Abstract
The avalanche current resulting from the release of a short (30 nsec) pulse of photoelectrons (5 × 104) at the cathode of a uniform field gap (5 cm) is observed experimentally. By this technique the drift velocities of electrons (v-) and of positive ions (v+) are measured. In dry air in the range E/p = 50-100 v cm-1 torr-1 at temperature 20 °c, v- = 256 × 103 E/p + 5 × 106 and in water vapour in the range E/p = 55-85 v cm-1 torr-1 at 20 °c, v- = 255 × 103 E/p + 8.9 × 106, with units cm sec-1 and v cm-1 torr-1. The values of v- obtained for dry air fit the values calculated by Heylen and are higher than those measured with the magnetic-deflection method by Townsend and Tizard.
The mobility of positive ions in dry air, which is constant in the range E/p = 45-70 v cm-1 torr-1 and is equal to 2.2 × 103 cm2 v-1 sec-1 (at 1 torr and at 20 °c), gradually decreases at higher E/p values. In water vapour the mobility of positive ions was constant over the measured range E/p = 50-90 v cm-1 torr-1 and is equal to 0.61 × 103 cm2 v-1 sec-1 (at 20 °c and at 1 torr). In humid air the value of mobility is lower than the value calculated from Blanc's law. This suggests a clustering of ions.