How does increasing distance affect radiation dose, and what happens if distance is doubled?

• A. Dose falls with the square of distance; doubling distance reduces dose to one-quarter.
• B. Dose falls linearly with distance; doubling distance halves dose.
• C. Dose remains constant; doubling distance has no effect.
• D. Dose increases with distance; doubling distance doubles dose.

Answer: (A)

The main idea is the inverse square behavior: radiation intensity from a point source spreads out over the surface of a growing sphere, so the dose rate falls as the square of the distance. The energy that leaves the source is distributed over an area that increases with r^2, making the dose per unit area proportional to 1/r^2.

If you double the distance, the surface area over which the energy is spread becomes four times larger, so the dose at that new location is about one-quarter of the original dose. This geometric spreading is the reason doubling distance reduces dose to a quarter.

This explanation assumes a point source in open space. Real-world factors like extended sources, shielding, and scatter can modify exact values, but the inverse-square relationship is the fundamental rule.