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Chapter 1: The Radiographic Process

Nature of X-Rays

X-rays are a form of electromagnetic radiation (EMR), as is light. Their distinguishing feature istheir extremely short wavelength--only about 1/10,000 that of light, or even less. Thischaracteristic is responsible for the ability of x-rays to penetrate materials that absorb or reflectordinary 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 beingfocused 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 arerequired to detect this phenomenon. Hence, it is impractical to focus x-rays. It would be possibleto illustrate the other similarities between x-rays and light but, for the most part, the effectsproduced are so different--particularly their penetration--that it is preferable to consider x-rays andgamma 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, anddifferences 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 radiationcannot be controlled by the user. Some gamma-ray-emitting radioactive isotopes, such asradium, 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 throughan 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 exposedbecome dark when the film is immersed in a developing solution, the degree of darkeningdepending on the amount of exposure. After development, the film is rinsed, preferably in aspecial bath, to stop development. The film is next put into a fixing bath, which dissolves theundarkened portions of the sensitive salt. It is then washed to remove the fixer and dried so that itmay be handled, interpreted, and filed. The developing, fixing, and washing of the exposed filmmay be done either manually or in automated processing equipment.

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Figure 2: Schematic diagram showing the fundamentals of a radiographic exposure. Thedark 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 focalspot 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 rayspass through and others are absorbed-the amount transmitted depending on the nature of thematerial and its thickness. For example, if the object is a steel casting having a void formed by agas 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 surroundingmetal. A dark spot, corresponding to the projected position of the void, will appear on the filmwhen 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 moreopaque to x- or gamma-radiation.

Figure 3: Diagram of setup for making an industrial radiograph with x-rays.

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Intensifying Screens

X-ray and other photographic films are sensitive to the direct action of the x-rays, but thephotographic effect can be increased very appreciably, and exposure time can be decreased bythe 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 evenlycoated 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 totalphotographic 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 actiondepends on the fact that it converts some of the x-ray energy into light, to which the film is verysensitive.

The decision as to the type of screen to be used-or whether a screen is to be used at all-dependson 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 varyingdegrees, but also to scatter them--as radiation of longer wavelength--in all directions. Inradiography, 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, andclarity of the radiographic image. Lead screens, in contact with the film, lessen the relative effectof 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 FilmSeveral special types of x-ray film have been designed for the radiography of materials. Sometypes work best with lead screens, or without screens. Other types are intended primarily for usewith fluorescent intensifying screens. X-ray films are commonly coated with emulsion on bothsides of the support--the superposition of the radiographic images of the two emulsion layersdoubles 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 mightcause confusion.