Irradiation is the process of exposing an object to nuclear radiation.

The idea

Irradiation is being exposed to radiation from a source that stays outside the object. The source is separate; nothing is transferred. Radiation travels from it, some is absorbed by the object, and the moment the source is removed or shielded, the exposure stops. That is the crucial practical difference from contamination, where the material has moved onto or into the object and keeps decaying there. Irradiation is used deliberately: gamma sources sterilise surgical instruments and food by killing bacteria, and radiotherapy beams irradiate a tumour to destroy unwanted tissue. The exposure is controlled by time, distance and shielding, which is why sources are kept in lead-lined boxes and handled at arm's length with tongs.

What earns the marks

• Irradiation being the exposure of an object to nuclear radiation from a source outside it.
• The radioactive source remaining separate, with no radioactive material transferred.
• The exposure ending as soon as the source is removed or shielded.
• A named use or control measure, such as sterilising instruments or storing the source in a lead-lined box.

Common mistakes and how to put them right

• Writing that irradiation makes the object radioactive.

• Using irradiation and contamination as though they mean the same thing.

• Saying gloves prevent irradiation.

• Assuming irradiated food carries radioactive material and is unsafe to eat.

• Describing irradiation as harmless because nothing is transferred, ignoring the damage the radiation does while it passes through.

Exam tips

• Keep 'exposure to radiation' as your opening phrase; it is the wording the mark scheme uses.
• If both terms appear in a question, define each in one sentence rather than describing only the difference.
• Name time, distance and shielding as the three ways exposure is reduced, since questions often want one of each.