What is the most effective material to block gamma radiation?

Lead is the most effective material for blocking gamma radiation due to its high atomic number and density, which enable it to attenuate high-energy photons more effectively than lighter materials. Gamma radiation is a type of electromagnetic radiation that can penetrate deeply into materials; thus, a material's ability to stop or reduce gamma rays is primarily determined by its density and atomic structure.

Lead's high density means that it can absorb a significant amount of gamma radiation relative to its thickness. Other materials, such as aluminum or plastic, lack the necessary density and atomic characteristics to provide effective shielding against gamma rays. While cardboard may provide some minimal protection due to its thickness, it is far less effective compared to dense metals like lead. Therefore, lead is widely used in applications requiring radiation shielding, such as in medical settings, nuclear facilities, and industrial radiography, making it the clear choice for blocking gamma radiation effectively.