low electric field strengths and in the critical region

The mobility of excess electrons in argon has been determined from drift time measurements using the pulse-shape method. The excess electrons were generated by pulsed emission from a photocathode. The measurements were performed in liquid argon in the temperature range T_c-35 K ⩽T⩽T_c and in gaseous argon in the temperature range T_c⩽T⩽T_c+2 K at field strengths between 3 V/cm and 50 V/cm. At the mobility maximum, observed at about T_c-5 K, an increase of the mobility down to a field strength of 3 V/cm is observed; outside the mobility maximum the low field mobility is independent of field strength. A precision temperature regulation system made it possible to approach T_c within 2 mK. The low field mobility at T_c is determined to μ_c≈160 cm² /Vs