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Measuring radiation and risk

Different units are used when discussing radiation and risk quantitatively.

  • The ‘activity’ of a radioactive material is the number of ionizing radiations emitted every second. This is measured in becquerels (Bq).
  • The amount of harm caused by ionizing radiation will depend on the energy it deposits, called the ‘absorbed dose’. The unit of absorbed dose, the gray, is the number of joules absorbed per kg of material.

Two additional factors affect the potential harm from ionizing radiation

  • the type of radiation. For example, alpha radiation is highly ionizing and gamma radiation is weakly ionizing.
  • the type of tissue exposed. The sensitivity to radiation of different tissue types varies. Cells which naturally divide rapidly are most at risk.

Irradiation or contamination?

‘Irradiation’ is the exposure to radiation from a source located at some distance outside the body. Irradiation from a source of alpha radiation presents a low risk because alpha radiation is easily absorbed – by air, clothing, or the layer of dead skin cells on your skin.

‘Contamination’ is the exposure to radiation from a source inside your body, or on your skin or clothes. This is generally more dangerous, because cells are unprotected.

Dose equivalent

Radiation ‘dose equivalent’ is a measure of the potential damage from radiation exposure, or risk. Its unit is sieverts (Sv). The UK average annual dose of 2.6 millisieverts comes almost entirely from eating and breathing naturally radioactive materials.

  Radiation and decision making


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Workers in danger of radiation exposure can carry one of these, a dosimeter, which records how much radiation the wearer is exposed to
Photo: CC: Rama

Alpha Decay

Gamma Decay