APPLICATIONS OF NUCLEAR FOOD AND AGRICULTURE
APPLICATIONS OF NUCLEAR FOOD AND AGRICULTURE
Nuclear science is all around us. It’s not as obvious as we think at first, but take a closer look – you’ll find aspects of nuclear science everywhere you go. Everywhere from power generation to medicine to daily life, most people will encounter some form of nuclear science in action, making their lives easier.
Radioisotopes and Radiopharmaceuticals: In medicine, substances containing radioisotopes are given the specialised name radiopharmeceuticals. Many nuclear medicine procedures rely upon radiopharmeceuticals to work..
Radiopharmeceuticals can act as tracers.A huge array of different compounds are found in the human body, and doctors can follow these through the body using radiopharmeceuticals. Radiopharmeceuticals are simply the ordinary compounds found within the body – except they are synthesised to contain an atom which is radioactive. Thus, a radiopharmeceutical is just a normal compound that has been radioactively ‘tagged’, and can thus be tracked with radiation measuring devices.
Consider a molecule of glucose (a type of sugar found in the body). When it is artificially synthesised as a tracer, scientists replace a stable carbon atom with a radioactive one, consequently tagging it. This resultant tagged molecule is a radiopharmeceutical. The trail of radiation that it emits while it passes through the body can be monitored using Geiger meters or scintillation counters. This trail can then be mapped into a computer to show doctors how the compound is processed by the body. The tracing technique is particularly useful for monitoring digestion of food. It has also been used in the study of how amino acids, critical substances in our bodies, are formed.
Radioisotopes can also be used to calculate the permeability of cell walls – that is, how easily cell walls can be penetrated. Radioactively tagged molecules are injected into the fluid surrounding the cell. The presence of radioactivity inside or outside of the cell is monitored over time, allowing the permeability to be determined. For example, nerve impulses in our bodies involve sodium and potassium ions passing through ‘sodium-potassium pumps’. By placing radioactive sodium ions outside cell walls of nerve cells and monitoring the level of radioactivity, the activity of the pump can be determined.
Radiopharmeceuticals can also be used to determine activity of organs. Specific compounds concentrate in certain parts of the body. Therefore in the analysis of a certain part of the body, the patient is given the corresponding radiopharmeceutical that will accumulate most there.
Once in the body, the distribution of the radioactively labelled compounds is monitored by a network of “gamma cameras” around the patient, which work by detecting the radiation from the compounds. The data collected by the cameras is processed by a computer, allowing doctors to analyse the patient’s body. High concentrations of the radioactively tagged compound in or around an organ can indicate overactivity; low concentrations can indicate underactivity.
One example of this is the common test for thyroid gland functionality. Patients are given a capsule of often iodine-123, and a scan takes place usually on the next day to assess the condition of the thyroid.
Magnetic Resonance Imaging: Magnetic resonance imaging is based on the vibrations that nuclei experience when bombarded by radio waves in a magnetic field. See Nuclear Magnetic Resonance in science.
The alternative name, magnetic resonance imaging (MRI) arises from a modification of nuclear magnetic resonance (NMR). The word “nuclear” is omitted so as to avoid the negative connotations. In fact, NMR has nothing to do with radiation or radioactivity and is relatively safe compared to other medical imaging techniques. “Nuclear” is relevant because of the process’ relation with nuclei.
Magnetic resonance imaging is used as an analytical tool in scanning various parts of the body. It is especially used in brain scans and is one of the best techniques known for such procedures. The technique is also used in obtaining highly accurate images of body tissues – much more precise than computer aided tomography (CAT) scans or ultrasound. The resolution of magnetic resonance images typically is of the order of 0.5 to 1 mm, one of the most precise techniques available.
In a hospital, magnetic resonance imaging machines consist of a large hollow cylinder where the patient is placed. In this cylinder, there are several kilometres of wire wrapped around in a coil. When a current is passed through this wire, a very strong magnetic field is generated, and is especially concentrated in the centre of the cylinder. This provides the magnetic field for the resonance of nuclei.
The patient to be scanned is placed in the cylinder, and then the machine generates a magnetic resonance image, which is then used by doctors to determine the patient’s condition.
