GB638833A - Improvements in and relating to electric-discharge-tube apparatus - Google Patents

Abstract

638,833. Detecting gamma radiations. FRANKLIN, E. Sept. 2, 1947, No. 24253. [Class 37] [Also in Group XL (b)] In a device for detecting gamma radiations a Geiger-Muller tube (or other device sensitive to gamma radiations) is arranged to trigger a cold cathode valve causing a condenser to discharge through the valve until it blocks, whereupon the condenser recharges through an integrating circuit containing a meter to indicate the sum of the charging pulses. As shown, ionization of the Geiger-Muller tube cause positive pulses to appear across resistor 11 which are applied to the normally biassed trigger electrode of the cold cathode valve 7 by way of condenser 8. The trigger-cathode gap is broken down, the trigger current being limited by resistor 6. This limiting of the trigger-current reduces the recovery time. Condenser 16 then discharges through the main gap and the tube is extinguished, resistor 9 being of too high a value to allow discharge to continue via battery 10. The condenser charging pulses are then integrated by condenser 15 and resistor 13 and indicated on meter 14. If the Geiger-Muller tube is not of the self-quenching type the connection to the bias position on battery 10 may be removed and the upward voltage excursion of the trigger - electrode produced when the cold cathode valve fires may be used to quench the tube. If it should be desired to earth the cathode of the Geiger-Muller tube the output signal must be taken from its anode. It is stated that it has been found possible to employ a cold cathode valve in which the trigger electrode is situated close to the anode so that discharge takes place when the anode of the Geiger-Muller tube pulses negatively. The cathode load resistor 9 is now replaced by an anode load resistor and the downward voltage swing produced at the trigger electrode when the valve fires used to quench the Geiger- Muller tube. An arrangement is described wherein three Geiger-Muller tubes are employed so as to increase the overall triggering rate. Each of these tubes has a circuit as shown in the Figure, but they all share the same integrating circuit and meter. Each condenser 16 is in five parts any one of which may be switched into circuit to vary the counting rate ; and since the accuracy of the instrument depends on the product of the counting rate and the integrating time, the circuit constants of the integrating circuit are varied by the same switch to maintain the same accuracy. A further condenser, switched in and out of the meter circuit provides a choice of two accuracies. The Geiger-Muller tubes may be arranged in a hand probe in which the tubes are grouped round a central cavity in which the specimen may be placed. Specification 421,341 is referred to.