Ngas determination for a parallel-plate ion chamber.

A parallel-plate ion chamber has been investigated with a view to determining Ngas, the cavity-gas calibration factor, introduced by the Task Group 21 protocol for dose determination. In-air and in-phantom irradiation of a Memorial Hospital parallel-plate ion chamber in a 60Co beam led to the determination of ion collecting volume and a calibration factor for the chamber. When these quantities were used to deduce Ngas factors for the chamber, an agreement within 1% was found. An average of the cavity-gas factors obtained from 60Co data agreed to within 0.5% with the factor determined through force fitting the doses received by the parallel-plate chamber and a cylindrical chamber in a 22-MeV (nominal) electron beam. Additionally, the parallel-plate chamber was utilized to study the effectiveness, in electron beam dosimetry, of the electron fluence perturbation correction factors introduced when the phantom material is replaced by a cylindrical ion chamber.