Radiation Therapy: Radiation can cause cancer. Normally, cells that are exposed to radiation die. However, if the radiation does not kill the cell, they cell may be mutated by the radiation, and can start to reproduce. These multiplying mutant cells constitute cancer.
However, although radiation can cause cancer, it can also be used to treat it. Rapidly multiplying cancer cells are particularly prone to radiation. Doctors can administer radiation to kill these cancerous cells. However, some surrounding normal cells are unavoidably killed as well, due to the large doses of radiation required. Thus patients who undergo radiation therapy also experience side effects such as radiation sickness. To try to minimise these side-effects, narrow beams of radiation are used, and the beam continuously rotates around the body, minimising sc the radiation to normal areas but concentrating it around the cancerous region.
Radiation is also used to sterilise surgical equipment and bandages. By bombarding the object to be sterilised with radiation, bacteria and viruses can be killed. This procedure is very common in hospitals for cleaning reusable surgical equipment between operations and sterilising blood for transfusions. It is also used in the manufacture of disposable medical products such as bandages and syringes where sterilisation is required before sale or use. Radiation-based sterilisation is especially useful in sterilising heat-sensitive or steam-sensitive materials.
Emission Tomography: The technique of single photon emission tomography, also known as SPET, involves administering the patient with a radiopharmeceutical, and then rotating a gamma camera or other radiation detector around the patient to detect the emitted radiation from many different angles. These different angles are put together by a computer to produce an image useful for the doctors.
The general concept of moving a detector around a patient to gain data from different angles is known as computer-aided tomography, or CAT. Thus SPET is a type of CAT.
Another technique called positron emission tomography, or PET, makes use of positron emitting atoms such as carbon-11, nitrogen-13 and oxygen-15. These atoms are again incorporated into radiopharmeceuticals that concentrate themselves in the part of the body to be studied once injected or consumed by the patient. When these atoms beta-decay, the emitted positron will move and eventually collide with an electron elsewhere in the body. Once this happens, the positron and electron are both destroyed, producing two gamma rays. These resultant gamma rays radiate out in exactly opposite directions.
A ring of detectors is placed around the patient, and when the gamma rays are detected, a computer can calculate the time difference and therefore work out where the annihation event occurred. However, with today’s technology, these time differences cannot be measured accurately enough to generate useful data. (The best possible resolution is about 8cm). Nevertheless, if this could be refined, the biggest advantage of PET would be the lower scanning time and that a lower dose of radiopharmeceutical could be used. This is because the detection in PET is done capturing two gamma rays at once with one large circular detector, whereas with other CAT techniques one small detector that rotates around the patient is used.
FOOD AND AGRICULTURE
Tracers like those used in medicine are also used in agriculture to study plants and their intake of fertilisers. The usage of tracers allows scientists and farmers to optimise the use of fertilising and weed killing chemicals. Optimisation of these chemicals is desirable because it saves money, and reduces chemical pollution. When fertilisers are used in overly excessive amounts, the excess “will run off and pollute rivers nearby, as well as possibly seeping through to the water table underground and polluting the water supply. To prevent this, studies are conducted to find out the optimal amount of chemical required, with fertilisers and weedkillers often tagged by nitrogen-15 or phosphorus-32 radioisotopes. These radioisotopes are analysed in the crops to see how much of the original chemical was actually consumed by the plants, compared to how much was given.
The ionising radiation from radioisotopes is also used to produce crops that are more drought and disease resistant, as well as crops with increased yield or shorter growing time. This practice has been in place for several decades, and has helped feed some third-world countries. The collection of crops that have been modified with radiation include wheat, sorghum, bananas and beans.
About 10% of the world’s crops are destroyed by insects. In efforts to control insect plagues, authorities often release sterile laboratory-raised insects into the wild. These insects are made sterile using ionising radiation – they are irradiated with this radiation before they hatch. Female insects that mate with sterile male insects do not reproduce, and the population of the insect pests can be quickly curbed as a consequence. This technique of releasing sterile insects into the wila, called the sterile insect technique (SIT), is commonly used in protecting agricultural industries in many countries around the world.
The technique is considered to be safer and better than conventional chemical insecticides. Insects can develop resistance against these chemicals, and there are health concerns about crops treated with them.
The largest application of this technique so far was conducted in Mexico against Mediterranean fruit-fly and screwworm in 1981. It was highly successful, and over the next 10 years the eradication program yielded about US$3 billion in economic benefits to the country.
