A Solid State Integrator for a Digital Dosemeter

Abstract

Methods of integrating dose during radiotherapy treatment range from electromechanical to electronic, each method being satisfactory for all or part of the range of current produced by the ionization chamber associated with the therapy machine. Farr (1957) describes how a d.c. low-inertia integrating motor whose speed of rotation is proportional to applied voltage can be used. Ionization chamber current is converted to a voltage which is amplified and applied to the motor; the number of revolutions made by the motor over a period of time indicates the integrated current or dose delivered by the machine. An alternative method is to integrate the ionization current in a capacitor and measure the resulting voltage with an electrometer valve or similar device. Watson (1963) describes a circuit of this type which employs a moving coil relay to determine a level of voltage to which the capacitor has became charged. When the relay operates it causes a second relay to discharge the capacitor and the cycle of events is repeated; hence for each operation of the relay a signal is available to operate a counter. A similar arrangement is described by Naylor (1964), but in place of the moving coil relay a cold cathode trigger tube is used. When the voltage excursion on the capacitor has reached the same value as that required to ignite the tube, the resulting change in anode potential is used via a cathode follower to discharge the capacitor. The application of semiconductors to the integration of ionization current has been investigated by Kemp, Bottrill and Klevenhagen (1968).