How many half-value layers (HVL) are needed to reduce beam intensity from 100% to 12.5%?

To determine how many half-value layers (HVL) are required to reduce beam intensity from 100% to 12.5%, it is important to understand how the concept of half-value layers works. Each half-value layer reduces the intensity of the beam to half of its previous amount.

Starting with an initial intensity of 100%, the following reductions occur with each half-value layer:

1. After the first half-value layer, the intensity is reduced to 50% (100% / 2).

2. After the second half-value layer, the intensity is further reduced to 25% (50% / 2).

3. After the third half-value layer, the intensity is reduced to 12.5% (25% / 2).

Thus, it takes three half-value layers to reduce the original intensity of 100% down to 12.5%. This illustrates how each HVL successively halves the intensity, leading to the final value of 12.5% after three layers.