In the inverse square law, doubling distance from a radiation source does what to the dose rate?

• A. Doubles the dose rate.
• B. Divides the dose rate by four.
• C. Triples the dose rate.
• D. Leaves dose rate unchanged.

Answer: (B)

The main concept is the inverse square law for a point radiation source: the dose rate falls off with the square of the distance. As energy spreads over a sphere, its surface area grows with r^2, so intensity goes as 1/r^2. If you double the distance, the dose rate becomes 1/(2r)^2 = 1/4 of the original. In other words, the dose rate is reduced to one quarter (divided by four). For example, if the rate is D at 1 unit of distance, it becomes D/4 at twice that distance, D/9 at three times, and so on. This rules out the ideas of doubling, staying the same, or tripling the dose rate.