SIT is in use in several countries, with support from the UN Food and Agriculture Organisation (FAO) and the International Atomic Energy Agency (IAEA). Australia is a large producer of many fruits and sterilises up to 25 million fruit fly pupae per week.
Food Treatment and Preservation
Ionising radiation is used as an alternative to chemicals in the treatment and preservation of foods. A French scientist first discovered that radiation could be used to prolong food shelf life in the 1920s and it became more widely used in World War II. Today, astronauts often eat radiation-preserved food while on space missions.
In meats and other foods of animal origin, irradiation destroys the bacteria that causes spoilage as well as diseases and illness such as salmonella poisoning. This allows for a safer food supply, and meats that can be stored for longer before spoilage. Additionally, irradiation also inhibits tubers that cause fruits and vegetables to ripen. The result is fresh fruits and vegetables that can be stored for longer before ripening.
The irradiation technique is particularly important when exporting to countries with tropical climates, where foods can be spoiled easily due to the warm temperatures.
Irradiation of food is carried out using accelerated electrons (beta radiation), and ionising radiation from sources such as the radioisotopes cobalt-60 and cesium-137. X-rays are also sometimes used. None of these sources of radiation used have enough energy to make the exposed foods radioactive
Radiation dose Purpose
“low” up to 1 kGy . inhibits fruit and vegetable ripening, destroys bacteria in meat including salmonella, shigella, Campylobacter and yersini-ainhibits mold growth on fruit
“medium” 1-10 destroys insects and bacteria in spices sterilise food to the same extent achieved by high heat
“high” more than 10 kGy
Inside the food treatment plant there is a conveyor belt or similar system that transports the food to the radiation source, so that workers do not have to move close to the radiation. The source is packaged in a pencil like device, about 1cm in diameter. The room where irradiation takes place is shielded by concrete walls to prevent radiation from escaping into the environment, although the radiation risk is considerably much less than that from a nuclear reactor. Where gamma radiation is used from a radioisotope source, the radioisotope is stored in a pool of water while not in use, to also help prevent radiation from escaping. However, the plant is in many ways similar to any other – refrigeration is still important. No process can make food completely spoil-proof.
Food treatment plants of this kind are monitored closely by government health and occupational safety authorities to ensure safe working conditions for employees, as well as safety to any nearby residents.
Food irradiation is a well-tested process. Scientists have performed numerous decades of research, and it has been shown that irradiation will not cause significant chemical changes in foods that may affect human heal nor will it cause losses that may affect the nutritional content of food. (Chemical residues left behind by irradiation are in concentrations equivalent to about 3 drops in a swimming pool. Chemical-based preservatives and treatments usually leave more residues.) Taste is usually unaffected. The World Health Organisation and food safety authorities in many countries have approved irradiation as a safe method of food treatment and preservation.
Radiation-treated food is still not very widely used today. Despite the scientific evidence and approvals, many activist organisations claim that irradiation is unsafe and exploit the lack of public awareness and concerns about food safety and nuclear issues. Some even say that irradiation is a way that governments can utilise nuclei wastes left over from weapons testing or power generation. (However, the wastes left cannot be used in food processing because they do not provide the right type of ionising radiation.) Consequently, these scare tactics deter the public and some food producers are reluctant to use irradiation for fear of consumer boycotts. However a recent survey conducted in mid-1998 by the Food Marketing Institute (a United States organisation) revealed that less than one percent of all those surveyed identified irradiation as a concern. Most said that spoilage and microbial hazards were of great concern – they very problem that irradiation addresses. Another study by an academic revealed that about 99% of consumers were willing to buy irradiated food after they were shown scientific data and irradiated food samples. This compared to 50% before shown this data.
Irradiation poses less of a risk to human health than many chemical treatments that are used today, which include the addition of chemical preservatives. The use of radiation is sometimes favoured to using chemical preservatives, because no allergic side-effect results. It is also better than heat-sterilisation because irradiation does not destroy nutrients and vitamins, whereas heat treatment does.
Irradiation is inexpensive – typical costs are about V20 cents per kilogram of food irradiated.
About 40 countries worldwide allow irradiation of foods. Depending on the country, irradiated foods may need to be labelled.
