1. The problem statement, all variables and given/known data The ‘half-thickness’ tells us the thickness of a given material needed to absorb half the incident photons from a particular source. Find the average distance of a gamma ray in lead before its first collision. I have a value for the half thickness (and equivalent half thickness) and the density of lead. 2. Relevant equations 3. The attempt at a solution I'm not sure how to go from the half thickness to a distance. Since this is the distance needed to absorb half the photons, is this also the average distance the photons will travel?