Stopping Invisible Energy
Gamma rays and neutrons carry enough energy to ionize atoms and damage biological tissue, but they are invisible, odorless, and penetrating. A 1 MeV gamma ray can traverse 10 cm of lead. A fast neutron can pass through a meter of steel. Shielding design is the engineering discipline of placing enough material between radiation sources and people to reduce dose rates to acceptable levels — typically below 1 mSv per year for the public and 20 mSv per year for radiation workers.
Exponential Attenuation
Gamma-ray intensity decreases exponentially with shield thickness: each additional centimeter removes the same fraction of the remaining beam. This is characterized by the linear attenuation coefficient μ, which depends on photon energy and shield material. At 1 MeV, μ is about 0.15/cm for concrete and 0.77/cm for lead — meaning lead is roughly five times more effective per centimeter, though concrete is far cheaper per unit area.
The Buildup Problem
Simple exponential attenuation underestimates actual dose because Compton-scattered photons can still reach a person behind the shield. The buildup factor corrects for this scattered radiation and depends on shield material, thickness in mean free paths, and photon energy. For thick concrete shields, the buildup factor can exceed 10 — meaning scattered photons deliver ten times more dose than the direct, unscattered beam alone.
Designing the Shield
This simulation calculates gamma-ray attenuation through four common shield materials using the buildup factor method. Adjust gamma energy, shield thickness, material type, and source strength to explore how each parameter affects the transmitted dose rate. Compare the half-value layers of different materials and notice how higher gamma energies require dramatically thicker shields due to reduced photoelectric absorption.