T.O. 33B-1-16-66.1.4 UniquePropertiesofGammaRays.6.1.4.1 General.Gamma-ray radiography is basically the same as X-ray radiography. Differences in material properties and effectsbetween X-rays and gamma rays are largely a matter of degree. The major advantage of using gamma rays is the factthat gamma ray sources are small and provide access to small spaces, thereby simplifying exposure technique.Exposure periods are, however, generally longer with gamma ray sources.6.1.4.2 SourcesOfRadiation.Many atoms exhibit a property called radioactivity, which is a phenomenon of spontaneous disintegration or decay.This characteristic is believed to be caused by the instability of the complex structure of the atom under the action of theelectric, magnetic and gravitational forces existing within. Radium is one of the elements with a natural unbalance thatreleases energy in the form of gamma rays to achieve a more stable condition. In addition to the gamma rays, somealpha particles (helium nuclei) and beta particles (electrons) are allowed to escape. The alpha and beta particles arereadily absorbed, but the gamma rays are more penetrating since their energy extends above 1,000,000 electron volts(eV). This energy release is uncontrolled and is a result of forces in the atom. Many of the atomic structures can beartificially made to release energy by subjecting them to strong fields of neutrons generated in nuclear reactors. Theseneutron fields add energy to the atom, which upsets the balance within the nucleus and causes the atom to emit one ormore types of energy. Cobalt is one element commonly made artificially radioactive and used in NDI since the energyit releases is a very penetrating form of gamma rays. An example of nuclear disintegration and the release of energy isshown in Figure 6-4.Figure 6-4. Diagram of Nuclear Disintegration.SECTION IIX-RAY GENERATORS6.2 X-RAYGENERATORS.6.2.1 Definition.X-ray generators are man-made electronic devices designed to produce X radiation. Many types of X-ray generatorsmay be obtained commercially. X-ray equipment may be either portable or stationary. Portable X-ray generators areused for inspection of test objects that are either impossible or very difficult to transport. Stationary X-ray generatorsare used in shielded facilities where the objects to be tested can be readily transported to the X-ray equipment.
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