Patent GB-598191-A (GB)
Title: Electrical apparatus for indicating the counting rate of a detector of penetrative radiation
IPC: G01R23/09
Filed: 1945-05-02
Abstract: 598,191. Determining physical qualities and dimensions of materials ; current and voltage measuring apparatus. TEXACO DEVELOPMENT CORPORATION. May 2, 1945, No. 11130. Convention date, May 27, 1944. [Class 37] Apparatus for indicating the counting rate of a detector of penetrative radiation comprises a resistance and a condenser in series with the output of the detector, means for producing a voltage opposing that collected on the condenser due to the pulses of the detector, voltage dividing means for adjusting the opposing voltage so as to balance the condenser voltage, and means for indicating the amount of adjustment of the voltage dividing means necessary to balance the condenser voltage. Such apparatus is applicable for measuring the thickness of plates, pipe walls, &c. from one side of the wall only by directing penetrative radiation such as gamma rays into the object and detecting by means of a radiation detector such as a Geiger-Muller counter the amount of radiation scattered within the material and returned so as to be intercepted by the detector. The intercepted radiation causes electrical pulses to be generated, their number depending upon the intensity of the intercepted radiation, which depends on the thickness of the object. The level of a liquid and the density of a fluid may be similarly determined. Pulses from the radiation detector are applied to a circuit comprising a resistance R and condenser C, Fig. 1, the pulses being collected to form a charge on the condenser. A circuit comprising a D.C. source such as a battery B and a potentiometer P provides a voltage in opposition to that on the condenser C, which is adjusted by varying the potentiometer until a micro ammeter A in an electromic voltmeter indicates zero. The position of the slider of the potentiometer P is read on a scale and is a measure of the voltage on the condenser C and thus of the counting rate of the radiation detector. As shown in Fig. 2 the output from the radiation detector or from a preamplifier is connected to an input plug 10, the pulses being equalised in width and amplitude by amplification stages 12, 14, 16. At the output of the amplifier stage 16 a double rectifier 18 is connected. This in one of its halves suppresses the positive part of each pulse because the cathode 20 of this half is earthed. The negative part of each pulse passes through the other half of the rectifier to the integrator 22 consisting of a high resistance 24 and a condenser 26, which is charged by the individual pulses. The voltage to which the condenser 26 is charged is opposed by the voltage tapped off from a potentiometer 32 in series with a battery 30, equality being indicated by zero reading on a microammeter 40 connected across the plates of the pushpull valve 38 of an electronic voltmeter. Resistances 41 and a condenser 41a form a preliminary integration circuit. With a twoway switch 42 in the upper position shown, pulses arrive at the condenser 26 through a low resistance 43, forming a second integration circuit in which the condenser is charged in a short time. By throwing the switch 42 to its lower position the high resistance 24 is substituted for the low resistance 43, thereby increasing the integration time and giving a stable voltage across the condenser 26 in spite of fluctuations due to the statistical nature of the pulses. The switch 42 is first put in its upper position and the condenser 26 reaches its average correct voltage in a short time. The switch is then changed to introduce the higher integration time, the final reading then being obtained in less than this time. The condenser 26 is shunted by a resistance 44 smaller than its natural leakage resistance so as to establish a fixed value for the unavoidable leakage. The resistances 24, 44 act as a voltage divider for the voltage across the condenser 41a, when the switch 42 is in its lower position. In order that the voltage across the condenser 26 shall be the same if the second or third integration circuit is used, a voltage divider formed by resistances 43, 46 with an equal ratio is introduced when the switch 42 is in its upper position, so that the change from short to long time constant does not produce any change in voltage across the condenser 26. The voltmeter consists of a double triode 38 the halves of which are in opposition. A potentiometer 52 is adjusted until the currents in the halves are exactly alike when the grids are earthed, this being indicated by zero reading on the meter 40.