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Industrial radiography

Radiography is the use of ionising electromagnetic radiation to view objects in a way that can't be seen otherwise. Radiography in general should not be confused with the use of ionizing radiation to change or modify objects; radiography's purpose is strictly for viewing. Industrial radiography has grown out of engineering, and is a major element of nondestructive testing (NDT). It is a method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation to penetrate various materials.

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History

Radiography started in 1895 with the discovery of X-rays (later also called Röntgen rays after the man who first described their properties in rigorous detail), a type of electromagnetic radiation. Soon after the discovery of X-rays, radioactivity was discovered. By using radioactive sources such as radium, far higher photon energies could be obtained than those which can be obtained from normal X-ray machines. Soon these found various applications, from helping to find shoes that fit, more lasting medical uses and the examination of non-living objects. X-rays and gamma-rays were put to use very early, before the dangers of ionising radiation were discovered. After World War II new isotopes such as caesium-137, iridium-192 and cobalt-60 were made available for use in industrial radiography, hence the use of radium and radon decreased.

Uses

Inspection of welds

The beam of radiation must be directed to the middle of the section under examination and must be normal to the material surface at that point, except in special techniques where known defects are best revealed by a different alignment of the beam. The length of weld under examination for each exposure shall be such that the thickness of the material at the diagnostic extremities, measured in the direction of the incident beam, does not exceed the actual thickness at that point by more than 6%. The specimen to be inspected is placed between the source of radiation and the detecting device, usually the film in a light tight holder or cassette, and the radiation is allowed to penetrate the part for the required length of time to be adequately recorded.

The result is a two-dimensional projection of the part onto the film, producing a latent image of varying densities according to the amount of radiation reaching each area. It is known as a radiograph, as distinct from a photograph produced by light. Because film is cumulative in its response (the exposure increasing as it absorbs more radiation), relatively weak radiation can be detected by prolonging the exposure until the film can record an image that will be visible after development. The radiograph is examined as a negative, without printing as a positive as in photography. This is because, in printing, some of the detail is always lost and no useful purpose is served.

Before commencing a radiographic examination, it is always advisable to examine the component with one's own eyes, to eliminate any possible external defects. If the surface of a weld is too irregular, it may be desirable to grind it to obtain a smooth finish, but this is likely to be limited to those cases in which the surface irregularities (which will be visible on the radiograph) may make detecting internal defects difficult.

After this visual examination, the operator will have a clear idea of the possibilities of access to the two faces of the weld, which is important both for the setting up of the equipment and for the choice of the most appropriate technique.

Airport security

Both hold luggage and carry-on hand luggage are normally examined by X-ray machines using X-ray radiography. See airport security for more details.

Non-intrusive Cargo Scanning (aka Non-Intrusive Inspection - NII)

Gamma Radiography and High-Energy X-ray radiography are currently used to scan intermodal freight cargo containers in US and other countries. Also research is being done on adapting other types of radiography like Dual-Energy X-ray Radiography or Muon Radiography for scanning intermodal cargo containers.

Sources

X-ray sources

A high energy X-ray machine can be used. It is often important to use a high accelerating voltage to provide the electrons with a very high energy. This is because in a braking radiation source the maximum photon energy is determined by the energy of the charged particles. A recent development is the betatron, which is a device similar to a cyclotron that acts as a very intense photon source. [1][2]

Radioisotope sources

These have the advantage that they do not need a supply of electrical power to function, but they do have the disadvantage that they can not be turned off. Also it is difficult using radioactivity to create a small and compact source which offers the photon flux which is possible with a normal sealed X-ray tube. One of the leading makers of radiographic equipment is the Source Production & Equipment Co., Inc. [3]

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 cesium isotopes. This means that it is difficult to get a physcially 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 cesium-137 have only a few gamma energies which make them close to monochromatic. The photon energy of cobalt-60 is higher than that of cesium-137, which allows cobalt sources to be used to examine thicker sections of metals than those which 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 enegies), 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 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 needs to be transported to the site where the work is to be conducted 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 which accepts photons to record a picture, the "camera" in industrial radiography is the radioactive photon source.

Torch design of radiographic cameras

One design is best thought of as being like a torch. The radioactive source is placed inside a shielded box, a hinge allowed part of the shielding to be peeled back exposing the source so allowing the photons to leave the radiography camera.

Another design for a torch is one where the source is placed in a metal wheel, this can turn inside the camera to move between the exposed and storage sites.

Cable based design of radiographic cameras

One group of designs use a radioactive source which comes out on a cable from a shielded container.[4] One such unit was involved in an accident [5] which occurred in Bolivia. This type of radiography could be compared to the remote afterloading method in Brachytherapy. In one design of equipment the source is stored in a block of lead or DU metal which has a S shaped tube like hole which passes through the block. In the safe position the source is in the centre of the block and is attached to a metal wire which extends in both directions, to use the source a guide tube is attached to one side of the block while a drive cable is attached to the other end of the short cable. Using a hand operated winch the source is then pushed out of the shield and along the guide tube to were it is needed. It was one of these cable-based systems which was involved in an accident [6] which occurred in Bolivia.

Contrast agents

Defects such as delaminations and planar cracks are difficult to detect using radiography, which is why penetrants are often used to enhance the contrast in the detection of such defects. Penetrants used include silver nitrate, zinc iodide, chloroform and diiodomethane. Choice of the penetrant is determined by the ease with which it can penetrate the cracks and also with which it can be removed. Diiodomethane has the advantages of high opacity, ease of penetration, and ease of removal because it evaporates relatively quickly. However, it can cause skin burns.

Neutrons

In some rare cases, radiography is done with neutrons. This type of radiography is called neutron radiography (NR, Nray, N-Ray) or neutron imaging. Neutron radiography can see very different things than X-rays, because neutrons can pass with ease through lead and steel but are stopped by plastics, water and oils. Neutron sources include radioactive (²⁴¹Am/Be and Cf) sources, electrically driven D-T reactions in vacuum tubes and conventional critical nuclear reactors. It might be possible to use a neutron amplifier to increase the neutron flux.[7]

Since the amount of radiation emerging from the opposite side of the material can be detected and measured, variations in this amount (or intensity) of radiation are used to determine thickness or composition of material. Penetrating radiations are those restricted to that part of the electromagnetic spectrum of wavelength less than about 10 nanometres.

Safety

Industrial radiography appears to have one of the worst safety profiles of the radiation professions, possibly because there are many operators using strong gamma sources (> 2 Ci) in remote sites with little supervision when compared with workers within the nuclear industry or within hospitals.[8] Many of the "lost source" accidents commented on by the International Atomic Energy Agency involve radiography equipment. Lost source accidents have the potential to cause a considerable loss of human life. One scenario is that a passerby finds the radiography source and not knowing what it is, takes it home.[9] The person shortly afterwards becomes ill and dies as a result of the radiation dose. The source remains in their home where it continues to irradiate other members of the household.[10] Such an event occurred in March 1984 in Casablanca (Mohammedia) which is part of Morocco; this is related to the more famous Goiânia accident, where a related chain of events caused members of the public to be exposed to radiation sources. Also see List of civilian radiation accidents.