This EOE article is adapted from an information paper published by the World Nuclear Association (WNA). WNA information papers are frequently updated, so for greater detail or more up to date numbers, please see the latest version on WNA website (link at end of article).
Many of the chemical elements have a number of isotopes. The isotopes of an element have the same number of protons in their atoms (atomic number) but different masses due to different numbers of neutrons. In an atom in the neutral state, the number of external electrons also equals the atomic number. These electrons determine the chemistry of the atom. The atomic mass is the sum of the protons and neutrons. There are 82 stable elements and about 275 stable isotopes of these elements.
When a combination of neutrons and protons that does not already exist in nature is produced artificially, the atom will be unstable and is called a radioactive isotope or radioisotope. There are also a number of unstable natural isotopes arising from the decay of primordial uranium and thorium. Overall there are some 1800 radioisotopes.
At present, there are up to 200 radioisotopes used on a regular basis, and most must be produced artificially. Radioisotopes can be manufactured in several ways. The most common is by neutron activation in a nuclear reactor. This involves the capture of a neutron by the nucleus of an atom resulting in an excess of neutrons (neutron rich). Some radioisotopes are manufactured in a cyclotron in which protons are introduced to the nucleus resulting in a deficiency of neutrons (proton rich).
The nucleus of a radioisotope usually becomes stable by emitting an alpha and/or beta particle. These particles may be accompanied by the emission of energy in the form of electromagnetic radiation known as gamma rays. This process is known as radioactive decay.
Radioisotopes have very useful properties. Radioactive emissions are easily detected and can be tracked until they disappear leaving no trace. Alpha, beta and gamma radiation, like x-rays, can penetrate seemingly solid objects, but are gradually absorbed by them. The extent of penetration depends upon several factors including the energy of the radiation, the mass of the particle, and the density of the solid. These properties lead to many applications for radioisotopes in the scientific, medical, forensic and industrial fields.
Neutron Techniques for Analysis
Neutrons can interact with atoms in a sample causing the emission of gamma rays that, when analyzed for characteristic energies and intensity, will identify the types and quantities of elements present. The two main techniques are Thermal Neutron Capture (TNC) and Neutron Inelastic Scattering (NIS). TNC occurs immediately after a low-energy neutron is absorbed by a nucleus, NIS takes place instantly when a fast neutron collides with a nucleus.
Most commercial analyzers use californium-252 neutron sources together with sodium iodide detectors and are mainly sensitive to TNC reactions. Other use americium-beryllium-241 sources and bismuth germanate detectors that register both TNC and NIS. NIS reactions are particularly useful for elements such as carbon, oxygen, aluminum, and silicon that have a low neutron capture cross section. Such equipment is used for a variety on on-line and on-belt analysis in the cement, mineral and coal industries.
A particular application of NIS is where a probe containing a neutron source can be lowered into a bore hole where the radiation is scattered by collisions with surrounding soil. Since hydrogen (the major component of water) is by far the best scattering atom, the number of neutrons returning to a detector in the probe is a function of the density of the water in the soil.
To measure soil density and water content, a portable device with an americium-beryllium-241 combination generates gamma rays and neutrons which pass through a sample of soil to a detector. The neutrons arise from alpha particles interacting with beryllium-9. A more sophisticated application of this is in borehole logging.
Gamma and X-ray Techniques in Analysis
Gamma ray transmission or scattering can be used to determine the ash content of coal on-line on a conveyor belt. The gamma ray interactions are atomic number dependant, and the ash is higher in atomic number than the coal combustible matter. Also the energy spectrum of gamma rays that have been inelastically scattered from the coal can be measured (Compton Profile Analysis) to indicate the ash content.
X-rays from a radioactive element can induce fluorescent x-rays from other non-radioactive materials. The energies of the fluorescent x-rays emitted can identify the elements present in the material, and their intensity can indicate the quantity of each element present.
This technique is used to determine element concentrations in process streams of mineral concentrators. Probes containing radioisotopes and a detector are immersed directly into slurry streams. Signals from the probe are processed to give the concentration of the elements being monitored, and can give a measure of the slurry density. Elements detected this way include iron, nickel, copper, zinc, tin and lead.
X-ray Diffraction (XRD) is a further technique for on-line analysis but does not use radioisotopes.
Gamma Radiography works in much the same way as x-rays screen luggage at airports. Instead of the bulky machine needed to produce x-rays, all that is needed to produce effective gamma rays is a small pellet of radioactive material in a sealed titanium capsule.
The capsule is placed on one side of the object being screened, and some photographic film is placed on the other side. The gamma rays, like x-rays, pass through the object and create an image on the film. Just as x-rays show a break in a bone, gamma rays show flaws in metal castings or welded joints. The technique allows critical components to be inspected for internal defects without damage.
Gamma sources are normally more portable than x-ray equipment so have a clear advantage in certain applications, such as in remote areas. Also, while x-ray sources emit a broad band of radiation, gamma sources emit at most a few discrete wavelengths. Gamma sources may also be much higher energy than all but the most expensive x-ray equipment, and hence have an advantage for much radiography. Where a weld has been made in an oil or gas pipeline, special film is taped over the weld around the outside of the pipe. A machine called a "pipe crawler" carries a shielded radioactive source down the inside of the pipe to the position of the weld. There, the radioactive source is remotely exposed and a radiographic image of the weld is produced on the film. This film is later developed and examined for signs of flaws in the weld.
X-ray sets can be used when electric power is available and the object to be x-rayed can be taken to the x-ray source and radiographed. Radioisotopes have the supreme advantage in that they can be taken to the site when an examination is required—and no power is needed. However, they cannot be simply turned off, and so must be properly shielded both when in use and at other times.