Quarantine and Exportation
Ionising radiation is used to rid goods of parasites and bugs before they are exported out of a country. The radiation kills these parasites that may be quarantine hazards in other countries.
The technique is used in Australia to clear primary produce materials such as raw wool and wood for export. It is also used worldwide in transporting archival and historical documents. This is beneficial in that any microorganisms existing in the paper that cause paper deterioration are destroyed.
Maintenance is a vital part of industry. For example, aircraft welding needs to be checked regularly for cracks and other faults, as well as gas and oil pipelines. The checking for cracks can be done using the radiation emitted by radioisotopes.
Using radioisotopes to check for cracks can be accomplished with a technique called gamma radiography. It is similar to how X-rays are used in hospitals to check for fractures and cracks in bones. In this process, the patient places their arm in front of an X-ray source, and the rays that penetrate through body tissue darken a photographic plate to reveal any cracks in the bones of the arm, etc.
Similarly, gamma radiography involves placing a radiation source on one side of the gas pipeline, and a photographic plate on the other. The radiation that can pass through cracks will show up on the photographic plate. This checking process could have been done using X-rays like in the hospital, except there is a lot of equipment required for X-rays, whereas using gamma radiography requires only a radiation source which can simply be a small pellet of radioactive material. X-rays also require electricity, whereas radioisotopes do not.
The exact process is done by taping a special photographic film over the weld or suspected crack of a pipeline. A pipe crawler carrying the sealed radiation source is then dispatched down the pipe to the weld position.
The field technician performing the check then sends a remote control message to the crawler to tell it to expose the radiation. When this happens, the radiation passes through any cracks that may exist, onto the photographic film taped on the outside. This film is then developed and checked for cracks or welding deterioration.
Some industrial machinery contains parts that have small amounts of radioactive materials. This allows easy observation and detection of wear and tear. However, this practice is not common because of public fears about the radiation.
Measurements and Industrial Analysis
Radioisotopes are commonly used for measuring viscosity, density and thickness in conditions where other methods would be difficult or impossible to apply. Since radiation does not require direct contact (unlike, for example, using a scale or tape-measure) it is used where high heat or corrosive chemicals may exist.
Radiation is reduced in intensity when it passes through many materials. Therefore the amount of stuff between a radiation detector and emitter can be determined by calculating the difference between the intensity of emitted radiation and the intensity of the received radiation. This concept is applied in the manufacture of thin plastic films. The produced film is passed through a radioisotope gauge – the thicker the film, the lower the detected radiation. Changes in the detected level of radiation correspond to a change in thickness of the plastic, so this is a form of quality control.
(The decrease in intensity of visible light as opposed to radiation is used in many photoelectric devices including some smoke detectors. However, light cannot be always used to measure thickness of materials unlike radiation, because light is completely blocked by opaque surfaces or completely transmitted by clear surfaces.)
Radioisotopes can also be used to calculate the efficiency of large mixers, or the flow of materials through blast furnaces. Leaks and other defects can also be detected in building cooling towers and power station heat exchangers.
Small Power Sources
Radioisotopes are used as power sources for applications requiring small amounts of portable energy, such as for remote weather stations and weather balloons, and navigation beacons and buoys. They are more environmentally friendly than batteries because once all the energy from radioactivity is used, there are few left-over wastes except for the stable atoms formed, whereas used batteries will always contain toxic heavy metals that are hazardous to the environment.
All substances that exist are likely to have radioactive atoms in them occurring naturally. Water is an example of this. Underground bore water in particular is likely to have radioactive minerals and gases present. The rocks underground will often contain small amounts of radioactive elements such as uranium. The decay of uranium will produce other radioactive elements, and these may leak into the bore water.
The concentration of these radioactive elements in the water can be analysed to determine whether the bore water deposits are being used faster than they are being replenished, as well as the “age” of the water (how long it has been untouched).
Ocean pollution can be analysed for radioactivity to trace the source that caused it. Similarly, the dispersion of a known factory’s pollutants can be monitored this way.
The rate and extent of soil erosion can be determined through the use of radioisotopes. See the table below for a list of radioisotopes used in environmental monitoring and other industrial processes. In general, it is the labelling property of radioisotopes that allows them to be used in a wide range of environmental assessment techniques.
