Industrial X-Rays

Author: E.E. Kimberly

X-rays are a form of invisible radiation similar to heat rays or light rays, but their wave lengths are shorter (frequencies higher). Because of their short wave lengths, X-rays are able to penetrate substances which are opaque to light rays, and so may be used to photograph in silhouette substances such as animal tissue, wood, and metals. Fig. 28-24 shows the range which X-rays occupy in the radiation spectrum.

Fig. 28-24. Position of X-Rays in the Radiation Spectrum

Fig. 28-25. Radiograph of Casting, With Blowhole Shown as Shadow near Center

Radiography is somewhat similar to photography, but the rays must be made to pass through the specimen to be studied and to strike a photographic plate after the passage. The number of X-rays which reach and affect the plate depends on the thickness and density of the specimen and, of course, on the time of exposure. The greater the thickness or density of the specimen to be X-rayed, the more powerful must be the X-ray beam directed at it, in order to obtain an acceptable exposure of the plate in a reasonable length of time. Brass is almost eight times, and steel is about two and one-half times, as opaque to X-rays as is aluminum. Fig. 28-25 shows a typical radiograph of a casting in which there is a gas pocket or blowhole that is invisible from the outside.

X-rays may be produced in any of several ways, but available industrial X-ray apparatus produces them by directing electrons against a metallic target. The electrons are produced by an electron gun similar to that shown in Fig. 28-23; but, instead of striking a fluorescent screen, they strike a slanted metallic anode as shown in Fig. 28-26. The X-rays produced by this collision are deflected at a right angle to the electron beam and, unhampered by the glass of the tube side, emerge as a narrow-angle beam directed toward the specimen, as shown in Fig. 28-26.

Fig. 28-26. Schematic Diagram of X-Ray Tube and Object to Be X-Rayed

X-ray equipment is rated according to the maximum voltage which can safely be applied to the X-ray tube. In the present state of development, industrial X-ray equipment is available from 140 kv to 10,000 kv; the voltage is selected according to the thickness and density of the object to be X-rayed. Fig. 28-27 shows a typical X-ray machine with power supply for 150 kv.

The use of industrial X-ray equipment involves a certain amount of personal hazard, if it is not properly shielded. Stray emission while the tube is in operation may reach near-by persons and, by its cumulative effect, may cause ray "burns."

Fig. 28-27. Industrial X-Ray Machine With Transformer

To prevent such injury to the operator and others, it is necessary to enclose the radiating portions of the equipment in walls of lead sheet or equivalent thickness of other substances, such as earth or water. The required thickness of the protecting wall depends on the power of the X-ray, and recommendations are offered by makers of the equipment.