Which of the following materials would be least effective in shielding against high-energy gamma radiation?

The least effective material for shielding against high-energy gamma radiation is a thin sheet of plastic. Gamma rays possess very high penetrating power and require dense materials to reduce their intensity effectively. Plastic, being lightweight and not dense, provides minimal attenuation to gamma radiation. High-energy gamma radiation can easily pass through thin non-dense materials like plastic, making it unsuitable for protection in environments where strong gamma sources are present.

In contrast, materials like thick concrete, thick steel, and lead are much denser and can absorb or scatter gamma radiation more effectively due to their atomic structure. Thick concrete walls and steel layers block gamma rays by utilizing their density to dissipate energy, while lead is particularly effective because of its high atomic number and density, which significantly enhances its ability to shield against radiation. Thus, the effectiveness of shielding materials against gamma rays is determined by their density and atomic composition, with thin plastic falling short in both aspects.