Here are some examples of how nuclear radiation is used:
- in chemical tracers to help with medical diagnosis
- to investigate chemical and biological processes in plants.
- for killing cancer cells
- for sterilising medical instruments
- for measuring the thickness of materials in, for example, a paper factory
- carbon dating
- for dating rocks and materials such as archaeological finds
- for measuring the thickness of materials in, for example, a paper factory
- in smoke detectors
Tracers in medical diagnosis and industry
Radioisotopes are used as tracers in industry and hospitals. They're used to find out what is happening inside objects without the need to break into the object.
In industry they can be used to:
find leaks or blockages in underground pipes
find the route of underground pipes
track the dispersal of waste
To
locate a leak in an underground pipe a very small amount of radioactive
material that gives off gamma rays is put into the pipe. A detector is
moved along the ground above the pipe. The reading on the detector increases
when above a leak as the radiation can escape through the hole in the
pipe more easily and also there will be an accumulation of the radioactive
fluid in the ground around the leak.
Radioisotopes used to find leaks are ones that emit gamma radiation with a short half-life. Gamma rays can easily penetrate pipes (even if they are underground) and reach the detector. Both alpha and beta particles would not pass through pipes, so could not be used.
Radioactive sources (most commonly Technetium-99 or sometimes Iodine-123) can be used by doctors to find out if different organs in our bodies are working properly (eg. checking for a blocked kidney). Doctors can inject a small amount of radioactive substance into a patient's body. This substance is carried by the blood to the organ, eg the kidneys or the thyroid gland, that the doctor wants to examine. These radioactive chemicals are called tracers. Tracers are chosen which will concentrate in the part of the body to be investigated, and the tracer's radiation will also be concentrated there. Radiation detectors placed outside the body detect the radiation emitted and, with the aid of computers, build up an image of the inside of the body on a computer screen. The image helps the doctor to diagnose what is wrong with the organ.
When a radioactive chemical is used in this way it is not normally harmful, because:
- It has a short half-life and so decays before it can do much damage.
- It is not poisonous.
Emitters of beta radiation or gamma radiation are used because these types of radiation readily pass out of the body, and they are less likely to be absorbed by cells than alpha radiation.
Radioactive isotopes are also used in industry, to detect leaking pipes. Some of the isotope is injected into the pipe and then detected with a Geiger counter above ground.
Radiotherapy (killing cancer cells)
Although
ionising radiation can cause cancer, high doses of gamma (or X-ray)
radiation can target and kill cancerous cells. This is called radiotherapy.
About 40% of people with cancer undergo radiotherapy as part of their
treatment. It is administered in two main ways:
- From outside the body using X-rays or the radiation from radioactive cobalt.
- From inside the body by putting radioactive materials into the tumour, or close to it.
Some normal cells are also damaged by the radiation, but they can repair themselves better than the cancer cells are able to.
Sterilising medical equipment and food
Surgical
instruments are sterilised using high doses of gamma radiation. This technique
is a means of eliminating contaminants such as fungi, bacteria, viruses,
mould, insects and eggs. Cobalt-60 is usually used as the gamma ray source.
Food can also be sterilised by gamma radiation - even after the food has been packaged. The radiation kills microbes, prolongs the food's shelf-life.
Irradiated food does not itself become radioactive; however in some cases there may be subtle chemical changes and this may affect the taste. In the UK only correctly labelled irradiated herbs, spices or vegetable seasonings are allowed. Major supermarkets say they will not stock irradiated foods because people are reluctant to buy them, even though the food itself is not radioactive.
Monitoring the thickness of materials
Beta
radiation is used in industry in detectors that monitor and control the
thickness of materials such as paper, plastic and aluminium. The thicker
the material, the more radiation is absorbed and the less radiation reaches
the detector. The detector then sends signals to the equipment that adjusts
the thickness of the material.
Alpha radiation is not used because it gets absorbed too quickly even by paper, whereas gamma radiation hardly gets absorbed at all.
A source with a fairly long half-life (a few years)) is chosen so that it does not need to be replaced often.
Smoke detectors
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One type of smoke detector (also called smoke alarm) operates using ionisation. It contains a tiny amount (1/5000th of a gram) of Americium-241, a weak radioactive source. The Americium emits alpha particles that ionise the air between the two charged metal plates, resulting in a small current flowing.
If smoke particles enter the alarm, they (1) gather up and neutralise the ions and (2) absorb some of the alpha particles, so both the ionisation rate and the number of ions gets less, the current reduces, a microchip notices this and triggers the alarm. (The half-life of Am-241 is 460 years, so has many years of useful service to offer!)
Radioactive dating
Carbon dating
Carbon-14 is a radioactive isotope of carbon (it has two extra neutrons in its nucleus making it unstable).
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| Drawing of heads of cave lions in the Chauvet cave
(south-east France). Dated by radiocarbon dating of the charcoal used to produce the drawings to around 36,000 years ago. [Reproduced from J. Clottes.] |
The amount of carbon-14 in the atmosphere has not changed in thousands of years. Even though it decays into nitrogen, new carbon-14 is always being formed when cosmic rays hit atoms high in the atmosphere.
Plants absorb carbon dioxide from the atmosphere and animals eat plants. This means all living things have radioactive carbon-14 in them. When an organism, eg a tree, dies it stops taking in carbon dioxide. The amount of carbon-14 in the wood decreases with time as it decays with a half-life of about 5700 years into nitrogen. By comparing how much carbon-14 there is in the dead organism with the amount in a living one, the age of the dead organism can be estimated.
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Graph shows the amount of carbon-14
decreasing over time. It halves every 5700 years.
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Dating rocks
Rocks can be dated in a similar way, by measuring the proportion of radioactive uranium in some cases, or potassium in others. These isotopes have much longer half-lives than carbon-14, so are suitable for investigating ancient rocks.
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Graph showing how the % of uranium-238
decreases over time. The amount halves every 4500 years.
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The half-life of uranium-238 is 4500 million years. When it decays, it forms thorium-234 which is also unstable. After a series of radioactive isotopes are formed it eventually becomes lead-206, which is stable.
The age of the rock can be calculated if the ratio of uranium to lead is known. As the rock gets older the proportion of lead increases. If half of the uranium-238 has turned into lead-206 the rock will be 4500 million years old.
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science
]