A nuclear medicine technologist stands 1 metre from an unshielded point source of radioactivity and records an exposure rate of 400 microsieverts per hour. If she steps back to 2 metres, the expected exposure rate is:

• A 200 microsieverts per hour
• B 50 microsieverts per hour
• C 100 microsieverts per hour ✓
• D Unchanged, because distance does not affect gamma emissions

Explanation

For a point source, exposure follows the inverse square law: intensity varies as 1 divided by distance squared. Doubling the distance from 1 m to 2 m reduces the rate to one quarter, giving 100 microsieverts per hour. Distance is the most effective of the three cardinal protection measures of time, distance and shielding, since even modest increases in distance produce large dose reductions.

Reference: Khan, The Physics of Radiation Therapy, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.