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Aviation History
1941
1941 - 1065.PDF
MAY 8TH, 1941. SEEING IT THROUGH (Continued The first is that the tube must be capable of operatingcontinuously for indefinite periods at maximum voltage and with a reasonable tube current. Since the exposure time is directly proportional to thetube current (radiation density), and since the density of T.Tie X-ray beam varies inversely with the square of thedistance from the tube target, the necessity for a reason- ably high tube current (5 to 15 mA.) will be apparent.This is particularly the case where the need for Jong target film distances arises. • Owing to the fact that an X-ray tube tends to passless current with a reduction in operating voltage, it is, in addition, essential that a tube for industrial use becapable of passing sufficient current, also at very low voltages, in order that radiographs may be taken withoutunduly lengthy exposure periods. Target Area The second feature called for is that the tube shouldpossess a very small target area. Fig. 3 illustrates, dia- gram matically, the physical arrangement of a modern self-protective tube. The tube comprises an evacuated glass vessel into which are sealed two electrodes—an anode andcathode. The cathode (negative electrode) comprises an incandescent filament constituting a source of free elec-trons. The anode (positive electrode) forms—over a small portion of its area—the target to which the electrons areprojected (by the very high voltage across the tube) and from which the X-rays generated by electronic bombard-ment radiate outwards in straight divergent lines. This type of tube has a hooded anode which traps all unwantedradiation and permits only the required beam to emerge. Referring now to the target, it will be appreciated thatconsiderable energy is dissipated at this point. For ex- ample, a modern tube operating continuously at 200 kV.10 mA. may dissipate 2 kilowatts over an area of only 5 millimetres square, necessitating adequate cooling.Should the cooling of the target become inadequate, it might very quickly become incandescent and proceed toemit electrons which, during the negative half-cycle with an A.C. operated tube, would fly across and bombard therelatively delicate filament, thereby wrecking the tube as previously described. What is the reason for limiting to an absolute minimumthe area of the target? Fig. 4 makes this point clear. It will be observed that when projecting an image of a speci-men "S" upon a film "F" the quality of definition increases as the source of radiation becomes smaller. Withthe very small flaws encountered in industrial work it is, therefore, absolutely essential that the X-ray tube becapable of producing a clear image of such flaws, and the use of a tube with a coarse or large target might easily maska dangerous defect. In the case of inter-crystalline porosity, tubes with target areas of only 0.6 mm. to 1.5 mm. squareare sometimes used The effect of a point source of radiation is furtherincreased by positioning the X-ray tube at a considerable distance from the specimen, and target-film distances of5oin. are not uncommon. As far as structural design is concerned attention is drawnto the lay-out. It will be seen that the various items of equipment are compact and easily transportable. A veryimportant component in an aircraft X-ray unit is the tube stand, and this should be capable of enabling the X-raytube to be rotated about all its axes. Although most radio- graphic work is carried out with the X-ray beam projectingvertically downwards, the necessity frequently arises for "angle shots" where the form of a particular castinginvolves special radiographic technique. The tube stand must also be structurally rigid to prevent vibration or acci-dental displacement oi the tube during operation. Radiography and Fluoroscopy According to the importance of the part they play, air-craft components are either examined radiographically or INSIGHT : An operatorinspecting a radiograph of a light metal casting.The image is clipped in front of an illuminatedopal glass. visually with the aid ofa fluorescent screen. The process of radio-graphy, as we saw in Fig. 1, consists of re-cording, on suitable film, an X-ray " shadowgraph " of the internal structure of the speci-men. Film manufacturers now produce special films* lor industrial work, and one well-known concern hasrecently perfected and marketed a new type of film enabling radiographs of the highest degree of definitionto be obtained. In general, the flaw discrimination ol X-ray film under well-controlled conditions can be as fineas 0.5 to 1 per cent. That is, flaws which represent only one-half or one per cent, of the total thickness of thespecimen can be detected. When dealing with aluminium and similar light metalsthe X-ray film is, in most cases, activated by the X-ray beam only. When, dealing with steel and other heavymetals, however, the exposure time is substantially reduced by employing "intensifying screens." These screens consist of crystals or zinc-tungstate thinly deposited upon card backings and possess the property of fluorescing under theinfluence of X-rays. The X-ray film is sandwiched between a pair of intensifying screens and is, therefore,activated by the fluorescent light in addition to the X-radiation. Fluorescent View-screens The fluorescent screen consists of a film of bariumplatino-cyanide crystals which fluoresce under X-radiation. A specimen interposed between tube and screen castsa shadow upon the screen, the brightness of which varies inversely with the thickness of the specimen.This method of inspection is much less revealing than the radiographic method. Flaw discrimination of between5 and 10 per cent, is usually achieved under practical conditions. One of the disadvantages of fluoroscopic examinationconcerns the question of safety, and screening apparatus has to be very carefully designed and constructed to ensure thatthe operator is protected against stra'y and scattered radiation. It is found that maximum efficiency in inspection con-flicts with the provision of adequate safety. Latterly, however, X-ray manufacturers have perfected variousscreening units which are entirely safe and also highly efficient. It remains to be pointed out that the evolution of theprecise and highly efficient methods of X-ray inspection of aircraft components now established in this country arevery largely due to the specialised applications developed in the Aeronautical Inspection Directorate of the AiiMinistry. In conclusion, a compliment might be paid to the BritishX-ray industry. Before the outbreak of war specially designed industrial X-ray apparatus was obtainable onlyfrom foreign sources Since the outbreak of war certain British manufacturers have not only designed, developedand perfected X-ray apparatus for industrial work with per- formance superior to that of foreign equipment, but havealso made it commercially available. In fact, a number of these newly developed equipments are to-day fulfillingtheir most important role in the war effort.
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