Other Industrial Uses
Radiation is also used in determining the nature and extent of termite infestation in buildings. This is done by feeding the termites a sample of artificially synthesised radioactive wood. The termites then disperse the radiation when they bury into the building structures, and the spread of radiation is used to give an indication of the nature of termite infestation. This process sometimes is better than physically removing and examining parts of the building, because the radiation can be detected easily through the building materials without needing to dismantle anything.
Radioisotope Industrial Uses
hydrogen-3 water age measurement, study of sewage
carbon-14 water age measurement
chlorine-36 water age measurement
scandium-40 study of blast furnace efficiency
manganese-54 study of environmental impact of mining
chromium-57 study of coastal erosion
zinc-65 study of environmental impact of mining
caesium-137 soil erosion monitoring
iridium-192 study of coastal erosion, checking of aircraft welding faults
gold-198 study of sewage and sources of water pollution, monitoring of sand movements in ocean floors and river beds, coastal erosion, study of blast furnace efficiency
lead-210 soil and sand age measurement
Household Uses of Radiation
Smoke Detectors: Smoke detectors are important safety devices that are in many modern homes. Two types of smoke detectors exist. One type uses a photoelectric sensor to detect changes in light caused by smoke, and the other uses radioisotopes – usually americium-241. (American spelling: americum) The photoelectric type is more expensive and is less effective, whereas the other type is much cheaper and more sensitive to a wider range of fire conditions. These radioisotope-based detectors are the most common.
Inside a radioisotope-based smoke detector, the americium-241 is placed inside an air chamber. The ionising alpha radiation emitted collides with air molecules in the air chamber, ionising them (making the air particles charged). The ionised air molecules are then able to conduct an electrical current between two electrodes on either side of the air chamber.
When smoke enters the air chamber, the ionised air molecules attract the smoke particles, causing a decrease in the current conducted. (The ionised air molecules are now “carrying” extra smoke particles, decreasing the flow of current.) This current decrease is then detected by electrical circuitry, activating an alarm.
Smoke Detectors and Americium: The americium-241 used in smoke detectors exists in the form of americium oxide Am02. This chemical is expensive – the US Atomic Energy Commission sells it for about US$1500 per gram. However, 1 gram of americium-241 is enough for over 5000 household smoke detectors. The americium-241 radioisotope was first discovered 50 years ago as part of the Manhattan project – the US’s attempt to build the first nuclear bomb. Americium-241 is formed in nuclear reactors and has a half life of 432 years.
Typical smoke detectors use less than 35 kilobecquerels of americium-241, which is a very small amount. The radiation emitted by a smoke detector containing this amount of americium poses less danger to human health than background radiation in the atmosphere, so smoke detectors being “radioactive” are not a health hazard. Even still, Australian standards require that detectors be labelled as containing radioactive material, and must not be disposed through general garbage collection, although in some states smoke detectors are compulsory in all homes.
Warfare: Nuclear weapons are many times more effective than conventional weapons in destruction of military targets. Conventional weapons are commonly based on chemical explosives (for example TNT, trinitrotoluene). These explosives derive their destructive capability from chemical reactions, which are to do with electrons orbiting atoms, whereas nuclear weapons are based on nuclear reactions. The energy required by a nucleus to keep an electron orbiting it is much less than that contained inside the nucleus itself. As a result, nuclear-based reactions release more energy than chemical-based reactions.
The United States developed its nuclear weapons from the so-called Manhattan Project, established during World War II. The Project was a secret operation that took place in isolated Los Alamos, New Mexico. Nuclear bombs used by the United States against Japan in World War II killed about 150,000 people instantly, with many others dying soon after due to radiation.
Archaeology and Geology
The principals of radioactive decay are employed widely in many fields of archaeology and geology to determine the nature and age of materials, artefacts and rocks.
Radioactive Dating: The principals of radioactive decay are applied in the technique of radioactive dating, a process widely used by geologists and archaeologists to determine the age of materials and artifacts.
Radioactive Carbon: 14 atoms exist naturally. They are everywhere around us: in our clothes, in the food we eat, even in the air we breathe. However, there are not many of these – only 1.3 x 10-12 percent of all carbon atoms are the carbon-14 isotope. This is why they do not pose danger to us – there are so few of them.
