Differences between signal currents for both polarities of applied voltages on cavity ionization chambers

A difference between the surface potential of the charge collecting electro de and that of the guard electrode of an ionization chamber changes the cha rge collecting volume depending on the applied voltage. If the difference i s large, the saturation curve of the signal current shows a maximum at a lo w applied voltage. Even when there is no electrical or mechanical defect, t he signal current from a parallel plate ionization chamber irradiated with (CO)-C-60 gamma -rays increases or decreases with the applied voltage beyon d the recombination region depending on the polarity of the applied voltage . The variation in the signal current is explained as a result of the chang e in the stopping power of air due to the acceleration or deceleration of s econdary electrons. These electrons are emitted from the polarizing electro de towards the collector as a result of Compton scattering. In a range of l ow applied voltages, the signal current from a cylindrical ionization chamb er is expected to be smaller for a negative applied voltage than for a posi tive applied voltage. This is because epithermal electrons are expected to have a higher probability of being lost by back diffusion than positive ion s which are originally produced in a thermal equilibrium condition. An expe rimental result, however, showed no difference in the polarities of the app lied voltage. The result may be explained as a consequence of the fact that epithemal electrons do not drift for long distances and maintain their ene rgies. (C) 2001 Elsevier Science Ltd. All rights reserved.