### Correct Option: A. Lead #### Detailed Explanation: Gamma radiation is a form of electromagnetic radiation that is highly penetrating and can pass through most materials. To effectively shield against gamma radiation, we need materials that have both high density and high atomic number. This is because: 1. **High Density**: A denser material has more mass per unit volume, which means there are more atoms in a given volume to interact with the gamma photons. This increases the likelihood of the gamma rays being absorbed or scattered. 2. **High Atomic Number**: Materials with a higher atomic number have more protons and neutrons in their nuclei, which increases the probability of interaction with gamma radiation. The interaction can occur through processes such as photoelectric absorption, Compton scattering, and pair production. **Lead** is the most commonly used material for gamma radiation shielding because: - **High Density**: Lead has a density of about 11.34 g/cm³, making it one of the densest materials available for practical use. - **High Atomic Number**: Lead has an atomic number of 82, which is relatively high compared to many other materials. When gamma rays encounter lead, they are more likely to be absorbed or scattered, significantly reducing the intensity of the radiation that can pass through. #### Why Other Options Are Incorrect: **B. Aluminum**: - Aluminum has a lower density (about 2.7 g/cm³) and a lower atomic number (13) compared to lead. While it can provide some shielding against lower-energy radiation, it is not effective against gamma rays, which require denser materials for effective attenuation. **C. Plastic**: - Plastic is generally low in density and has a low atomic number. While certain types of plastic can be used for neutron shielding, they are not effective against gamma radiation. The lack of density means there are fewer atoms to interact with the gamma photons. **D. Wood**: - Wood is also low in density (about 0.6 to 0.9 g/cm³) and has a low atomic number (around 6 for carbon, which is a major component of wood). Like plastic, wood is not effective for gamma radiation shielding due to its insufficient density and atomic number. ### Summary of Key Points: - **Lead** is the most effective material for shielding against gamma radiation due to its high density (11.34 g/cm³) and high atomic number (82). - Other materials like aluminum, plastic, and wood are less effective because they have lower densities and atomic numbers, making them inadequate for gamma radiation protection. - Effective shielding requires materials that can absorb or scatter gamma rays, which is best achieved with dense, high-Z materials like lead. ### Revision Summary: - **Lead** is the best material for gamma radiation shielding due to its high density and atomic number. - **Aluminum**, **plastic**, and **wood** are ineffective for gamma radiation shielding. - Shielding effectiveness increases with material density and atomic number. - Understanding the properties of materials is crucial for effective radiation protection.