X Ray

B

Compton Effect

The Compton effect, discovered in 1923 by the American physicist and educator Arthur Holly Compton, is an important manifestation of the absorption of X rays of shorter wavelengths. When a high-energy photon collides with a stationary electron, both particles may be deflected at an angle to the direction of the path of the incident X ray. The incident photon, having delivered some of its energy to the electron, emerges from the impact with a lower frequency and a correspondingly longer wavelength. These deflections, accompanied by a change of wavelength, are known as Compton scattering.

C

Pair Production

In the third type of absorption, especially evident when elements of high atomic weight are irradiated with extremely high-energy X rays, the phenomenon of pair production occurs. When a high-energy photon penetrates the electron shell close to the nucleus, it may create a pair of electrons, one of negative charge and the other positive; a positively charged electron is also known as a positron. This pair production is an example of the conversion of energy into mass. The photon requires at least 1.2 MeV of energy to yield the mass of the pair. If the incident photon possesses more energy than is required for pair production, the excess energy is imparted to the electron pair as kinetic energy. The paths of the two particles are divergent.

VI

Applications of X Rays

The principal uses of X radiation are in the field of scientific research, industry, and medicine.

A

Research

The study of X rays played a vital role in theoretical physics, especially in the development of quantum mechanics. As a research tool, X rays enabled physicists to confirm experimentally the theories of crystallography. By using X-ray diffraction methods, crystalline substances may be identified and their structure determined. Virtually all present-day knowledge in this field was either discovered or verified by X-ray analysis. X-ray diffraction methods can also be applied to powdered substances that are not crystalline but that display some regularity of molecular structure. By means of such methods, chemical compounds can be identified and the size of ultramicroscopic particles can be established. Chemical elements and their isotopes may be identified by X-ray spectroscopy, which determines the wavelengths of their characteristic line spectra. Several elements were discovered by analysis of X-ray spectra.

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A number of recent applications of X rays in research are assuming increasing importance. Microradiography, for instance, produces fine-grain images that can be enlarged considerably. Two radiographs can be combined in a projector to produce a three-dimensional image called a stereoradiogram. Color-radiography is also used to enhance the detail of X-ray photographs; in this process, differences in the absorption of X rays by a specimen are shown as different colors (see Color). Extremely detailed and analytical information is provided by the electron microprobe, which uses a sharply defined beam of electrons to generate X rays in an area of specimen as small as 1 micrometer (about 1/25,000 in) square.

B

Industry

In addition to the research applications of X rays in physics, chemistry, mineralogy, metallurgy, and biology, X rays are used in industry as a research tool and for many testing processes. They are valuable in industry as a means of testing objects such as metallic castings without destroying them. X-ray images on photographic plates reveal the presence of flaws, but a disadvantage of such inspection is that the necessary high-powered X-ray equipment is bulky and expensive. In some instances, therefore, radioisotopes, which emit highly penetrating gamma rays, are used instead of X-ray equipment. These isotope sources can be housed in relatively light, compact, and shielded containers. Cobalt-60 and cesium-137 have been used widely for industrial radiography. Thulium-70 has been used in small, convenient, isotope projectors for some medical and industrial applications.

Many industrial products are inspected routinely by means of X rays so that defective products may be eliminated at the point of production. Other applications include the detection of fake gems and the detection of smuggled goods in customs examinations. Ultrasoft X rays are used to determine the authenticity of works of art and for art restoration.

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