If the exposure rate to a body standing 5 feet from a radiation source is 12 mR/min, what is the dose rate at 12 feet?

If the exposure rate to a body standing 5 feet from a radiation source is 12 mR/min, what is the dose rate at 12 feet?

• A. 2 mR/min
• B. 12 mR/min
• C. 0.5 mR/min
• D. 8 mR/min

Answer: (A)

Exposure rate from a radiation source follows the inverse square law, meaning the dose rate falls with the square of the distance from the source. If you know the rate at one distance, you multiply by (initial distance / new distance) squared. Here, the rate at 5 feet is 12 mR/min. At 12 feet, the rate is 12 × (5/12)² = 12 × (25/144) ≈ 2.08 mR/min, about 2 mR/min. This shows how moving farther away greatly reduces the dose rate; the factor (5/12)² is roughly 0.174, so the rate is about one-sixth of the original.

Exposure rate from a radiation source follows the inverse square law, meaning the dose rate falls with the square of the distance from the source. If you know the rate at one distance, you multiply by (initial distance / new distance) squared.

Here, the rate at 5 feet is 12 mR/min. At 12 feet, the rate is 12 × (5/12)² = 12 × (25/144) ≈ 2.08 mR/min, about 2 mR/min. This shows how moving farther away greatly reduces the dose rate; the factor (5/12)² is roughly 0.174, so the rate is about one-sixth of the original.