Industrial Radiography - Sources

Sources

A high-energy X-ray machine can be used. As an alternative, radioisotope sources have the advantage that they do not need a supply of electrical power to function, but they can not be turned off. Also it is difficult using radioactivity to create a small and compact source that offers the photon flux possible with a normal sealed X-ray tube.

It might be possible to use caesium-137 as a photon source for radiography but this isotope is always diluted with inactive caesium isotopes. This makes it difficult to get a physically small source, and a large volume of the source makes it impossible to capture fine details in a radiographic examination.

Both cobalt-60 and caesium-137 have only a few gamma energies, which makes them close to monochromatic. The photon energy of cobalt-60 is higher than that of caesium-137, which allows cobalt sources to be used to examine thicker sections of metals than those that could be examined with Cs-137. Iridium-192 has a lower photon energy than cobalt-60 and its gamma spectrum is complex (many lines of very different energies), but this can be an advantage as this can give better contrast for the final photographs.

It has been known for many years that an inactive iridium or cobalt metal object can be machined to size. In the case of cobalt it is common to alloy it with nickel to improve the mechanical properties. In the case of iridium a thin wire or rod could be used. These precursor materials can then be placed in stainless steel containers that have been leak tested before being converted into radioactive sources. These objects can be processed by neutron activation to form gamma-emitting radioisotopes. The stainless steel has only a small ability to be activated and the small activity due to 55Fe and 63Ni are unlikely to pose a problem in the final application because these isotopes are beta emitters, which have very weak gamma emission. The 59Fe isotope which might form has a short half-life, so by allowing a cobalt source to stand for a year much of this isotope will decay away.

The source is often a very small object, which must be transported to the work site in a shielded container. It is normal to place the film in industrial radiography, clear the area where the work is to be done, add shielding (collimators) to reduce the size of the controlled area before exposing the radioactive source. A series of different designs have been developed for radiographic "cameras". Rather than the "camera" being a device that accepts photons to record a picture, the "camera" in industrial radiography is the radioactive photon source.