Which substance is most effective at shielding against gamma rays?

Lead is the most effective material for shielding against gamma rays due to its high density and atomic number. Gamma rays are a form of electromagnetic radiation with high energy, and they can penetrate many materials. The effectiveness of a shielding material against gamma radiation is largely determined by its ability to absorb and scatter the gamma photons.

Lead's density allows it to provide significant attenuation of gamma rays, meaning that a relatively thin layer of lead can reduce the intensity of gamma radiation considerably. This property makes it a popular choice in various applications, such as radiation protection in medical imaging and nuclear facilities.

While concrete, steel, and aluminum can also provide shielding against gamma rays, they are less effective than lead. Concrete requires a much thicker layer to achieve the same level of protection as lead, while steel, although stronger than aluminum, does not have the same level of effectiveness in gamma radiation shielding as lead. Aluminum, being much lighter and less dense, offers minimal protection against gamma rays, thus making lead the preferred material in situations where effective shielding is necessary.