Applications of radioactive isotopes are varied in several areas of science. Application of radioactive isotopes ranges from the field of agriculture to the diagnosis of diseases.
Protection of seeds and plants
Radioisotopes are being widely used in plant nutrition studies and several researches are being conducted in many countries for using radiation sources to preserve certain types of agricultural produce, such as potatoes. However, vital use of radioisotopes is in production of new varieties of plants by radiation-induced genetic changes or by directly stimulating plant growth. Hardier and more disease resistant crops (peanuts, tomatoes, onions, rice, soybeans, and barley) have been developed using radioactive materials in the agricultural research.
One of the most important applications of radioisotopes is for determining the function of fertilizers used in different plants and producing high yielding crop seeds. Nutritional value, baking and melting qualities of some crops, and cooking times have been improved using radioisotopes. Radiations emitted from some radioisotopes are used for killing insects that damage the food grains. Certain seeds and canned food are stored for longer periods (preserved) by gentle radiation exposure. Irradiation destroys disease-causing bacteria and reduces the incidence of food borne illness. Isotopes help farmers and scientists control pests. For example, in California radiation sterilization is used since the mid-70s to control Mediterranean fruit fly infestations. Various research at Taiwan for the development of plant varieties had led to production 37 promising lines derived from certain varieties of rice by means of irradiation with x-rays, all new strains being better than mother varieties. Development of 23 high yielding varieties of pulses, oilseeds, rice and jute are done with help of radioisotopes.
Encouraging results for different crop plants variety are found in many countries such as, India, Italy, Japan, the Netherlands and Sweden. Tracer technique has also been used to study rate and direction of movement of molecules in plant using the autoradiography. The dark areas developed in autoradiograph indicate movement of these molecules. This provides the information of optimum season for crop fertilization and weed poisoning. Phosphorous is an important constituent of fertilizers so, 32P is used is used to determine affect of fertilizer on the plant.
Radioisotopes are used to increase the yields of milk and eggs and the shelf life of diary products. For example, pasteurization of milk and radiosterilization of food increases their shelf life. Strong gamma emitter such as 137Cs and 60Co are generally used.
Radioisotopes have several industrial applications, such as flow measurements, to detect gamma scanner leakage, to study sediment transport at ports and harbors, hydrology, and water resource management. Industries utilize radioactive sources for a wide range of applications. Automobile industry makes use of isotopes to test the quality of steel used in cars and aircraft manufacturers use radiation to check flaws in jet engines. Mining & petroleum companies use isotopes to locate and to quantify geological mineral deposits.
Gamma irradiation is a physical means of sterilization and decontamination, widely used to preserve food (extends the shelf life and reduces the risk of food-borne diseases), wool, sterilizing medical products, and microbial decontamination of spices, herbs and vegetable seasonings.
Radiation sterilization is performed using either gamma rays from a radioisotope source (usually cobalt-60) or electron-beam or X-ray irradiation. However, gamma-ray irradiation is by far the most common method used. High-energy gamma irradiation is used mainly in the healthcare industries to sterilize disposable medical devices and supplies such as syringes, gloves, and clothes. Pharmaceutical companies use radioisotopes to sterilize ophthalmic preparations, topical ointments, and veterinary products. Smaller gamma irradiators are used to treat blood for transfusions and for other medical applications.
ANSTO in Australia sterilizes up to 25 million Queensland fruit fly pupae per week for NSW agriculture by gamma irradiation.
Radioisotopes are used in gauging. The phenomenon that intensity of radiation coming from radioisotopes (source) reduces till it reaches the detector (measures the reduction of intensity) is used to gauge presence or absence, and to measure the quantity (density) of material between the source and the detector.
Advantage of this technique is that there is no contact with the material being gauged. In paper manufacturing, beta gauges are used to monitor thickness of the paper at speeds of up to 400m/s. Plastic film manufacturing machines use radioisotope gauging (with beta particles) to measure the thickness of the plastic film. Variation on this technique is used to measure material coatings by analyzing ‘back-scattered’ radiation.
This non-invasive process uses gamma-ray radioisotopes to test materials for flaws, such as invisible cracks, defects, or occlusions in the welds. To produce effective gamma rays, a small pellet of radioactive material is required in a sealed titanium capsule. Gamma radiography works similarly to x-rays to screen luggage at airports. Basic step involved is placing the titanium capsule on one side of object being screened, and a photographic film on the other side. Gamma rays pass through the object and create image on the film. Image obtained shows flaws in metal castings or welded joints. This technique allows study of critical components for internal defects without damage.
