IGCSE Physics / Applications of radioactivity
Contamination and irradiation revision notes
Edexcel International GCSE Physics (4PH1), chapter 24: Applications of radioactivity, specification point 7.15. Free lesson.
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Irradiation is exposure to radiation from a source outside an object. Contamination is when radioactive material gets onto or into it, so knowing the difference helps you explain when an object or person remains at risk.
What irradiation means
Irradiation is exposure to radiation from a source that stays outside the object. The rays reach the object, but the radioactive material does not. So the object does not become radioactive. Irradiation stops when the object moves away from the source or is shielded from it. Food sterilised with gamma rays is irradiated, but no radioactive material is left behind.
What contamination means
Contamination happens when radioactive material itself gets onto or into an object or person. The material is the source of the radiation, so moving away does not remove the danger. Radioactive dust on skin or breathed into the lungs can contaminate a person. Contamination continues until the radioactive material is removed or decays. A contaminated object carries radioactive material. It is not just being exposed to rays from a separate source.
How to tell irradiation and contamination apart
The key difference is where the source is. For irradiation it remains outside the object. For contamination the radioactive material is on or inside it. This also explains why the two hazards stop in different ways. Irradiation: the object does not become radioactive; move away or use shielding to stop exposure. Contamination: the object carries radioactive material; remove the material or wait for it to decay.
Which radiation is most dangerous
The hazard depends on whether the source is outside or inside the body. Alpha is stopped by the dead outer layer of skin, but it is highly ionising. This means it can cause strong damage to cells close to an internal source. With a source outside the body, alpha is least dangerous because skin stops it. Beta reaches the skin and eyes, so it presents some danger from outside. Gamma reaches the organs, making it the most dangerous from outside the body. With a source inside the body, alpha is most dangerous because it is highly ionising and close to living cells. Beta presents some danger…
Apply the difference to situations
A fruit exposed to gamma rays is irradiated, not contaminated. The source stays outside it, and the rays leave no radioactive material behind, so the fruit does not become radioactive. A worker splashed with a liquid that emits alpha particles is contaminated because radioactive material is on her hands. The source stays with her and continues to irradiate her until it is washed off. It could also be swallowed and get inside her body. Inside the body, alpha is highly ionising and can damage nearby cells. This makes the spill more dangerous than standing near a sealed alpha source.