☀️ Irradiation
The radiation comes from a source outside. It passes into the object, but the radioactive atoms stay in the source. When the source is removed or shielded, the exposure stops. The object does not become radioactive.
Radioactivity » Contamination and Irradiation
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 7.15
- The difference between irradiation and contamination
- Why irradiated objects do not become radioactive but contaminated ones are
- Which radiation is most dangerous outside the body and which is most dangerous inside it
- The precautions that protect people against each
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Unlock This CourseTwo ways to be exposed
A radioactive source can affect an object or a person in two very different ways. Examiners love asking you to tell them apart, so learn the difference carefully.
Key terms:
- Irradiation: being exposed to radiation from a source that is outside the object.
- Contamination: having unwanted radioactive atoms on or inside the object.
💧 Contamination
Radioactive atoms have got on or inside the object, for example as dust or liquid. They keep giving out radiation until they are removed or have decayed. The object is carrying a radioactive source with it.
Irradiation: radiation from outside
Think of standing in front of a lamp. You are lit up while you stand there. Switch the lamp off, or put a screen in front of it, and the light on you stops. Irradiation works in the same way. Nothing is left behind afterwards.
- Irradiated fruit. Strawberries are exposed to gamma rays from a source. This kills the mould and bacteria on them, so they last longer. The gamma rays pass through and out again. The fruit is not radioactive, so it is safe to eat.
- Sterilised instruments. Surgical instruments sealed in packs can be irradiated to kill germs. The instruments do not become radioactive. These uses were covered in Uses of Radioactivity in Medicine and Uses of Radioactivity in Industry.
Contamination: radioactive atoms in the wrong place
Now think of getting paint on your hands. Switching off a lamp does not help, because the paint is on you. You must wash it off. In contamination the "paint" is radioactive atoms, and they go on emitting radiation wherever they are.
- Radioactive dust breathed in. Dust containing radioactive atoms is breathed into the lungs and stays there, giving out radiation next to the lung tissue.
- A leak into water. If a radioactive liquid leaks into a river or a water supply, the atoms spread into the water. Anyone who drinks it takes the radioactive atoms into their body.
The atoms only stop being a danger when they are removed (for example by washing) or when they have decayed. A source with a long half-life, from Activity and Half-Life, stays a danger for a very long time.
Which radiation is the most dangerous?
It depends on where the source is. Use what you know about penetrating power from Alpha, Beta and Gamma Radiation.
| Radiation | Source outside the body (irradiation) | Source inside the body (contamination) |
|---|---|---|
| Alpha | Least dangerous. It cannot get through a few centimetres of air or the outer layer of skin. | Most dangerous. Nothing lets it escape, so all of it is absorbed in the body, and it is strongly ionising. |
| Beta | Dangerous. It can pass through skin but is stopped by thin aluminium. | Dangerous. Much of it is absorbed close to where the atoms are. |
| Gamma | Most dangerous. It is the most penetrating, so it passes through the body and is hard to stop. | Least dangerous. Much of it passes straight out of the body without being absorbed. |
Why the order flips
Outside the body, what matters is how well the radiation gets in. Alpha is stopped, so it is the safest. Inside the body, what matters is how much of it is absorbed. Alpha is all absorbed, so it is the worst. Gamma is the opposite way round.
Precautions
The precaution must fit the type of exposure. Irradiation is stopped by keeping the radiation away from you. Contamination is prevented by keeping the radioactive atoms off and out of you.
🛡️ Against irradiation
- Shielding: put a barrier such as lead or thick concrete between you and the source.
- Distance: stay as far away as possible. Use tongs to handle a source instead of your hands.
- Time: spend as little time near the source as you can.
🧤 Against contamination
- Gloves so that atoms do not touch your skin.
- Masks so that you do not breathe in radioactive dust.
- Protective suits that are taken off before leaving the area.
- Washing to remove any atoms that do get on you.
Shielding, distance and time do not protect you from contamination once the atoms are on you or inside you, because they are now with you wherever you go. That is why the contamination precautions are about stopping the atoms from getting there.
Worked example
A worker handles a sealed gamma source with long tongs, behind a lead screen, for two minutes. Is this irradiation or contamination? Which precautions are being used?
The source is sealed and outside the worker, so no radioactive atoms can get on or into them. This is irradiation.
The lead screen is shielding, the long tongs give distance, and two minutes is a short time.
Common mistakes
- Saying that irradiated food is radioactive. The source was outside, and nothing radioactive is left in the food.
- Saying that irradiation and contamination mean the same thing. Irradiation is exposure to radiation from outside. Contamination is radioactive atoms on or in the object.
- Saying alpha is safe. It is safe outside the body, but the most dangerous inside it.
- Suggesting a lead screen as the answer to contamination. Gloves, masks, suits and washing are the precautions that keep the atoms off you.
- Writing "the person becomes radioactive" after irradiation. They do not.
Exam-style question
A technician works with a radioactive powder that gives out alpha particles.
(a) State the difference between irradiation and contamination. [2 marks]
(b) The technician accidentally breathes in some of the powder. Explain why this is more dangerous than standing 1 m away from a sealed alpha source. [3 marks]
(c) State two precautions that reduce the risk of contamination when working with the powder. [2 marks]
Model answer
(a) Irradiation is exposure to radiation from a source outside the body, and it stops when the source is removed (1). Contamination is having radioactive atoms on or inside the body, which keep giving out radiation (1)
(b) Alpha particles are stopped by air or skin, so a sealed source 1 m away cannot reach the body (1). Once breathed in, the alpha particles cannot escape from the body (1). All of their energy is absorbed in the body, and alpha is strongly ionising (1)
(c) Any two from: wear a mask (1), wear gloves (1), wear a protective suit (1), wash after the work (1)
Exam tip
In a "more dangerous" question, say where the source is, inside or outside the body, and link it to penetrating power or absorption. A bare "alpha is dangerous" scores nothing.