Radiographic equipment

This is a page devoted to the basic equipment used for radiographic work, both medical and industrial.

There are many types of sources for high energy X-ray and gamma photons.

X-ray generators are made of a high voltage power supply that is applied on a usually sealed X-ray tube. This produces the emission of electrons from the cathode of the tube and the emission of X-rays when these hit a target located at the anode side.

In industrial radiography, energy goes from 20 to several hundreds of kV according to the application. In medical radiography voltage from 20 kV in mammography up to 150 kV for chest radiography are used for diagnostic. Energy can go up to 250 kV for radiotherapy applications.

These have the advantage that they do not need a supply of electrical power to function, but they have the disadvantage that 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 Cs-137 as a photon source for radiography but this isotope has the disadvantage that it is always diluted with inactive caesium isotopes. This means that it is difficult to get a physically small source, a large radioactive volume of the source will make it impossible to get the finest detail from 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 within stainless steel containers, which are 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 that might form has a short half-life, so by allowing a cobalt source to stand for a year much of this isotope decays away.

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