A radiographer stands 2 metres from an X-ray tube during an exposure and receives an exposure rate of 16 mGy/hr. If the radiographer moves to 4 metres from the source, the new exposure rate will be:

• A 8 mGy/hr
• B 4 mGy/hr ✓
• C 2 mGy/hr
• D 1 mGy/hr

Explanation

The inverse square law states that exposure rate varies inversely with the square of the distance from a point source. Doubling the distance from 2 m to 4 m reduces the exposure rate by a factor of 2 squared, that is 4, giving 16 divided by 4 equals 4 mGy/hr. Distance is the cheapest form of radiation protection because even small increases produce large dose reductions.

Reference: Radiologic Science for Technologists (Bushong), 12th ed.

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