The ratio of radioactive carbon-14 atoms to stable carbon-12 atoms in the atmosphere has remained constant over thousands of years. Although carbon-14 naturally decays, it is also continually being formed. Carbon-14 atoms are formed when neutrons from the sun’s cosmic radiation collide with nitrogen-14 atoms in the atmosphere:
Thus the decay of carbon-14 is reasonably balanced with its production, resulting in a constant ratio of carbon-14 to carbon-12.
Carbon dioxide (C02) molecules in the air can contain either isotope of carbon. This C02 is continually used by plants to grow. Because the ratio of carbon-14 to carbon-12 in atmospheric C02 is constant, the intake of C02 by a plant results in a constant ratio of the two isotopes in the plant’s body while it is alive. However, when the plant dies it will no longer take in C02. As a result, the carbon-14 decaying in the dead plant will not be replenished by a “fresh supply” of more C02, resulting in the ratio of carbon-14 to carbon-12 decreasing over time.
Because animals eat plants, the ratio of carbon-14 to carbon-12 in them also decreases once they die, since the carbon-14 cannot be replenished.
This process of dating using carbon-14 is used by paleontologists. Paleontologists burn a small sample of a fossil to react the carbon in it with oxygen, to form C02. The C02 that contains carbon-14 will be radioactive, and the amount can be easily measured using a radiation counter. Burning is done to facilitate measuring the level of carbon-14.
Carbon-14 has a half life of about 5730 years. This means that in a given sample of a carbon-containing substance, (without the carbon-14 being replenished) the ratio of carbon-14 to carbon-12 will decrease by half every 5730 years. Suppose for example, some archaeologists uncovered ancient manuscripts and found that the ratio of carbon-14 to carbon-12 in the paper was half of that found in living trees. This would mean that the manuscripts would be about 5730 years old.
The use of radioactive carbon-14 for dating was first done by William Libby, an academic at the University of Chigaco, USA, in 1947.
The relatively short half-life of carbon-14 (5730 years) means that the amount of carbon-14 remaining in materials and objects older than about 80,000 years is too small to be measured with today’s equipment. Thus carbon dating is limited to objects which are not older than this. However, the abundance of other atoms with longer half-lives, such as uranium-238 (half-life 4.5 x 109 years) can be measured in place of carbon-14. Geologists measure the amounts of other radioactive metal isotopes such as uranium-238, rubidium-84 and potassium- 40 (see below) found in rocks to determine their age. Measurements show that the oldest rocks on Earth are about 4.6 billion years old – which is a reasonably accurate estimate of the Earth’s age. Similarly, analysis of fossilised plants shows that they first occurred on Earth about 3 billion years ago.
The major problem using radiocarbon dating is the chance of getting carbon from the samples mixed up with “fresh” carbon.
As well as using carbon-14 to carbon-12 decay, geologists also measure the decay of potassium-40 to argon in dating rocks. However, this method is not accurate for rocks that have been heated above 120°c (250°f) because the argon diffuses out from the rock at these temperatures. The decay of rubidium-87 to strontium-87 is used to check potassium-argon dates, and is much more accurate because neither isotope is diffused by heat. This rubidium technique was used by scientists to determine the age of the moon. Measurements using uranium- 238 were used to determine the age of the Earth.
Geology and Element Identification
Radioactivity is used to identify the location of deposits of uranium and other radioactive minerals. This is useful in mining exploration. The intensity of detected radiation also is an indication of the amount of uranium that may be located there.
The mining industry employs radioactivity in its routines. One example is identification of rocks and minerals. X-rays from a radioactive material can induce other materials to emit fluorescent X-rays. These subsequent X-rays can have their energies measured, and then this gives an indication of the elements present in the original material. The intensity of these X-rays also is an indicator of the amount of the element present.
This technique is done by placing probes into the slurry water that contains sediments of minerals, etc. The probes contain a radioisotope and a detector. The radiation from the isotope causes metals in the slurry to emit fluorescent X-rays – these are identified by the detector also located on the probe. The probe’s input is then analysed to give an indication of the types and amount of metals present in the slurry. Metals that are detected this way include lead, copper, tin, zinc, nickel and iron.
Elements that can absorb neutrons will release gamma rays. These gamma rays can be analysed for their energies. Specific energies correspond to specific elements – thus this is another way of identifying the metals and minerals that may be present.