What is one primary method for shielding against gamma rays?

Lead is considered one of the most effective materials for shielding against gamma rays due to its high density and atomic number. Gamma rays are a form of electromagnetic radiation that can penetrate various types of matter, and lead's dense structure helps to absorb and scatter this radiation more effectively than lighter materials. The effectiveness of gamma ray shielding is often measured in terms of the half-value layer (HVL), which indicates the thickness of a material required to reduce the intensity of radiation by half. Lead has a relatively low HVL for gamma rays, meaning that a thinner layer can adequately reduce exposure compared to other materials.

In contrast, the other materials listed, such as cardboard, wood, and glass, are considerably less effective at shielding against gamma rays. These materials have lower densities and atomic numbers, making them less capable of attenuating the powerful radiation emitted by gamma sources. As a result, while they may provide some level of protection against lower-energy radiation, they are not suitable for high-energy gamma radiation. Thus, for effective gamma radiation shielding, lead is the preferred choice.