Radiographic Examination – Radiation Sources

Both x-rays and gamma rays can be used as the incident radiation. Energy and intensity are the most important characteristics of the incident radiation. The energy of the incident radiation determines its ability to penetrate the workpiece. Higher energy radiation is required to penetrate thicker workpieces but it reduces the scatter of radiation passing through the workpiece. The intensity of the radiation reaching the photographic film after it has passed through the workpiece controls the length of time required to properly expose the film. Higher intensity radiation is required for thicker workpieces to obtain reasonable exposure times, because attenuation of the radiation passing through the workpiece increases with material thickness.

X-rays are usually produced by x-ray tubes, which are electronic devices that convert electrical energy into x-rays. The voltage of the x-ray tube determines the energy of the x-rays produced, and the current controls the intensity of the radiation. Consequently, higher voltages and currents are required for thicker workpieces, which necessitate larger x-ray tubes and electrical power sources. The bulk of the equipment required to produce high energy-high intensity radiation with x-ray tubes severely restricts portability. Therefore, RT systems suitable for pressure vessels are generally not satisfactory for shell thicknesses greater than 2 inches.

Linear accelerators (LINAC) can also be used to produce x-rays. They are much more practical than x-ray tubes for producing the high energy-high intensity radiation required for thick workpieces, and some systems are sufficiently portable. However, the equipment is still quite cumbersome to handle.

Gamma-rays are created by the radioactive decay of unstable isotopes of naturally occurring or artificially produced elements. Cobalt-60 and iridium-192 are the two isotopes most commonly used for RT. The radiation has a relatively high energy for penetrating thick workpieces. However, the intensity of the radiation is generally lower than that produced by x-ray tubes or LINACs, and it diminishes with time as the radioactive isotope decays. Therefore, the time required to properly expose the film can be quite long. Nevertheless, the equipment required for RT with gamma rays from a radioactive isotope is much less cumbersome than that required to produce x-rays, and it is therefore more suitable.

A “point” source of radiation would provide the sharpest images of flaws on the photographic film exposed to the radiation passing through the workpiece. However, actual sources of radiation used for RT are provided with an aperture of approximately 1/10 to 1/4 inch to obtain sufficient intensity of radiation to expose the film in a reasonable length of time. Consequently, some “geometric unsharpness” results in the image of the flaw, because the source of radiation is not a true point source. The unsharpness is reduced by large source-to-film distances relative to the thickness of the workpiece. The source-to-film distance should be at least five times the thickness of the workpiece to give satisfactory clarity.

It is extremely important to recognize that the radiation sources used for RT have much higher energies and intensities than those used for medical x-rays and therefore can very severely damage animal tissues. Severe disability or death can result from exposure to the radiation. Therefore, it is essential to provide proper shielding of the radiation source and to prevent unauthorized access to the area when RT is being performed. Guidance for shielding and restriction of access should be obtained from knowledgeable safety and health specialists before performing the RT.

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