X-ray machines may be either fixed or mobile, depending on the specific uses for which they are intended. When the material to be radiographed is portable, the x-ray machine is usually permanently located in a room protected against the escape of x-radiation. The x-ray tube itself is frequently mounted on a stand allowing considerable freedom of movement. For the examination of objects that are fixed or that are movable only with great difficulty, mobile x-ray machines may be used. These may be truck-mounted for movement to various parts of a plant, or they may be small and light enough to be carried onto scaffolding, through manholes, or even self-propelled to pass through pipelines. Semiautomatic machines have been designed for the radiography of large numbers of relatively small parts on a "production line" basis. During the course of an exposure, the operator may arrange the parts to be radiographed at the next exposure, and remove those just radiographed, with an obvious saving in time.
Gamma-Ray Sources
Radiography with gamma rays has the advantages of simplicity of the apparatus used, compactness of the radiation source, and independence from outside power. This facilitates the examination of pipe, pressure vessels, and other assemblies in which access to the interior is difficult; field radiography of structures remote from power supplies; and radiography in confined spaces, as on shipboard.
In contradistinction to x-ray machines, which emit a broad band of wavelengths (see "X-rays"), gamma-ray sources emit one or a few discrete wavelengths. Figure 8 shows the gamma-ray spectrum of cobalt 60 and the principal gamma rays of iridium 192. (The most intense line in each spectrum has been assigned an intensity of 1.0.)
Figure 8: Gamma-ray spectrum of cobalt 60 (solid lines) and principal gamma rays of iridium 192 (dashed lines).
Note that gamma rays are most often specified in terms of the energy of the individual photon, rather than in the wavelength. The unit of energy used is the electron volt (eV)--an amount of energy equal to the kinetic energy an electron attains in falling through a potential difference of 1 volt. For gamma rays, multiples--kiloelectron volts (keV; 1 keV = 1,000 eV) or million electron volts (MeV; 1 MeV = 1,000,000 eV)--are commonly used. A gamma ray with an energy of 0.5
MeV (500 keV) is equivalent in wavelength and in penetrating power to the most penetrating radiation emitted by an x-ray tube operating at 500 kV. The bulk of the radiation emitted by such an x-ray tube will be much less penetrating (much softer) than this (see Figure 18). Thus the radiations from cobalt 60, for example, with energies of 1.17 and 1.33 MeV, will have a penetrating power (hardness) about equal to that of the radiation from a 2-million-volt x-ray machine.
For comparison, a gamma ray having an energy of 1.2 MeV has a wavelength of about 0.01 angstrom (A); a 120 keV gamma ray has a wavelength of about 0.1 angstrom.
The wavelengths (or energies of radiation) emitted by a gamma-ray source, and their relative intensities, depend only on the nature of the emitter. Thus, the radiation quality of a gamma ray source is not variable at the will of the operator.
The gamma rays from cobalt 60 have relatively great penetrating power and can be used, under some conditions, to radiograph sections of steel 9 inches thick, or the equivalent. Radiations from other radioactive materials have lower energies; for example, iridium 192 emits radiations roughly equivalent to the x-rays emitted by a conventional x-ray tube operating at about 600 kV.
The intensity of gamma radiation depends on the strength of the particular source used-- specifically, on the number of radioactive atoms in the source that disintegrate in one second. This, in turn, is usually given in terms of curies (1 Ci = 3.7 x 1010s-1). For small or moderate- sized sources emitting penetrating gamma rays, the intensity of radiation emitted from the source is proportional to the source activity in curies. The proportionality between the external gamma- ray intensity and the number of curies fails, however, for large sources or for those emitting relatively low-energy gamma rays. In these latter cases, gamma radiation given off by atoms in the middle of the source will be appreciably absorbed (self-absorption) by the overlying radioactive material itself. Thus, the intensity of the useful radiation will be reduced to some value below that which would be calculated from the number of curies and the radiation output of a physically small gamma-ray source.
