CHARGE COLLECTION CHARACTERISTICS OF AN AIR ION CHAMBER FOR 0.2-[Jisec PULSES OF 25 MeV ELECTRONS

The charge collection characteristics of a parallel-plate transmission air ion chamber have been measured for 0.2-/isec pulses of 25 MeV electrons over a range of 3 initial charge densities from 0.6 to 430 esu/cm . Plate spacings of 0.07, 0.40 and 0.73 cm were used with E/p values of 1 and 6; 1, 3 and 6; and 1 and 3 V/cm-mmHg, respectively. Electron beam monitoring was done with a graphite Faraday cup. The results of the experiment were compared with calculated values of charge collection efficiency from a model that accounts for negative ion collection by means of Boag's theory and electronic charge collection by an empirical model. The relative proportion of negative ions to electron charge carriers was calculated from a theory due to Thomson and to Boag and Wilson. Charge collection efficiency as calculated from the model was in good agreement with experimental values.