The intensity of radiation at 0.1 meters of a source is 300 mCi, what is the intensity at 5 meters?

Question

Very unsure of the setup of the problem?

Answer 1

Intensity of radiation drops off with the square of the distance, I is proportional to 1/r². So set up a ratio I(5) / I(0.1) = (0.1)² / (5)² = 0.01/25, then I(5) = 300 * 0.01/25 = 0.12 mCi.