Patent GB-948413-A (GB)
Title: X-ray apparatus
IPC: H05G1/60
Filed: 1962-01-31
Abstract: 948,413. X-ray apparatus. LABORATORY FOR ELECTRONICS Inc. Jan. 31, 1962 [Jan. 31, 1961], No. 3657/62. Heading H5R. [Also in Division H2] An X-ray apparatus comprises a gridcontrolled X-ray tube, means for converting the X-ray image of an object into a visible image, a camera for photographing said visible image, means for producing an output pulse of fixed duration each time the camera shutter opens, and means for pulsing the grid of the X-ray tube for the duration of each output pulse so as to generate a pulse of X-rays for the said duration. Fig. 1 illustrates the general scheme. X-rays from a generator tube 11 pass through an object 12 to form an image on the end of a converter tube 12. This produces a visible image on the farther end 14 which can be viewed through a lens system 20, 21. A partially reflecting mirror 22 between the lenses reflects some of the light to a movie camera 25. The camera is coupled to a pulse generator 26 in such a way as to produce a single output pulse each time the camera shutter opens. This pulse acts as a trigger for a multi-vibrator 27, which produces a square wave output pulse of a fixed predetermined duration. The square wave pulse acts through the X-ray tube control 30 to impose a square wave on the grid of the X-ray tube 11, so that each time the camera shutter opens, a fixed duration pulse of X-rays is produced. Fig. 2 (not shown) sets out in more detail the scheme between the multivibrator and the X-ray tube. The square wave output from the former is applied to a differentiating unit which produces two outputs, one of which is inverted and lagging behind the other by the width of the multi-vibrator pulse. These two outputs are utilized to apply, and then cut off, a bias voltage across a resistance (shown as 120 in Fig. 4) in series with the grid of the X-ray tube. For the duration of this bias voltage, the tube is conducting, so that an output pulse of X-rays is produced for a specific time equal to the duration of the multi-vibrator pulse and coincident with its occurrence. Fig. 3 (not shown) illustrates in more detail the part of the circuit up to and including the multivibrator, showing how a single triggering pulse is applied to the multi-vibrator at each opening of the camera shutter, independently of the repetition rate of the shutter opening. Fig. 4 shows in detail the remainder of the circuit from the multi-vibrator onwards. The output of differentiator 80 is applied across the primary of a transformer 82 which has a centre tap secondary winding, so that the differentiated pulses, 180 degrees out of phase with each other, are produced across the two secondary windings These are connected respectively to the grids of thyratrons 90 and 91 (" turn on " and " turn off " thyratrons), which provide the " bias on " and then " bias off " resistance 120 referred to above. The two thyratrons have their anodes coupled respectively to the opposite ends of the primary of a transformer 93, and their cathodes connected to an A.C. common lead 94. The arrangement is such as to produce an output pulse across the resistance 120 (connected across the secondary of transformer 93), a positive pulse of about 2000 volts. (A third (" restoration ") thyratron, with its associated circuitry as shown, is fired on the negative half-cycle of the A.C. voltage supply whenever the other two thyratrons have been fired on the previous positive half-cycle, in order to prevent the transformer core from being saturated.) A transformer 123 is coupled across a 220 volt R.M.S. A.C. line, and, together with a diode 124, capacitors 131, 132 and a resistance 125, connected as shown, provides a bias D.C. voltage of - 2000 volts at the junction between resistances 120 and 125. When the positive 200 volt pulse is generated across resistance 120 in response to the multi-vibrator pulse, the - 2000 volts, which serves as a negative bias on the control grid of the X-ray tube 121, is bucked out and the X-ray tube fires, generating X-rays for the duration of the + 2000 volt pulse.