**Correct Option: C. Lead** ### Detailed Explanation When it comes to shielding against high-frequency electromagnetic radiation, such as X-rays and gamma rays, the effectiveness of a material is determined by its density and atomic number. High-frequency radiation has high energy and can penetrate materials easily. Therefore, materials that are dense and have high atomic numbers are more effective at absorbing or scattering this type of radiation. 1. **Understanding Radiation Penetration**: - X-rays and gamma rays are forms of electromagnetic radiation with very short wavelengths and high energy. Because of their high energy, they can penetrate many materials that would otherwise block lower-energy radiation (like visible light). - The ability of a material to shield against this radiation is often quantified by its "half-value layer" (HVL), which is the thickness of the material required to reduce the intensity of the radiation by half. 2. **Material Properties**: - **Lead (C)**: Lead has a high atomic number (82) and is very dense (11.34 g/cm³). This combination makes it extremely effective at attenuating X-rays and gamma rays. The high density means that there are more atoms in a given volume to interact with the radiation, and the high atomic number increases the likelihood of interactions that absorb or scatter the radiation. - **Aluminum (B)**: Aluminum has a lower atomic number (13) and is less dense (2.70 g/cm³) compared to lead. While aluminum can provide some shielding against X-rays, it is not nearly as effective as lead, especially for high-energy gamma rays. - **Wood (A)**: Wood is much less dense (about 0.6-0.9 g/cm³) and has a low atomic number (6). It provides minimal shielding against high-frequency radiation. While it can attenuate some lower-energy radiation, it is not suitable for X-rays or gamma rays. - **Plastic (D)**: Similar to wood, plastic is also low in density (about 0.9-1.5 g/cm³) and has a low atomic number (varies, but generally around 6-8). It is not effective for shielding against high-energy radiation like X-rays and gamma rays. ### Why Other Options Are Incorrect or Weaker - **A. Wood**: - Wood is not dense enough to provide significant shielding against high-energy radiation. It may block some lower-energy radiation but is ineffective against X-rays and gamma rays. - **B. Aluminum**: - While aluminum can provide some level of protection, it is not nearly as effective as lead. Its lower density and atomic number mean that it cannot attenuate high-energy radiation as effectively as lead can. - **D. Plastic**: - Plastic is also ineffective for high-frequency radiation. Its low density and atomic number mean that it does not provide adequate shielding against X-rays and gamma rays. ### Summary of Key Points - **Lead is the most effective material for shielding against high-frequency electromagnetic radiation** due to its high density and atomic number. - **Aluminum, wood, and plastic are less effective** because they have lower densities and atomic numbers, making them inadequate for blocking high-energy radiation. - **Understanding the properties of materials** is crucial when selecting appropriate shielding for radiation protection. ### Revision Summary - **Lead (C)** is the best material for shielding against X-rays and gamma rays due to its high density and atomic number. - **Aluminum (B)**, **wood (A)**, and **plastic (D)** are less effective due to their lower densities and atomic numbers. - **High-frequency radiation** requires materials that can absorb or scatter energy effectively, which lead does best. - **Always consider the half-value layer** when evaluating the effectiveness of a shielding material.