If the air kerma measured 6 feet from the x-ray table is 5 Gray, what is the air kerma measured at a distance of 3 feet?

If the air kerma measured 6 feet from the x-ray table is 5 Gray, what is the air kerma measured at a distance of 3 feet?

• A. 20 Gray
• B. 10 Gray
• C. 1.25 Gray
• D. 2.5 Gray

Answer: (A)

The important idea here is the inverse square relationship. Radiation intensity (and thus air kerma from a point source) drops with the square of the distance. Halving the distance from the source makes the kerma four times larger, because (6/3)² = 4. So, if at 6 feet the air kerma is 5 Gy, at 3 feet it becomes 5 Gy × 4 = 20 Gy. (For context, if you were at 12 feet, the kerma would be 5 Gy × (6/12)² = 5 Gy × (0.5)² = 1.25 Gy, illustrating the same principle.)

The important idea here is the inverse square relationship. Radiation intensity (and thus air kerma from a point source) drops with the square of the distance. Halving the distance from the source makes the kerma four times larger, because (6/3)² = 4.

So, if at 6 feet the air kerma is 5 Gy, at 3 feet it becomes 5 Gy × 4 = 20 Gy.

(For context, if you were at 12 feet, the kerma would be 5 Gy × (6/12)² = 5 Gy × (0.5)² = 1.25 Gy, illustrating the same principle.)