This technique is suitable for the detection of internal defects in ferrous and non- ferrous metals and other materials.
X-rays, generated electrically, and Gamma rays emitted from radio-active isotopes, are penetrating radiation which is differentially absorbed by the material through which it passes; the greater the thickness, the greater the absorbtion. Furthermore, the denser the material the greater the absorbtion.
X and Gamma rays also have the property, like light, of partially converting silver halide crystals in a photographic film to metallic silver, in proportion to the intensity of the radiation reaching the film, and therefore forming a latent image. This can be developed and fixed in a similar way to normal photographic film.
Material with internal voids is tested by placing the subject between the source of radiation and the film. The voids show as darkened areas, where more radiation has reached the film, on a clear background. The principles are the same for both X and Gamma radiography.
In X-radiography the penetrating power is determined by the number of volts applied to the X-Ray tube - in steel approximately 1000 volts per inch thickness is necessary. In Gamma radiography the isotope governs the penetrating power and is unalterable in each isotope. Thus Iridium 192 is used for 1/2" to 1" steel and Caesium 134 is used for 3/4" to 21/2" steel.
In X-radiography the intensity, and therefore the exposure time, is governed by the amperage of the cathode in the tube. Exposure time is usually expressed in terms of milliampere minutes. With Gamma rays the intensity of the radiation is set at the time of supply of the isotope. The intensity of radiation from isotopes is measured in Becquerel’s and reduces over a period of time. The time taken to decay to half the amount of curies is the half life and is characteristic of each isotope. For example, the half life of Iridium 192 is 74 days, and Caesium 134 is 2.1 years. The exposure factor is a product of the number of curies and time, usually expressed in curie hours. The time of exposure must be increased as the isotope decays - when the exposure period becomes uneconomical the isotope must be renewed.
As the isotope is continuously emitting radiation it must be housed in a container of deleted uranium or similar dense shielding material, whilst not exposed to protect the environment and personnel.
To produce an X or Gamma radiograph, the film package (comprising film and intensifying screens - the latter being required to reduce the exposure time - enclosed in a light tight cassette) is placed close to the surface of the subject.
This technique is suitable for the detection of internal defects in ferrous and non- ferrous metals and other materials.X-rays, generated electrically, and Gamma rays emitted from radio-active isotopes, are penetrating radiation which is differentially absorbed by the material through which it passes; the greater the thickness, the greater the absorbtion. Furthermore, the denser the material the greater the absorbtion.X and Gamma rays also have the property, like light, of partially converting silver halide crystals in a photographic film to metallic silver, in proportion to the intensity of the radiation reaching the film, and therefore forming a latent image. This can be developed and fixed in a similar way to normal photographic film.Material with internal voids is tested by placing the subject between the source of radiation and the film. The voids show as darkened areas, where more radiation has reached the film, on a clear background. The principles are the same for both X and Gamma radiography.In X-radiography the penetrating power is determined by the number of volts applied to the X-Ray tube - in steel approximately 1000 volts per inch thickness is necessary. In Gamma radiography the isotope governs the penetrating power and is unalterable in each isotope. Thus Iridium 192 is used for 1/2" to 1" steel and Caesium 134 is used for 3/4" to 21/2" steel.In X-radiography the intensity, and therefore the exposure time, is governed by the amperage of the cathode in the tube. Exposure time is usually expressed in terms of milliampere minutes. With Gamma rays the intensity of the radiation is set at the time of supply of the isotope. The intensity of radiation from isotopes is measured in Becquerel’s and reduces over a period of time. The time taken to decay to half the amount of curies is the half life and is characteristic of each isotope. For example, the half life of Iridium 192 is 74 days, and Caesium 134 is 2.1 years. The exposure factor is a product of the number of curies and time, usually expressed in curie hours. The time of exposure must be increased as the isotope decays - when the exposure period becomes uneconomical the isotope must be renewed.As the isotope is continuously emitting radiation it must be housed in a container of deleted uranium or similar dense shielding material, whilst not exposed to protect the environment and personnel.To produce an X or Gamma radiograph, the film package (comprising film and intensifying screens - the latter being required to reduce the exposure time - enclosed in a light tight cassette) is placed close to the surface of the subject.
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