A term often used in speaking of radioactive sources is specific activity, a measure of the degree of concentration of a radioactive source. Specific activity is usually expressed in terms of curies per gram. Of two gamma-ray sources of the same material and activity, the one having the greater specific activity will be the smaller in actual physical size. Thus, the source of higher specific activity will suffer less from self-absorption of its own gamma radiation. In addition, it will give less geometrical unsharpness in the radiograph or, alternatively, will allow shorter source- film distances and shorter exposures (See the calculation under "General Principles").
Gamma-ray sources gradually lose activity with time, the rate of decrease of activity depending on the kind of radioactive material (See Table II). For instance, the intensity of the radiation from a cobalt 60 source decreases to half its original value in about 5 years; and that of an iridium 192 source, in about 70 days. Except in the case of radium, now little used in industrial radiography, this decrease in emission necessitates more or less frequent revision of exposures and replacement of sources.
Table II - Radioactive Materials Used in Industrial Radiography
Radioactive
Element
|
Half-Life
|
Energy of Gamma Rays (MeV)
|
Gamma-Ray Dosage Rate (roentgens1 per hour per curie
at 1 metre)
|
Thulium 170
|
127 days
|
0.084 and 0.542
|
--
|
Iridium
192
|
70 days
|
0.137 to 0.6513
|
0.55
|
Cesium 137
|
33 years
|
0.66
|
0.39
|
Cobalt 60
|
5.3 years
|
1.17 and 1.33
|
1.35
|
1 The roentgen (R) is a special unit for x- and gamma-ray exposure (ionization of air): 1 roentgen = 2.58 x.10-4 coulombs per kilogram (Ckg-1). (The International Commission on Radiation Units and Measurements [ICRU] recommends that the roentgen be replaced gradually by the SI unit [Ckg-1] by about 1985.)
2 These gamma rays are accompanied by a more or less intense background of much harder radiation. The proportion of hard radiation depends upon the chemical nature and physical size of the source.
3 Twelve gamma rays.
The exposure calculations necessitated by the gradual decrease in the radiation output of a gamma-ray source can be facilitated by the use of decay curves similar to those for indium 192 shown in figure 9. The curves contain the same information, the only difference being that the curve on the left shows activity on a linear scale, and the curve on the right, on a logarithmic scale. The type shown on the right is easier to draw. Locate point X, at the intersection of the half- life of the isotope (horizontal scale) and the "50 percent remaining activity" line (vertical scale). Then draw a straight line from the "zero time, 100 percent activity" point Y through point X.
Figure 9: Decay curves for iridium 192. Left: Linear plot. Right: Logarithmic plot.
It is difficult to give specific recommendations on the choices of gamma-ray emitter and source strength (See Figure 10). These choices will depend on several factors, among which are the type of specimen radiographed, allowable exposure time, storage facilities available, protective measures required, and convenience of source replacement. The values given in Table III for practical application are therefore intended only as a rough guide and in particular case will depend on the source size used and the requirements of the operation.
Figure 10: Typical industrial gamma-ray arrangement. Gamma-ray source in a combination "camera" and storage container.
Table III - Industrial Gamma-Ray Sources and Their Applications
Source
|
Applications and Approximate Practical Thickness Limits
|
Thulium 170
|
Plastics, wood, light alloys. 1/ -inch steel
or equivalent.
2
|
Iridium 192
|
11/ - to 21/ -inch steel
or equivalent.
2
2
|
Cesium 137
|
1 to 31/ -inch steel
or equivalent.
2
|
Cobalt 60
|
21/ - to 9-inch steel or equivalent.
2
|
1The atomic number of an element is the number of protons in the nucleus of the atom, and is equal to the number of electrons outside the nucleus. In the periodic table the elements are arranged in order of increasing atomic number. Hydrogen has an atomic number of 1; iron, of 26; copper, of 29; tungsten, of 74; and lead of 82.