Radiography is the creation of radiographs, photographs made by exposing a photographic film or other image receptor to x-rays. Since x-rays penetrate solid objects, but are slightly attenuated by them, the picture resulting from the exposure reveals the internal structure of the object. The most common use of radiography is in the medical field (where it is known as medical imaging), but veterinarians and engineers also use radiography. X-ray radiography has been in use for quite some time. Neutron radiography was later addition. X-ray radiography and neutron radiography are complementary to each other.
The type of electromagnetic radiation of most interest to radiography is x-ray and gamma radiation. This radiation is much more energetic than the more familiar types such as radio waves and visible light. It is this relatively high energy, which makes gamma rays useful in radiography but potentially hazardous to living organisms. They are produced by X-ray tubes, high-energy X-ray equipment or natural radioactive elements, such as Radium and Radon, and artificially produced radioactive isotopes of elements, such as Cobalt 60 and Iridium 192. Electromagnetic radiation consists of oscillating electric and magnetic fields. It is generally pictured as a single sinusoidal wave.
It is characterised by its wavelength (the distance from a point on one cycle to the point on the next cycle) or its frequency (the number of oscillations per second). All electromagnetic waves travel at the same speed, the speed of light (c). The wavelength (W) and the frequency (?) are all related by the equation:
Wv = c
This is true for all electromagnetic radiation. Electromagnetic radiation is known by various names, depending on its energy. The energy of these waves is related to the frequency and the wavelength by the relationship:
E = hv = hc / W
Where h is a constant known as Plancks constant.
Gamma rays are indirectly ionising radiation. A gamma ray passes through matter until it undergoes an interaction with an atomic particle, usually an electron. During this interaction, energy is transferred from the gamma ray to the electron, which is a directly ionising particle. As a result of this energy transfer, the electron is liberated from the atom and proceeds to ionise matter by colliding with other electrons along its path. For the range of energies commonly used in radiography, the interaction between gamma rays and electrons occurs in two ways. One effect takes place where all the gamma rays energy is transmitted to an entire atom. The gamma ray no longer exists and an electron emerges from the atom with kinetic (motion in relation to force) energy almost equal to the gamma energy. This effect is predominant at low gamma energies and is known as the photoelectric effect. The other major effect occurs when a gamma ray interacts with an atomic electron, freeing it from the atom and imparting to it only a fraction of the gamma rays kinetic energy. A secondary gamma ray with less energy (hence lower frequency) also emerges from the interaction. This effect predominates at higher gamma energies and is known as the Compton effect. In both of these effects the emergent electrons lose their kinetic energy by ionising surrounding atoms. The density of ions so generated is a measure of the energy delivered to the material by the gamma rays. The most common means of measuring the variations in a beam of radiation is by utilising its effects onto a photographic film. This effect is the same as that of light, and the more intense the radiation is, it will produce a darker film, or a more exposed film. Other methods are in use, such as the ionising effect measured electronically, its ability to discharge an electrostatically charged plate or to cause certain chemicals to fluoresce as in fluoroscopy.
X-rays are, after blood tests, the most commonly used medical investigations performed. Bone and some organs (e.g. lungs) especially lend themselves for imaging by X-ray. It is a relatively low-cost investigation with a very high diagnostic yield, although CT scans or other more specialised modalities are occasionally necessary to delineate disease processes. Ultrasound, by comparison, requires more expertise to perform.
Industrial radiography is a nondestructive method of inspecting materials for hidden flaws by utilising the ability of short wavelength electromagnetic radiation to penetrate various materials. The value of this ability lays in the fact the material to a degree dependent upon its composition and thickness absorbs penetrating radiation. 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 about 10 nanometres. The beam of radiation shall be directed to the middle of the section under examination and shall be normal to the material surface at that point, except on special techniques where known defects would be 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%. So, to follow up on what has just been said, 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 (becoming greater by successive additions) in its response, relatively weak radiation can be detected by prolonging the exposure until the film can record an image, which 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 visually before hand, in order to eliminate, as a preliminary, any external defects, if the surface of a weld metal is too irregular, it may be desirable to grind it to obtain a sufficiently smooth finish, nevertheless, this is likely to be limited to those cases which the surface irregularities (which will be visible on the radiograph) may lead to difficulties in detecting any internal defects. 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.
|Posted : 10/26/2005|