If a technologist doubles their distance from an X-ray source, by what factor does the beam dose change?

If a technologist doubles their distance from an X-ray source, by what factor does the beam dose change?

• A. Dose doubles
• B. Dose quadruples
• C. Dose halves
• D. Dose decreases by a factor of 2^2

Answer: (D)

The key idea is the inverse square law: X-ray beam dose (or intensity) falls off with the square of the distance from the source. If you double the distance, the dose doesn’t just halve or stay the same—it becomes one quarter of what it was. That’s because the beam spreads over a surface area that grows with distance squared, so the energy per unit area drops proportional to 1/(2^2) = 1/4. So the dose decreases by a factor of 4, which matches the statement that the dose decreases by a factor of 2^2. The other options would imply different geometric behavior (doubling, quadrupling, or halving), which isn’t how the dose scales with distance.

The key idea is the inverse square law: X-ray beam dose (or intensity) falls off with the square of the distance from the source. If you double the distance, the dose doesn’t just halve or stay the same—it becomes one quarter of what it was. That’s because the beam spreads over a surface area that grows with distance squared, so the energy per unit area drops proportional to 1/(2^2) = 1/4. So the dose decreases by a factor of 4, which matches the statement that the dose decreases by a factor of 2^2. The other options would imply different geometric behavior (doubling, quadrupling, or halving), which isn’t how the dose scales with distance.