Non-destructive testing is an extension of gamma radiography used on a variety of products and materials. For instance, ytterbium-169 tests steel up to 15 mm thick and light alloys to 45 mm, while iridium-192 is used on steel 12 to 60 mm thick and light alloys to 190 mm.
The radiation that comes from a radioisotope has its intensity reduced by matter between the radioactive source and a detector. Detectors are used to measure this reduction. This principle can be used to gauge the presence or the absence, or even to measure the quantity or density, of material between the source and the detector. The advantage in using this form of gauging or measurement is that there is no contact with the material being gauged.
Many process industries utilize fixed gauges to monitor and control the flow of materials in pipes, distillation columns, etc., usually with gamma rays.
The height of the coal in a hopper can be determined by placing high-energy gamma sources at various heights along one side with focusing collimators directing beams across the load. Detectors placed opposite the sources register the breaking of the beam and hence the level of coal in the hopper. Such level gauges are among the most common industrial uses of radioisotopes.
Some machines that manufacture plastic film use radioisotope gauging with beta particles to measure the thickness of the plastic film. The film runs at high speed between a radioactive source and a detector. The detector signal strength is used to control the plastic film thickness.
In paper manufacturing, beta gauges are used to monitor the thickness of the paper at speeds of up to 400 m/s.
When the intensity of radiation from a radioisotope is being reduced by matter in the beam, some radiation is scattered back towards the radiation source. The amount of 'backscattered' radiation is related to the amount of material in the beam, and this can be used to measure characteristics of the material. This principle is used to measure different types of coating thicknesses.
Gamma irradiation is widely used for sterilizing medical products, for other products such as wool, and for food. Cobalt-60 is the main isotope used, since it is an energetic gamma emitter. It is produced in nuclear reactors, sometimes as a by-product of power generation.
Large-scale irradiation facilities for gamma sterilization are used for disposable of medical supplies such as syringes, gloves, clothing and instruments, many of which would be damaged by heat sterilization. Such facilities also process bulk products such as raw wool for export from Australia, archival documents, and even wood, to kill parasites. Currently, the Australian Nuclear Science and Technology Organisation (ANSTO) sterilizes up to 25 million Queensland fruit fly pupae per week for New South Wales Agriculture by gamma irradiation.
Smaller gamma irradiators are used for treating blood for transfusions and for other medical applications.
Food preservation is an increasingly important application, and has been used since the 1960s. In 1997, the irradiation of red meat was approved in USA. Some 41 countries have approved irradiation of more than 220 different foods, to extend shelf life and to reduce the risk of food-borne diseases.
Radioisotopes can also be used as tracers in medicine and industry. These radiotracers emit gamma rays and/or beta particles that can be detected and measured by a variety of different counters -- either in situ or from samples in labs. By proper analysis the quantity of the tracer can be determined at any point in a pathway through which it is traveling. The tracers used are specific to the use.
For example, one wouldn't want a long-lived tracer to measure pollution in a stream that only took a day to empty into the river. On the other hand, one might be interested in longer-lived species if one was also interested in how plants along the way took in the fluids and in what happened to them as a consequence. The activity selected would also depend on the decay time because you still need to take accurate measurements later in time. Then from a combination of the original characteristics of the tracer and its dilution and elapsed time and quantity of the measured sample, the absolute origin and time-of-passage are easily identifiable.
The radiotracers can be applied in different ways:
- Mixing efficiency of industrial blenders can be measured: radiotracers are added to various solutions that are to be mixed together to allow the manufacturer to determine when his mixture has reached uniformity.
- Radiotracers are used to trace down sources of pollution. For example, if one injects a known amount of radioactive tracer at a source of pollution (say at an outflow from an industrial plant or even a point of soil wash into a stream), its pathway downstream can be identified. In this way, it might be found that the industrial plant was the culprit for pollution washed ashore miles away, or (equally likely) that that particular pollutant came from a different source. Similarly, looking at soil washed into streams, it would be possible to determine which farmer (or even which cows) where the culprits by using different tracers. In the old days a colored dye might be used as an indicator, but no accurate measurements could be taken.
- Small leaks can be detected in complex systems such as power station heat exchangers or oil pipelines in a refinery.
- Flow rates of liquids and gases in pipelines can be measured accurately, as can the flow rates of large rivers.
- The extent of termite infestation in a structure can be found by feeding the insects radioactive wood substitute, then measuring the extent of the radioactivity spread by the insects. This measurement can be made without damaging any structure as the radiation is easily detected through building materials.
- Using tracers, research is conducted to examine the impact of human activities. The age of [water]] obtained from underground bores can be estimated from the level of naturally occurring radioisotopes in the water. This information can indicate if groundwater is being used faster than it is being replenished. Tracer radioactive fallout from nuclear weapons' testing in the 1950s and 1960s is now being used to measure soil movement and degradation. This is assuming greater importance in environmental studies of the impact of agriculture.
- Radioisotopes are used to test material parts and products such as metals, tire rubber, and engine oil for wear. Radioisotopes are added to these products, and then with the use of sensitive radiation detectors, the location and amount of wear of these products is determined. These tests help the manufacturer to produce the best quality and most reliable products.
In agricultural laboratories, radioisotopes are used to determine how plants take up nutritional materials or fertilizers to improve the efficiency. In the past, the improvement of plant species took several plant generation times as those with good characteristics (say, disease resistance, or nutritional value, or smell -- in herbs) were weeded out and propagated in favor of those with poor characteristics. Now by use of radioactive labeling, it is possible to shorten the time considerably and even arrange that a plant be generated with all the desirable characteristics (both disease resistance and oil flavor in the case of the peppermint plant).
- WNA paper on Radioisotopes in industry