Energy and particles · GCSE Physics
Irradiation and contamination
Teacher-written GCSE Physics revision on irradiation versus contamination, background radiation, risks of ionising radiation, and how time, distance and shielding reduce dose.
Irradiation is exposure from outside; the object does not become radioactive. Contamination is unwanted radioactive material on or in you, which then keeps emitting.
The important bits
What you need to know
- 1
Irradiation is exposure to radiation from an outside source. When the source is removed or switched off, irradiation stops. The object is not radioactive.
- 2
Contamination is unwanted radioactive material on or in an object. That material keeps decaying, so the object remains a source until it is cleaned or the isotope decays.
- 3
Background radiation is always present: radon gas from rocks, food, cosmic rays, medical uses and traces of fallout. It is usually low; radon in some buildings is the main domestic issue in the UK.
- 4
Risk comes from ionisation damaging DNA, which can cause cancer. Higher dose and longer exposure increase risk. Alpha is especially hazardous if you are contaminated internally.
- 5
Reduce dose with time (less time near the source), distance (intensity falls as you move away) and shielding (paper, aluminium, lead or concrete matched to the type).
- 6
Sterilising instruments with gamma is irradiation: the tools are not left radioactive. A spilled powder on a bench is contamination.
- 7
Tracers are a controlled contamination of a patient or a pipe; the isotope is chosen so half-life and type of radiation keep the risk acceptable.
- 8
Radiation dose is measured in sieverts (Sv) in real life; GCSE usually discusses count rate, activity in Bq, and qualitative risk rather than sievert calculations.
Quotations worth analysing
Short evidence. Real method.
“Irradiation does not cause an object to become radioactive.”
Food irradiated to kill microbes is not a radioactive meal. The gamma source stays in the plant; the food only received a dose.
“Contamination is the unwanted presence of radioactive atoms on or in an object.”
Those atoms continue to decay. Cleaning, isolation and waiting for half-lives to pass are the responses, not just stepping away for a minute.
Go deeper
The route matters as much as the type
Alpha on the skin from outside is usually stopped; alpha in the lungs from radon daughters or a dusty source is a serious internal irradiator of tissue. Gamma from a sealed source across the room is an irradiation problem: increase distance, add lead, limit time. The same gamma-emitting powder spilled on your hands is contamination: you carry the source with you, you can ingest it, and walking away from the lab does not help until you are decontaminated. Exam questions often give a scenario — a technician, a leak, a sterilising beam — and ask which term applies and why. Use both words correctly in one sentence: “The workers are irradiated by the beam but are not contaminated unless the source leaks.” That contrast is the mark.
Go deeper
Background is not zero, and it is not all the same
Everyone in the UK receives background radiation. Granite areas have more radon. Flying increases cosmic-ray dose. A chest X-ray is a medical irradiation, justified because the benefit outweighs the small extra risk. When you measure a source in class, the Geiger counter already clicks with no source present: that is background, and it must be subtracted for half-life work. Do not tell a story that “radiation is unnatural”. Do tell a story that extra dose should be justified, limited and shielded. Precautions in a lab: tongs, lead castle, do not eat, point sources away from the body, put sources back promptly. Those are time, distance and shielding in practical clothes.
See the idea in action
A sealed cobalt-60 gamma source is used to sterilise syringes on a conveyor. The syringes are irradiated: microbes are killed, but the plastic does not become radioactive, so hospitals can use them at once. A cracked capsule that leaked cobalt powder onto the belt would contaminate the syringes; they would keep emitting and would be unsafe. Background in the room is 20 counts/min. A monitor on a worker’s coat reads 20 counts/min after the shift — no contamination found. A wipe test on the belt reads 500 counts/min: that is contamination and must be cleaned, with the waste stored as radioactive until activity falls.
Exam technique
Turn knowledge into marks
Define both terms, then apply them to the situation. For safety, name time, distance and shielding, and match the shield to alpha, beta or gamma. Do not claim an irradiated object keeps emitting.
Common mistakes
Do not give these marks away
- 01
Confusing irradiation with contamination, or claiming an irradiated apple is now a radioactive source.
- 02
Saying all radiation is equally dangerous regardless of type and whether it is inside or outside the body.
- 03
Forgetting background when discussing “any reading on a counter”, or ignoring radon as a contamination risk in buildings.
A tray of medical instruments is exposed to gamma rays from a sealed source, then removed. Which statement is correct?
AThe instruments are contaminated and will keep emitting gamma rays
BThe instruments were irradiated and are not radioactive
CThe instruments have a shorter half-life
DGamma rays have made the instruments into alpha emitters
Show the answer
The instruments were irradiated and are not radioactive. The source stayed sealed. Irradiation can kill microbes without leaving radioactive material on the instruments. Contamination would mean radioactive atoms on or in them.
Quick questions
If this is the bit you searched
What is the difference between contamination and irradiation?
Contamination is radioactive material on or in an object, which continues to emit. Irradiation is exposure to a source outside; once the source is removed, the object is not radioactive.
How do you reduce the risk from a radioactive source?
Minimise time near it, maximise distance, and use suitable shielding. Handle with tongs, store in lead, and avoid eating or drinking in the lab.
What is background radiation?
The ionising radiation always around us from rocks, radon, food, cosmic rays and human sources such as medicine. It must be subtracted in count-rate experiments.
Why is alpha more dangerous inside the body than outside?
Outside, skin stops it. Inside, it is highly ionising and deposits a lot of energy in a short track through living tissue.