Which substance is known to be an effective shield for gamma rays?

Lead is an effective shield for gamma rays due to its high-density and high atomic number. Gamma rays are a form of electromagnetic radiation that have a very short wavelength, making them highly penetrating. To effectively attenuate gamma radiation, the shielding material must have a substantial density and atomic number to reduce the energy of the gamma photons as they pass through the material.

Lead's density (around 11.34 g/cm³) means that it is capable of effectively absorbing and scattering gamma radiation, thus reducing the intensity of the radiation that passes through it. The high atomic number of lead contributes to its effectiveness, as it has a larger number of electrons, which helps in interacting with the high-energy photons of gamma rays.

Other materials, such as aluminum, plastic, and concrete, can provide some level of shielding but are not as effective as lead against gamma radiation. Aluminum, for example, is less dense and thus offers limited attenuation. Concrete can block some radiation, but its effectiveness varies based on its density and thickness compared to lead. Plastic might be useful for lower energy radiation types but does not have the necessary properties to effectively block gamma rays. Therefore, lead stands out as the best choice for shielding against gamma radiation.