Gamma sources have advantages over x-ray sources, as gamma rays are more portable, emit discrete wavelengths, and have higher energy. Gamma radiography has another advantages over x-ray as they do not need power source and can be taken to any place or site for the examination. It is used to inspect flaw and weld in oil or gas pipeline. However, they cannot be simply turned off, and so it must be properly shielded to avoid exposure.
Americium-241 is used in the smoke detectors . A chamber inside smoke detector contains ionized air and electronics of detector is capable of sensing even small amount of current. Drop in current between the plates due to smoke causes alarm to set off. They are useful device for home as well as public places.
Neutron activation analysis (NAA)
NAA is an analytical technique based on the measurement of characteristic radiation from radionuclides produced directly or indirectly by the neutron irradiation of a sample. These neutrons are called thermal neutrons. NAA involves the two main techniques, Thermal Neutron Capture (TNC) and Neutron Inelastic Scattering (NIS). TNC occurs immediately after a low-energy neutron is absorbed by a nucleus, while NIS occurs instantly when a fast moving neutron collides with a nucleus.
It has been extremely useful for qualitative and quantitative determination of trace and minor elements in environmental, food, forensic science, geological, inorganic materials, and water analysis. To measure soil density and water content, a portable device with americium-241-beryllium combination is used that generates gamma rays and neutrons that passes through sample of soil to a detector.
Most commercial analyzers use californium-252 as a neutron source and sodium iodide detectors. NAA has been used for the characterization of reference materials. It has been widely used for the analysis of samples within environmental specimen banking programs.
Gamma & X-ray Techniques in analysis
Gamma ray transmission or scattering can be used to determine the ash content of coal (as gamma ray interactions are dependent upon atomic weight) on line on a conveyor belt. It is used for interpretation of sediment composition, provenance, and diagenesis. X-ray inducing fluorescence is used to quantify elements present in the sample.
This technique is used to determine concentrations of elements in mineral concentrators. The size and the shape of any compound can be determined by the diffraction of x-rays. X-ray diffraction does not use radioisotopes.
Tracers are used in the oil industry to gauge fluid flow through the reservoir qualitatively or quantitatively, and estimate residual oil saturation. The tracer technique is extremely sensitive in testing sealing process, flow rates and bulk flow measurements. It is also used for testing the uniformity of mixtures. It has also been applied to manufacture chocolate, soap, cement paints, and fertilizers.
Radioactive sources (generally short-lived) are utilized in various industries for a wide range of applications. When radioactive sources used by industry no longer emit sufficient radiation that can be utilized, they are treated as radioactive waste. Some industry handles raw materials (rocks, soils, and minerals) that contain naturally occurring radioactive material called NORM. NORM increases the risk of exposure to radioactive materials and radiations.
The main industries that result in NORM contamination are oil and gas industry, coal industry, fertilizer industry, wastewater treatment plants, and metal industry. Oil and gas exploration and production generates residues of radium and other radionuclides. The sulfate scale from an oil well is radium rich, while water, oil and gas from a well contains radon. The radon decays to form solid radioisotopes, which in turn form coatings on the inside of pipe work. Oil and gas operations are main sources of radioactive releases to waters in Northern Europe. Most coal contains uranium, thorium, or other radionuclides. Generally, they come from power station as fly ash. About 300 million tones of coal ash are produced globally. Similarly, processing of phosphate rocks results in the production of phosphate fertilizers that enhances level of uranium, thorium and potassium.
In wastewater treatment plants, the filters are used to remove impurities and radioactive wastes. They form part of NORM. Scrap metal from processing industry also contains enhanced levels of natural radionuclides. The exact nature and concentration of these radionuclides is dependent on the process from which the scrap originates.
Applications in Scientific Research
Scientific research facilities use variety of radioisotopes for the development of new products and other research (mentioned previously). Radioisotopes are employed in a range of applications, such as production of an extensive range of organic chemicals with a particular atom or atoms in their structure replaced with an appropriate radioactive equivalent, tracing flow of contaminants in biological system, and determination metabolic processes. The age of water (obtained from underground bores) can be estimated from level of naturally occurring radioisotopes present. Later, such trace levels of radioactive fallout from nuclear weapons testing is now being used to measure soil movement and degradation.
Carbon-14 Dating or Radiocarbon dating
Carbon-14 dating is probably one of the most widely used and best-known radiometric dating methods. It was developed by J. R. Arnold and W. F. Libby in 1949, and has become an indispensable part of the archaeologist’s tool kit since then. It relies on a simple natural phenomenon.
As the Earth’s upper atmosphere is bombarded by cosmic radiation, atmospheric nitrogen is broken down into an unstable isotope of carbon i.e., 14C. The unstable isotope is brought to earth by atmospheric activity, such as storms, and becomes fixed in the biosphere.
