Patent GB-576737-A (GB)
Title: Improvements in dose-meters for use in x-ray therapy
IPC: G01T1/15
Filed: 1944-01-05
Abstract: 576,737. Quantity meters ; thermionic valve voltmeters. FARMER, F. T. Jan. 5, 1944, No. 200. [Class 37] A circuit arrangement for indicating the condition of charge of a conductor, particularly applicable as an integrating dose-meter for X-ray therapy, includes a chamber adapted to have a charge applied to its central conductor, a relay actuated when the charge under the action of X-rays falls to a predetermined value, and means controlled by the relay for applying a charge of fixed value to the central conductor. The chamber ch has its central conductor cc connected through the inner conductor ic of a flexible cable FC to the grid of an electrometer valve EV, the anode of which is connected through a high resistance R1 to the positive high tension, e.g., 280 volts. The outer conductor oc of the chamber is connected to a point of zero potential, and the outer sheath os of the flexible cable is connected to a point of low positive potential, e.g., 70 volts, to which also is connected the cathode of the valve EV. The low positive potential may be derived from a voltage stabiliser VS. The anode of the electrometer valve EV is connected to the grid of an oscillator valve OV through the secondary winding sw of a feed-back transformer FT, the anode of the valve OV being connected through the tuned primary winding pw of the transformer and the winding of a relay Re to the positive high tension. The cathode of the valve OV is connected to a point on a voltage divider comprising resistors R2, R3. The relay Re controls the mechanical parts of the system including the counting gear, two sets of contacts re1, re2 being shown. The contacts re1 - short-circuit the secondary winding sw on operation of the relay. The contacts re2 connect a condenser cc alternatively between the high and low positive potential points for charging, or between the conductor ic and the 70 volt point for transferring its charge to the conductor cc of the chamber ch. For initially charging the central conductor cc it is connected through a thermal switch TS to an adjustable point on a potentiometer Pt which is connected in series with a milliammeter in A between the 70 volt and zero potential points. The switch TS is normally closed, but is opened when an element ts becomes heated after the main current has been switched on. Connected to the anode of the valve EV is the grid of an indicator valve IV, the cathode of which is connected through the milliammeter in A to the 70 volt point. After initial charging of the conductor cc it remains isolated from the potentiometer during the operating period. Irradiation of the chamber ch by X-rays causes loss of charge of the conductor cc which lowers the potential of the grid of the valve EV and thereby raises the potential at the anode of the valve EV and the grid of the valve OV, the current through which increases until oscillation occurs. The consequent sudden increase in anode current operates the relay Re, which shortcircuits the secondary winding sw to stop the oscillation and connects the condenser CC to recharge the central conductor cc, and then releases to establish the charging circuit for the condenser CC. At the same time as the potential of the grid of the valve OV is raised, the potential of the grid of the indicator valve IV is also raised, causing a gradually increasing current to flow through the valve IV. This flows through the milliammeter mA in the reverse direction to the potentiometer current so that a falling reading is obtained corresponding to the falling charge on the conductor cc.