radiography(redirected from subtraction radiography)
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radiography:see X rayX ray,
invisible, highly penetrating electromagnetic radiation of much shorter wavelength (higher frequency) than visible light. The wavelength range for X rays is from about 10−8 m to about 10−11
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a method for studying various objects, such as manufactured items and minerals, through the effect of the emissions of a radioisotope on photographic film. In radiography, external sources of ionizing radiation are used. These sources are specially produced radioisotopes placed in closed metal ampuls. In autoradiography, which is the main type of radiography, internal radiation sources are used; that is, the radioisotope is introduced into the object being studied (seeAUTORADIOGRAPHY).
If the ionizing radiation that penetrates an object registers on photographic material, the presence of areas of greater or lesser density can be discerned. This analysis derives from the uneven attenuation of radiation passing through areas free of defects as opposed to areas where defects are present. A photographic image, or shadowgraph, of the hidden defects is thus formed, which permits a determination of the shape and dimensions of these defects. In this way, radiography serves as a nondestructive technique for the quality control of cast, welded, soldered, and forged items and materials, that is, a technique for radioisotopic flaw detection.
X-ray films are most common for the purposes of radiography, whereas in autoradiography, various types of photographic materials are used. Among these materials are nuclear photographic emulsions, which permit the registering not only of the total effect, in the form of blackening, of the flux of ionizing particles on the film but also the effect of each individual particle. A chain of developed grains represents a trace, or track, of the particles on the film. The amount of radiation is measured using a characteristic curve found for each type of photographic emulsion and radiation, and the optical density of the photographic material is measured with such photometers as densitometers and microphotometers. A picture of the distribution of optical density is obtained by scanning the photographic image with the measurement aperture of a photometer. Regions of the object with a higher content of radioactive atoms correspond to regions of more intense blackening on the photographic image. This correspondence forms the basis for the radiographic study of the distribution of radioisotopes in solid objects.
The distribution of radioactive atoms in such microscopic objects as plant and animal cells and grains of metal is studied by using a microscope to examine the distribution of the particle tracks or individual developed grains of the photographic emulsion. The accuracy achieved in determining the location of isotopes in objects depends on such factors as the type and energy of the radiation, the thickness of the sample, the thickness of the film, and the distance between the sample and the photographic emulsion. Various radiographic techniques are available for different purposes. These techniques permit, for example, a detection of separate charged particles, a quantitative determination of radioactive atoms in separate regions of an object, and a measurement of the amount of ionizing radiation.
REFERENCESRadiografiia. Moscow, 1952. (Translated from English.)
Korobkov, V. I. Metodmakroavtoradiografii. Moscow, 1967.
Bruk, B. I. A vtoradiograficheskoe issledovanie metallov, primeniaemykh v sudostroenii. Leningrad, 1966.
Rogers, A. Avtoradiografiia. Moscow, 1972. (Translated from English.)
V. I. KOROBKOV