X-rays are a form of electromagnetic radiation (EMR), as is light. Their distinguishing feature is their extremely short wavelength--only about 1/10,000 that of light, or even less. This characteristic is responsible for the ability of x-rays to penetrate materials that absorb or reflect ordinary light.
X-rays exhibit all the properties of light, but in such a different degree as to modify greatly their practical behavior. For example, light is refracted by glass and, consequently, is capable of being focused by a lens in such instruments as cameras, microscopes, telescopes, and spectacles. X- rays are also refracted, but to such a very slight degree that the most refined experiments are required to detect this phenomenon. Hence, it is impractical to focus x-rays. It would be possible to illustrate the other similarities between x-rays and light but, for the most part, the effects produced are so different--particularly their penetration--that it is preferable to consider x-rays and gamma rays separately from other radiations. The figure below shows their location in the electromagnetic spectrum.
Figure 1: Portion of the electromagnetic spectrum. Wavelengths in angstrom units (1A = 10-8 cm = 3.937 x 10-9 inch)
Nature of Gamma Rays
Gamma rays are similar in their characteristics to x-rays and show the same similarities to, and differences from, visible light as do x-rays. They are distinguished from x-rays only by their source, rather than by their nature. Gamma rays are emitted from the disintegrating nuclei of radioactive substances, and the quality (wavelength or penetration) and intensity of the radiation cannot be controlled by the user. Some gamma-ray-emitting radioactive isotopes, such as radium, occur naturally. Others, like cobalt 60, are artificially produced. In industrial radiography, the artificial radioactive isotopes are used almost exclusively as sources of gamma radiation.
Making a Radiograph
A radiograph is a photographic record produced by the passage of x-rays or gamma rays through an object onto a film. See the figure below. When film is exposed to x-rays, gamma rays, or light, an invisible change called a latent image is produced in the film emulsion. The areas so exposed become dark when the film is immersed in a developing solution, the degree of darkening depending on the amount of exposure. After development, the film is rinsed, preferably in a special bath, to stop development. The film is next put into a fixing bath, which dissolves the undarkened portions of the sensitive salt. It is then washed to remove the fixer and dried so that it may be handled, interpreted, and filed. The developing, fixing, and washing of the exposed film may be done either manually or in automated processing equipment.
Figure 2: Schematic diagram showing the fundamentals of a radiographic exposure. The dark region of the film represents the more penetrable part of the object; the light regions, the more opaque.
The diagram in figure 3 shows the essential features in the exposure of a radiograph. The focal spot is a small area in the x-ray tube from which the radiation emanates. In gamma radiography, it is the capsule containing the radioactive material, for example, cobalt 60, that is the source of radiation. In either case the radiation proceeds in straight lines to the object; some of the rays pass through and others are absorbed-the amount transmitted depending on the nature of the material and its thickness. For example, if the object is a steel casting having a void formed by a gas bubble, the void results in a reduction of the total thickness of steel to be penetrated. Hence, more radiation will pass through the section containing the void than through the surrounding metal. A dark spot, corresponding to the projected position of the void, will appear on the film when it is developed. Thus, a radiograph is a kind of shadow picture--the darker regions on the film representing the more penetrable parts of the object, and the lighter regions, those more opaque to x- or gamma-radiation.
Figure 3: Diagram of setup for making an industrial radiograph with x-rays.
Intensifying Screens
X-ray and other photographic films are sensitive to the direct action of the x-rays, but the photographic effect can be increased very appreciably, and exposure time can be decreased by the use of an intensifying screen in contact with each side of the film.
One form of intensifying screen consists of lead foil, or a thin layer of a lead compound evenly coated on a paper backing. Under the excitation of x-rays of short wavelength and gamma rays, lead is a good emitter of electrons, which expose the sensitive film, thus increasing the total photographic effect.
Another form of intensifying screen consists of a powdered fluorescent chemical--for example, calcium tungstate, mixed with a suitable binder and coated on cardboard or plastic. Its action depends on the fact that it converts some of the x-ray energy into light, to which the film is very sensitive.
The decision as to the type of screen to be used-or whether a screen is to be used at all-depends on a variety of circumstances, which will be discussed in more detail later.
Scattered Radiation
It is a property of all materials not only to absorb and transmit x-rays and gamma rays in varying degrees, but also to scatter them--as radiation of longer wavelength--in all directions. In radiography, the film receives scattered radiation from the object, the film holder, and any other material in the path of the primary x-ray beam. The effect is to diminish the contrast, detail, and clarity of the radiographic image. Lead screens, in contact with the film, lessen the relative effect of this longer-wavelength scattered radiation. Under some circumstances, a filter of copper or lead, placed between the x-ray tube and the object, or between the object and the film, diminishes the effect of scattered radiation on the film. A lead mask that limits the volume of matter exposed to the primary radiation is sometimes helpful in lessening scatter.
Types of Film
Several special types of x-ray film have been designed for the radiography of materials. Some types work best with lead screens, or without screens. Other types are intended primarily for use with fluorescent intensifying screens. X-ray films are commonly coated with emulsion on both sides of the support--the superposition of the radiographic images of the two emulsion layers doubles the density and hence greatly increases the speed. X-ray films coated on one side only (single-coated films) are available for use when the superposed images in two emulsions might cause confusion.