Industrial Uses of Radioisotopes
Radioisotopes are used by industry for a number of purposes, as reactor startup fuel, for qualitative and quantitative analysis, as thickness and level gauges, and to insure uniform mixing of materials, to mention a few. The following is a discussion of some isotope applications.
Neutron Sources
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When we think of fuels we tend to think of coal, wood, and gasoline. But for a nuclear reactor used to generate electricity, uranium-235 is also fuel. Uranium-235 is an easily fissile isotope, but it needs to be struck by a neutron before it splits just as a match needs to be struck before it ignites. When it fissions, it releases a lot of energy that can be used to produce steam and drive a turbine to produce electricity. When the uranium fissions, it also releases neutrons that are used to split even more uranium-235 nuclei, thus starting a chain reaction and the continuous production of electricity. Uranium-235, however, does not spontaneously fission at a high enough rate; a chain reaction must be initiated by some other means. Because californium-252 does spontaneously fission, thereby releasing neutrons, it can be used as a neutron source–a startup fuel—to initiate the uranium-235 chain reaction. One inserts the californium-252 into the reactor to "fire" it up and start the production of electricity.
Californium-252 is also used in security devices as a means to detect the presence of hidden explosives such as one might uncover in an airport. Many explosives are composed of nitrogen-containing chemical compounds. Nitrogen-14 , the most abundant isotope of nitrogen, has a relatively large affinity for capturing neutrons. After the nitrogen captures the neutron, the nitrogen is in an excited state and loses energy by emitting a characteristic gamma ray. (Gamma rays are photons emitted by a nucleus and are part of the same electromagnetic spectrum as visible light–the light you see.) By irradiating a suspected item with neutrons, these gamma rays may be detected and inform the security agents that an explosive is possibly present.
Gamma Radiography
Gamma-emitting isotopes have important industrial applications in radiography. Radiography is a technique by which an external, sealed source of radioactivity is placed on one side of the material to be examined and a gamma detector, such as a piece of photographic paper, on the other. If one develops the photographic paper and sees that it was exposed, then this means gamma rays passed through the material rather than being absorbed or scattered by it. In this regard, gamma radiography is similar to taking an x-ray. Gamma radiography, however, can be used on denser, thicker materials that otherwise are completely opaque to x-rays because the gamma rays are of much higher energy and therefore penetrate further than x-rays. As an example, iridium-192, a gamma emitter, is used to nondestructively test the integrity of oil pipes and other cast or welded objects. The source is placed inside the pipe or on one side of a weld, and photographic film is set opposite to the source on the other side. By developing the photographic paper, one can determine if there are cracks in the pipe or if welded joints are flawed. Cobalt-60 and ytterbium-169 are two gamma sources in addition to iridium-192 that have found extensive use in radiography.
Uniform Mixing
Metal alloys such as vanadium steel are made of more than one material and must be thoroughly mixed in the molten stage to insure maximum strength. Radioisotopes can play an important role in guaranteeing that the metal is uniform throughout. In the case of vanadium steel, a known amount of radioactive vanadium-52 can be added to the molten metal during the mixing stage. If the metal is thoroughly mixed, then each sample of metal, whether drawn from the top, bottom, or middle of the batch, should show the same vanadium activity per unit volume of molten metal as measured by the high-energy gamma ray that is emitted when vanadium-52 decays. Obviously this technique can be applied to any process where uniform mixing is required.
Radioisotope Gauging
This is a technique that takes advantage of the fact that the more matter between the radioactive source and its detector, the less radiation gets through. By measuring the amount of radiation that passes through a material, such as a thin film of plastic, one can gauge the film's thickness with great precision. A simple way to calibrate the detector is to place known thicknesses of plastic, for example mylar, between the radioactive source and the detector, then measure how much radiation gets through. In this manner, one constructs a calibration curve that can be used to measure the thickness of an unknown sample of the same type of plastic. Similarly, a gamma source may be used to monitor the thickness of a film while it is being manufactured in order to insure a uniform thickness throughout the product during production.
If the film to be gauged is a surface coating on a material through which the radiation can not penetrate, then the thickness of the coating can be measured through detection of the back-scattered radiation. (If it does not go through, it is either absorbed or scattered.) How it back scatters depends on the thickness of the surface coating, say a silver plating or other precious metal, where conservation of material is highly important. As another example, optical coatings must be of very uniform thickness, and radioisotope gauges are used to measure their thicknesses.
Radioisotopes can be used as level gauges as well. One places a radioactive source such as cobalt-60, a gamma emitter, and a detector at the level not to be exceeded. If the height of the material exceeds this level, it will break the flow of radiation between the source and its detector. This interruption can be used to sound an alarm, or if the loading process is automated, it can be used to stop the process. One example where radioisotopes are used as level gauges is in coal hoppers. Once the height of the coal reaches the level gauge, the gauge initiates a sequence of events that stops loading the hopper.
Other isotopes found useful for thickness or level gauging are krypton-85 and cesium-137.
X-ray Fluorescence
Like gamma rays, x-rays are part of the electromagnetic spectrum that includes more commonly known ultraviolet radiation (the kind that causes sunburn), infrared radiation (used to enhance night-time vision), and visible light. Unlike gamma rays that originate from the nucleus, x-rays originate from energy transitions that take place in the electron configuration of an atom. Each element, such as carbon, cobalt, zinc, nickel, etc., can be induced to fluoresce, that is emit x-rays that uniquely define what element they came from. In this way, induced x-ray fluorescence can be used for qualitative analysis. Furthermore, the intensities of these x-rays can be used to quantify how much of an element is present so that the technique can be used for both qualitative and quantitative analysis.
Cadmium-109 is a radioisotope that emits a gamma ray that can be used to induce fluorescence in coal and other materials. The detected x-rays can then be used to determine sulfur content of the coal and thereby used to separate cleaner burning coals having less sulfur from lower grade coals. Likewise, it can be used for qualitative analysis of metal alloys.
Miscellaneous Uses of Radioisotopes
Radioisotopes have many other industrial uses in addition to those described above. For example, in addition to its use as a level gauge, cobalt-60 is used to sterilize surgical instruments and to preserve food. Americium-241 is an alpha emitter that is widely used in home smoke detectors. Makers of photographic film use polonium-210, an electron scavenger, to reduce the buildup of static charge. As a final example, thorium-229 is used to make fluorescent lights last longer.
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