For a point source, how does dose rate change with distance?

• A. Dose rate decreases with distance as 1/r
• B. Dose rate remains constant
• C. Dose rate increases with distance as r^2
• D. Dose rate decreases with distance as 1/r^2

Answer: (D)

The main concept is that dose rate from a point source follows the inverse-square law: it decreases with the square of the distance. Radiation energy emitted by the source spreads out uniformly in all directions, so at distance r it shines over the surface of a sphere with area 4πr^2. Since the total emission rate is the same, the dose rate (which is tied to the flux of energy per unit area) becomes proportional to 1/r^2. That’s why doubling the distance drops the dose rate to one quarter, and tripling distance drops it to one ninth. This behavior assumes free space with no shielding or attenuation.

So the correct relationship is that dose rate decreases with distance as 1/r^2. The other options don’t fit because constant would ignore geometric spreading, 1/r implies a slower falloff that doesn’t match a point-source geometry, and increasing with distance would contradict the way a point source distributes energy in space.