Patent GB-627557-A (GB)
Title: Improvings in or relating to measuring apparatus
IPC: G01F23/288
Filed: 1946-02-11
Abstract: 627,557. Measuring by means of y-rays. TEXACO DEVELOPMENT CORPORATION. Feb. 11, 1946, No. 4168. Convention date, Jan. 27, 1945. [Classes 97 (iii) and 98 (i)] [Also in Groups XXXVIII, XL (a), XL (b) and XL (c)] A device for measuring physical characteristics, such as thickness or density of an object, from one side thereof, of the kind having a radioactive source of y-rays, a γ-ray detector of elongated form, a shield member, formed of metal of high density and atomic number, between the source and the detector to prevent radiation from passing directly from one to the other, and means for indicating the intensity of radiation emitted from the source, scattered within the object, and thence impressed on the detector, is characterized in that the source lies on the longitudinal axis of the detector and is separated therefrom by the shield member. The source, shield and detector are mounted with a preamplifier in a cylindrical casing which may have means for attaching it to the object measured, with a cable connection to a unit which transforms the detector indications into a direct indication of radiation intensity. As shown, Fig. 5, within the cylindrical casing of the detector 24 are disposed a number of metal discs 74, electrically connected to the casing 60 and forming the cathode. The anode is formed by a number of wires 86, each passing through a series of aligned holes 76 in the discs 74. The anode wires are insulated from the casing and connected to a terminal 106 for external connection. The casing is filled with e.g. argon and petroleum vapour and sealed. The shield member 26, Fig. 2, is a cone of dense metal such as a tungsten alloy and is screwed by its base to the detector 24. The source of y-rays 22, e.g. 1 mg. of radium, is placed at the apex of the cone and surrounded by a lead shield 44 with a gap 42 restricting the rays to a pencil directed toward the object to be measured in order to reduce the effect of scattering from other objects. This effect is further reduced by a shield 46 placed around the detector with a gap adjacent to the object to be measured. The source, shield and detector, together with a preamplifier 28 mounted on a base 48, are enclosed in a cylinder 20. For air-tight sealing, end plates 34, 52 are urged by nuts threaded on bolts 32, 54 against rubber discs 36, 56 held by metal discs 30, 50 threaded in the casing. At one end an aperture 58 is provided for a multicore cable. At the other end is a bolt 38 for orienting the cylinder in a framework which clamps it to the object to be measured. Three types of framework are described for clamping the cylinder to the inside or outside of a tube or to a sheet of magnetic material respectively. Located at a distance from the cylinder and connected thereto by a multicore cable is a unit comprising an indicator and a power supply, which provides electrical supplies at the various voltages required. The detector produces negative-going voltage pulses when ionised by γ-rays; the frequency of these pulses is the measure of the intensity of the back-scattered radiation. These pulses are fed to a two-stage amplifier, Fig. 11, consisting of the two sections A and B of the twin-triode 172. The amplified negative-going pulses are fed to the indicator through the cable and to the cathode of the pentode 170. The pulses cause the anode potential of the pentode to fall, thus quenching the detector. In the indicator unit, Fig. 12, the pulses pass through a four-stage amplifier to a twin triode 184 connected as a multivibrator. The section B of this valve has negative bias on its grid which renders the multivibrator inoperative except when triggered by a pulse. The output, consisting of positive-going pulses of comparatively great, and equal widths, is taken from the anode of valve 184A. Part of this output is applied to a visual indicator 216. The whole output is applied to the grids of the twin triode 186, wherein section A is a phase inverter and section B actuates a mechanical counter connected to a jack 227. The negative going pulses from the.phase inverter are applied to an integration circuit, the diode 204 acting as a D.C. restorer. The integrating condenser 198 may be associated with either of the resistors 200 or 196 to provide different time constants and different degrees of accuracy. A measure of the pulse rate is indicated by the meter 208 in the valve voltmeter circuit formed by the twin triode 210 and associated components. U.S.A. Specifications 2,348,810, 2,304,910 and 2,277,756 are referred to.