GB638253A - Improvements in or relating to x-ray apparatus for measuring the thickness of material - Google Patents

Abstract

638,253. Measuring thickness by X-rays. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. April 20, 1948, Nos. 10859 and 10860. Convention dates, June 30, 1947, and Sept. 27, 1947. [Class 40 (iii)] Apparatus for determining the thickness of a material 1 by comparing its X-ray absorption with. that of a standard 11, comprises means for transmitting X-rays through each during alternate halfcycles of an A.C. supply 8, on to a fluorescent screen 21 which affects a photo-electric tube 22 of the electron multiplier type whose output thus consists of a series of pulses alternately due to each of the strips 1, 11. Those due to the standard, control the gain of the photo-electric tube, and those due to the test material are measured. As shown, Fig. 1, two X- ray tubes 4, 14 are used and the output pulses of the photo-electric tube 22 are amplified by a tube 42 and applied in parallel to two similar valves' 31, 32 whose auxiliary grids are biassed from a transformer 35 connected to the A.C. supply 8, so that each is rendered non-conductive for one half cycle, the valve 31 being conductive when pulses due to the standard material 11 are received. One dynode 37 of the photo-electric tube 22 is fed from a resistor in the anode circuit of valve 31, so that an increase in the magnitude of the pulse due to the standard material increases the anode current of valve 31 to reduce the voltage on dynode 37, thus restoring the magnitude of the pulses to the original value. A meter 34 in the anode lead of valve 32 indicates the relative thickness of the test material 1. Rectifier 48 short-circuits the input resistor 47 of valves 31, 32 in one direction to give an invariable standard voltage with respect to which the pulses are measured. In an alternative electronic circuit, Fig. 3, the amplified pulses derived from the photo-electric tube 22 are applied via a cathode-follower valve 141 to a rectifier bridge 146-149 fed from the A.C. supply 8 through a transformer, so that a meter 34 on the D.C. side of the rectifier bridge has its reading increased or decreased depending on which of the materials 1, 11 produces the largest pulse, the amplification of tube 22 being controlled by control of the potential of dynode 37 as before, to maintain a constant magnitude on the larger of the pulses. One X-ray tube or two fluorescent screens may be used with a mechanically-operated shutter. The photoelectric tube may be replaced by a Geiger counter or ionization chamber, with the gain control applied externally.