If X-ray intensity is 100 mrem/hr at 1 meter FFD, what is it at 4 meters?

The question involves understanding the concept of the inverse square law, which describes how the intensity of radiation decreases with distance from the source. According to this law, the intensity of radiation is inversely proportional to the square of the distance from the source.

In this scenario, if you have an intensity of 100 mrem/hr at a Source-to-Focal Distance (FFD) of 1 meter, to find the intensity at 4 meters, you calculate it based on the change in distance. The formula is:

Intensity at new distance = Original intensity × (Original distance / New distance)²

Here, the original intensity is 100 mrem/hr, the original distance is 1 meter, and the new distance is 4 meters.

When you plug the values into the equation:

Intensity at 4 meters = 100 mrem/hr × (1/4)²

= 100 mrem/hr × (1/16)

= 100 mrem/hr × 0.0625

= 6.25 mrem/hr

This calculation shows that at 4 meters, the intensity is indeed 6.25 mrem/hr. Therefore, the choice stating 6.25 mrem/hr represents the correct