What materials can shield gamma radiation most effectively?

The effectiveness of shielding materials against gamma radiation primarily depends on their density and atomic number. Dense materials, such as lead and steel, are highly effective at attenuating gamma radiation due to their ability to interact with high-energy photons through mechanisms like photoelectric absorption, Compton scattering, and pair production.

Lead is often the material of choice for radiation shielding because it provides a significant amount of attenuation in relatively thin layers, making it efficient in terms of weight and space. Steel, while slightly less effective than lead due to its lower atomic number, still offers substantial shielding capabilities because of its density.

In contrast, lighter materials like paper, aluminum, or lightweight plastic lack the necessary density to effectively reduce gamma radiation exposure. Water and paraffin, while they have some utility in neutron shielding, do not provide the same level of attenuation against gamma radiation as denser materials. Thus, when considering effective shielding against gamma radiation, the most suitable materials are indeed those with high density, such as lead and steel.