What type of radiation requires the most shielding to protect against?

Gamma rays require the most shielding to protect against due to their nature as high-energy electromagnetic radiation. Unlike alpha and beta particles, which are charged and can be stopped by relatively thin barriers (alpha particles by paper or skin and beta particles by plastic or glass), gamma rays are uncharged and can pass through most materials.

To effectively shield against gamma rays, materials with high atomic numbers and high density, such as lead or thick concrete, are commonly used. This is because gamma rays can penetrate deep into materials, necessitating considerable thickness to sufficiently reduce their intensity. In comparison, while neutrons also require significant protection, the approach to shielding is different; materials that can slow down (moderate) neutrons, such as water or polyethylene, are typically used.

The unique properties of gamma rays—specifically their ability to penetrate and their high energy—make them particularly challenging to shield against, thereby necessitating the most extensive measures in a safety context.