Which statement best describes how dose rate relates to distance according to the inverse-square law?

Which statement best describes how dose rate relates to distance according to the inverse-square law?

• A. Dose rate decreases with distance according to the inverse-square law
• B. Dose rate increases with distance
• C. Distance has no effect on dose rate
• D. Dose rate depends only on shielding

Answer: (A)

What this is testing is how dose rate changes with distance, and it follows the inverse-square law. For a point-like radiation source, the dose rate is inversely proportional to the square of the distance from the source. That means when you double the distance, the dose rate drops to one quarter, and when you triple the distance, it drops to one ninth. This decrease comes from the energy spreading over a larger area as you get farther away. Real sources aren’t perfect point sources forever, and shielding or materials between you and the source can also affect dose, but the fundamental distance relationship is about geometric spreading, not about shielding alone. So the statement that the dose rate decreases with distance according to the inverse-square law is the correct description, while the ideas that dose rate increases with distance, that distance has no effect, or that dose rate depends only on shielding don’t fit this relationship.

What this is testing is how dose rate changes with distance, and it follows the inverse-square law. For a point-like radiation source, the dose rate is inversely proportional to the square of the distance from the source. That means when you double the distance, the dose rate drops to one quarter, and when you triple the distance, it drops to one ninth. This decrease comes from the energy spreading over a larger area as you get farther away.

Real sources aren’t perfect point sources forever, and shielding or materials between you and the source can also affect dose, but the fundamental distance relationship is about geometric spreading, not about shielding alone. So the statement that the dose rate decreases with distance according to the inverse-square law is the correct description, while the ideas that dose rate increases with distance, that distance has no effect, or that dose rate depends only on shielding don’t fit this relationship.