Irradiation

1. When radiation from a radioactive source reaches an object, the object is irradiated.
2. The source stays where it is, and only the radiation travels from the source to the object.
3. Alpha, beta and gamma radiation are all forms of ionising radiation, so they can knock electrons out of atoms in the object and turn those atoms into ions.
4. Irradiation stops as soon as the source is taken away, sealed in a lead-lined container or blocked by suitable shielding.
5. An irradiated object does not become radioactive, because no unstable nuclei have been added to it.
6. Hospitals use this idea to sterilise surgical instruments, by irradiating sealed packs with gamma rays so that any bacteria on them are killed.
7. The instruments are safe to handle straight afterwards, because the gamma rays have passed through them without leaving any radioactive material behind.

Contamination

1. An object is contaminated when radioactive material, made of atoms with unstable nuclei, ends up on it or inside it.
2. The contaminating atoms keep decaying, so the object keeps being exposed to radiation for as long as the material stays there.
3. External contamination happens when radioactive dust, liquid or gas settles on the skin, hair or clothing.
4. Internal contamination happens when radioactive material is breathed in, swallowed or enters the body through a cut.
5. Moving away from the hazard does not help, because the contaminated person carries the radioactive material with them.
6. External contamination can often be reduced by removing contaminated clothing and washing the skin thoroughly.
7. Internal contamination is much harder to deal with, because the radioactive material cannot be washed off and may stay in the body for a long time.
8. Radioactive material can be passed on by touch to other people, surfaces and objects, so contamination can spread.
9. A contaminated person is also being irradiated, because the material on or in their body emits radiation into their tissues.

Comparing the hazards

1. Irradiation and contamination are both hazardous because ionising radiation can damage the cells of living tissue.
2. Ionisation inside a cell can damage its DNA, which may cause a mutation that leads to cancer.
3. A very large dose can kill cells outright, causing radiation burns or radiation sickness.
4. The hazard from irradiation lasts only while you are exposed to the source, so it ends when you move away or the source is shielded.
5. The hazard from contamination lasts for as long as the radioactive material stays on or in the body, which may be weeks, years or a lifetime.
6. Internal contamination is usually the most hazardous situation, because the source sits right next to living cells with no air or skin in between.
7. The type of radiation changes the comparison, because alpha, beta and gamma radiation penetrate matter by different amounts.
   1. An alpha source outside the body is a low irradiation hazard, because alpha particles are absorbed by a few centimetres of air or the outer layer of dead skin.
   2. An alpha source inside the body is extremely hazardous, because all of its strongly ionising alpha particles are absorbed by the living cells around it.
   3. Beta and gamma sources outside the body are a greater irradiation hazard than alpha sources, because their radiation passes through the skin and reaches internal organs.
   4. A gamma source inside the body is less damaging than an alpha source, because gamma rays are weakly ionising and most of them pass straight out of the body.
8. Neither hazard is always the greater one, so a full comparison considers the type of radiation, where the source is and how long the exposure lasts.

Reducing the risks

1. People who work with radioactive sources reduce irradiation by keeping the exposure time short, keeping their distance from the source and placing shielding such as lead or concrete between themselves and the source.
2. Sources are handled with long-handled tongs, held at arm's length and never pointed at anyone.
3. Sources are kept in lead-lined boxes when they are not being used.
4. Workers reduce contamination by wearing gloves and protective clothing, and by using sealed sources from which radioactive material cannot escape.
5. Workers wear a dosimeter badge that records the total dose of radiation they receive over time.
6. Rooms where open radioactive material is used are checked regularly with detectors, so that any spill can be found and cleaned up.