As it reacts identically to C-12 and C-13, C-14 gets incorporated to a complex organic molecule through photosynthesis in plants and becomes their part. Animals eating those plants in turn absorb carbon-14 as well as stable isotopes. The C-14 within an organism is continually decays into stable carbon isotopes, but since the organism is absorbing more C-14 during its life, the ratio of C-14 to C-12 remains about the same as the ratio in the atmosphere.
When organism dies, the ratio of C-14 within them begins to gradually decrease as they cease the metabolic function of the carbon uptake so there is no replenishment of radioactive carbon. As 14C decays, it emits nitrogen (enters back into atmosphere), a beta particle, or electron with 160keV energy. Libby, Anderson and Arnold were the first to measure the rate of this decay in 1949. Half-life (t 1/2) was the name given to this value, which Libby measured to be 5568±30 years. This was known as the Libby half-life. Major developments in radiocarbon method involve improvement in measurement techniques and research that includes dating with advanced techniques. For example, gas counting methods, liquid scintillation counting, and accelerator mass spectrometry.
Carbon-14 dating has played a significant role in paleontological studies, oceanography, archeology, calibration, dendrochronology, meteorology, and palaeoclimatology, such as discovery of ancient footprints of Acahualinca in Managua and determination of age of earth. Limitations of C-14 dating includes its utility in large sample sizes, extra care needed while collecting and packing samples to avoid contamination and cross-contamination, logarithmic decay rates, and inconsistency in atmospheric ratio of C-12 and C-14 leads to miscalculation, so calibration is needed before taking readings.
Nuclear Medicine is a branch of medicine that uses radioisotopes to diagnose and treat diseases. About one-third of all patients admitted to U.S. hospitals is diagnosed or treated using radioisotopes. Over 10,000 hospitals worldwide use radioisotopes in medicine, of which 95% are for the diagnostic purposes. Most commonly used radioisotope in medical diagnosis is technetium-99m, accounting for 80%. As previously mentioned, varieties of radiopharmaceuticals with specific functions are available. For example, 99mTc-INH is used as diagnostic agents for the detection of TB, or hyperthyroid conditions are successfully treated with the radioiodine therapy. Nuclear medicine was developed in the 1950s initially for using iodine-131 to diagnose and treat thyroid disease. Now, several hybrid-scanning techniques such as CT/PET, or SPECT/CT have emerged.
Diagnostic techniques in nuclear medicine use radiotracers (radiopharmaceuticals), which emit gamma rays. These tracers are generally short-lived isotopes are tagged to various chemical compounds, used to obtain images of tissues, or an organ. They are introduced in the body by injection, inhalation or given orally.
Gamma camera is used to build images from the radiations emitted by subject. Later, a computer enhances the images. These images are monitored for any indications of abnormal conditions. Gamma camera was the first instrument used to capture images in nuclear medicine (Figure 1).
More recent developments are Positron Emission Tomography (PET) and hybrid scanning techniques (Figure 2). As discussed in earlier section, PET is very precise and sophisticated technique and it uses cyclotron-produced radioisotopes. A positron-emitting radionuclide is injected into patients, which accumulates in the target tissue. It is detected by PET camera and images are acquired. PET is most important in oncology, neurology, and cardiology.
Fusion imaging techniques such as SPECT/CT and PET/CT are now used commonly in nuclear medicine to study anatomy as well as functions of tissues and organs, which would otherwise be unavailable, or would require a more invasive procedure, such as surgery.
Diagnostic medical imaging is a key procedure in the practice of modern medicine. As mentioned in previous section, diagnostic radiopharmaceutical is a drug that is intended for diagnosis of various diseases in humans. Diagnostic radiopharmaceuticals have been used extensively in nuclear medicine since long as noninvasive diagnostic imaging agents to study infection, inflammation, functioning of liver, lungs, heart and kidneys, to assess bone growth, and to predict therapeutic responses of drugs and effects of surgery. Various factors that make radioisotopes, and radiopharmaceuticals suitable imaging agents are discussed earlier in this chapter.
As mentioned ealier, most commonly used radionuclide for imaging in nuclear medicine is technetium-99m. But, in PET imaging 18F is most widely used.
The development of therapeutic radiopharmaceuticals is based on suitable choice of from different types of carrier molecule available and a variety of radioisotopes. The basic purpose of producing therapeutic radiophramaceuticals is destruction or weakening of malfunctioning cells using radiation. These therapeutic radiopharmaceuticals get localized in the desired organ in the same way as diagnostic radiopharmaceuticals. In most cases, β-radiation producing radionuclides are used that destroy damaged cells. This procedure is called radionuclide therapy (RNT) or radiotherapy. A short-range radiotherapy is known as brachytherapy.