A beam of gamma rays strikes a lead target with a half-value layer of $0.40 \mathrm{~cm}$. What fraction of the photons in the beam will penetrate to a depth of $1.20 \mathrm{~cm}$ or more?

Question

Quizplus · Accepted Answer

The half-value layer is the thickness of material that reduces the intensity of the radiation by half. Since the half-value layer for the lead target is $0.40 \mathrm{~cm}$, every $0.40 \mathrm{~cm}$ of lead will reduce the gamma ray beam's intensity by half. At a depth of $1.20 \mathrm{~cm}$, which is three half-value layers ($1.20 \mathrm{~cm} / 0.40 \mathrm{~cm} = 3$), the intensity of the beam will be reduced to $(1/2)^3 = 1/8$ or 0.125 of its original intensity.

A Beam of Gamma Rays Strikes a Lead Target with a Half-Value