Radiation dose (in sieverts) is a measure of the risk of harm resulting…

The idea

The sievert does not measure how much radiation is present; it measures the risk of harm to the body, which is why it combines the energy absorbed with how damaging that particular radiation is to living tissue. The same energy delivered as alpha therefore counts for far more sieverts than the same energy delivered as gamma. Dose depends on the type of radiation, on the time of exposure and on the activity or strength of the source, which is why standing further away or spending less time near a source lowers it. Typical exam figures are a background dose of about 2 to 3 mSv per year, a chest X-ray of about 0.02 mSv and a CT scan of several mSv; put any two on the same basis, per scan against per year, before comparing them. In passing: irradiation is exposure to a source, contamination is radioactive material on or inside you.

What earns the marks

• Stating that dose measures the risk of harm rather than the amount of radiation present.
• Saying that the dose depends on the type of radiation and on the time of exposure.
• Saying that the dose also depends on the activity or strength of the source.
• Putting two supplied doses on the same basis, per scan against per year, before comparing them.

Common mistakes and how to put them right

• Swapping the definitions of irradiation and contamination.

• Saying that an irradiated object becomes radioactive itself.

• Treating the sievert as a unit of energy or of activity.

• Comparing two doses without also comparing the exposure times.

Exam tips

• Name all three of source strength, exposure time and type of radiation when asked what the dose depends on.
• You are not required to recall the unit, but you must be able to use sieverts and millisieverts from data.
• When given a table of doses, compare like with like, per scan or per year, before drawing a conclusion.