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7/27/2019 Physics Applications of Radiation
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DIAGNOSE AND THERAPY
The most common of these medical procedures involve the use of x-rays— a type of radiation that can
pass through our skin. When x-rayed, our bones and other structures cast shadows because they are
denser than our skin, and those shadows can be detected on photographic film. The effect is similar to
placing a pencil behind a piece of paper and holding the pencil and paper in front of a light. The shadow
of the pencil is revealed because most light has enough energy to pass through the paper, but the
denser pencil stops all the light. The difference is that x-rays are invisible, so we need photographic film
to "see" them for us. This allows doctors and dentists to spot broken bones and dental problems.
Fluoroscopy
If the radiation is displayed visibly at the same time it is detected, the clinician can observe dynamic
processes, such as the beating heart or a probe moving through a cardiac artery or vein. The original
technique for this involved nothing more subtle than interposing a screen of suitable composition
between the patient and the observer. The material was chosen on the basis of visible light emission
upon bombardment with X-rays, a special case of fluorescence, which generally refers to the emission of
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longer-wavelength electromagnetic radiation upon bombardment by shorter-wavelength radiation.
Fluorescent lights, for example, emit visible light when the ultra-violet light from the mercury arc in the
interior strikes the coating on the inside surface of the glass tube.
Direct X-ray fluoroscopy has two major problems: first, the detection is not very efficient, so that a
larger dose of radiation to the patient is required. Second, there is often a great deal of scattered
radiation, or radiation that is not absorbed in the screen, which will irradiate unintended parts of the
patient, or the observer, or both. Both of these problems are addressed by using image intensifiers (see
chapter V, section C), to amplify the light from the fluorescent screen so that less radiation is required.
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FOOD
In irradiation, for instance, foods, medical equipment, and other substances are exposed to certain
types of radiation (such as x-rays) to kill germs without harming the substance that is being disinfected
— and without making it radioactive. When treated in this manner, foods take much longer to spoil, and
medical equipment (such as bandages, hypodermic syringes, and surgical instruments) are sterilized
without being exposed to toxic chemicals or extreme heat. As a result, where we now use chlorine— a
chemical that is toxic and difficult-to-handle—we may someday use radiation to disinfect our drinking
water and kill the germs in our sewage. In fact, ultraviolet light (a form of radiation) is already used to
disinfect drinking water in some homes.
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Radioactive material is also used in gauges that measure the thickness of eggshells to screen out
thin, breakable eggs before they are packaged in egg cartons. In addition, many of our foods are packaged in polyethylene shrinkwrap that has been irradiated so that it can be heated above its
usual melting point and wrapped around the foods to provide an airtight protective covering.
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 eatradiation-preserved food while on space missions.
In meats and other foods of animal origin, irradiation destroys the bacteria that causes spoilage aswell as diseases and illneses such as salmonella poisoning. This allows for a more safer food
supply, and meats that can be stored for longer before spoilage. Additionally, irradiation alsoinhibit tubers that cause fruits and vegetables to ripen. The result is fresh fruits and vegetables
that can be stored for longer before ripening.
the table shows the typical doses of radiation used for food treatment
Radiation dose (kilograys,
kGy)Purpose
"low" up to 1 kGy
inhibits fruit and vegetable ripening
controls some bacteria in meats
controls insects in grains
"medium" 1-10destroys bacteria in meat including salmonella, shigella,campylobacter and yersiniainhibits mold growth on fruit
"high" more than 10 kGydestroys insects and bacteria in spices
sterilises food to the same extent achieved by high heat
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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 diametre. The room where irradiation takes place
is shielded by concrete walls to prevent radiation from escaping into the environment, althoughthe 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 whilenot in use, to also help prevent radiation from escaping. However, the plant is in many wayssimilar to any other - refrigeration is still important. No process can make food completely spoil-
proof.
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AGRICULTURE
Plant seeds, for example, have been exposed to radiation to bring about new and better types of plants.
Besides making plants stronger, radiation can be used to control insect populations, thereby decreasing
the use of dangerous pesticides.
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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 wild, called thesterile insect technique
(SIT), is commonly used in protecting agricultural industries in many countries around the world.
Agricultural Tracers
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
weedkilling 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 tableunderground 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 collectionof crops that have been modified with radiation include wheat, sorghum, bananas and beans.
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