How many half value layers (HVL) are required to reduce an x-ray beam from 1 mGy/mAs to 0.05 mGy/mAs?

To understand how many half-value layers (HVL) are needed to reduce an x-ray beam intensity from 1 mGy/mAs to 0.05 mGy/mAs, we first need to recognize what HVL represents. Each half-value layer reduces the intensity of the radiation beam by half.

Starting with 1 mGy/mAs, the process of reducing it stepwise can be visualized as follows:

1. After the first HVL, the intensity reduces to 0.5 mGy/mAs.

2. After the second HVL, this is halved again to 0.25 mGy/mAs.

3. After the third HVL, the intensity is halved once more, resulting in 0.125 mGy/mAs.

4. Finally, after the fourth HVL, the intensity reduces to 0.0625 mGy/mAs.

At this point, the intensity of 0.0625 mGy/mAs is lower than 0.05 mGy/mAs. However, the question specifically asks for the point at which the intensity reaches or drops below 0.05 mGy/mAs. Therefore, it is the third HVL that first brings it below 0.125