Patent GB-957186-A (GB)
Title: Improvements in or relating to measuring devices
Inventor: BELL ERIC BRAYTON
IPC: G01N23/08
Filed: 1960-08-17
Abstract: 957, 186. Radio-active measuring devices. SAUNDERS-ROE & NUCLEAR ENTERPRISES Ltd. Nov.17, 1961 [Aug.17, 1960], No. 28467/60. Heading G1A. A measuring device comprises a sampling zone, a first radio-active isotope source radiating through said sampling zone, a second radio-active isotope source of similar characteristic to the first source, a radiation detector, a signal modi fying circuit and a recording means, wherein the signal modifying circuit is periodically calibrated comparing the emission received by the detector from the first source through the sampling zone, with the emission received by the detector from the second source. The device comprises, Fig. 2, a Caesium 137 radio isotope source 11 passing radiation through the test object housed in sampling zone 10, to a phosphor scintillator 12, to produce light flashes for detection by a scintillation counter 13. A D.C. current is thus produced by the counter and fed to a signal modifying and recording unit 2, Fig. 1, (not shown). Periodically a shutter unit 15 de- . scends and shuts off the measuring source 11. Simultaneously a second shutter unit 16 is withdrawn to allow radiation from a second, calibrating, isotope source 14, to fall on the scintillator 12 and produce a signal whereby the signal modifying and recording means is calibrated and corrected for any change in the intensity of the source 11 or the sensitivity of the detector 13. The change from the measurement state to the calibration state of the device may be manual, or, as in the described embodiment, automatic. The recorder chart motor 190, Fig. 5, operates a series of cams which intermittently operate switches S, T, U, V, W, X, Y and Z to change to the calibration state. In the drawing the filled-in contacts are those made during the measurement state. In the measurement state the output current from the photomultiplier type scintillation counter (see Fig. 6, not shown), is applied via terminals A and B, through resistors 153 and 154. A standard voltage source 155 produces a variable comparison potential at the wiper of potentiometer 156 under the control of measuring and standardizing potentiometers 156 and 160 respectively, the wipers of which are in turn controlled by measuring and standardizing motors 181 and 182, only motor 181 being active however at this time. The comparison potential is applied via switch U to one side of resistance 173, the potential across resistors 153. and 154 being applied, via resistors 171 and 174, to the other side thereof. The voltage difference across the resistor 173 is applied to the input of a phase sensitive amplifier 180 which applies an A.C. voltage to the measuring motor 181, via antihunting circuit 183,184,185, in such a manner as to cause the wiper of potentiometer 156 to be moved to bring the voltage across the resistor 173 to zero. The shaft of motor 181 is coupled to the recorder means and thus enables an indication of the density of the test object. The cams associated with the recorder chart motor now change the switches over to the calibration state. The recorder motor is thus stopped through switch Y opening, switch Z closes, activating'a delay valve 191 which reconnects the supply to the chart motor after an interval sufficient for the calibration process. The comparison potential applied to one side of resistor 173 is now taken from a fixed tap on resistor 157 and part of the potential across resistors 153 and 154 is shunted away by resistor 175, to compensate for the more powerful radiation from source 14. The voltage across resistor 173 is reduced to zero by motor 182 and potentiometer 160 thereby calibrating the device. If the full movement of the wiper of potentiometer 160 is insufficient to restore balance a contact P is closed lighting a warning lamp 192.