**Correct Option: B. Lead** ### Detailed Explanation Gamma radiation is a form of electromagnetic radiation that is highly penetrating and can pass through many materials. To effectively shield against gamma radiation, a material must have a high density and a high atomic number (Z). This is because: 1. **High Density**: Denser materials have 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 absorption or scattering of the gamma rays. 2. **High Atomic Number**: Materials with a high atomic number have a greater number of electrons surrounding the nucleus. Gamma rays interact with these electrons, and a higher number of electrons increases the probability of interaction, leading to better shielding. **Lead** is an excellent choice for gamma radiation shielding because: - It has a high density (approximately 11.34 g/cm³). - It has a high atomic number (Z = 82). When gamma rays encounter lead, they can be absorbed or scattered, significantly reducing their intensity. This makes lead one of the most effective materials for shielding against gamma radiation. ### Why the Other Options Are Weaker **A. Wood** - **Density**: Wood is much less dense than lead (density varies but is typically around 0.5 to 1 g/cm³). - **Atomic Number**: Wood is composed mainly of carbon, hydrogen, and oxygen, which have low atomic numbers (C = 6, H = 1, O = 8). This means it has fewer electrons to interact with gamma rays. - **Effectiveness**: Due to its low density and atomic number, wood is not effective at shielding against gamma radiation. **C. Aluminum** - **Density**: Aluminum has a density of about 2.7 g/cm³, which is significantly lower than lead. - **Atomic Number**: Aluminum has an atomic number of 13, which is still much lower than lead. - **Effectiveness**: While aluminum can provide some shielding against lower-energy radiation (like beta particles), it is not effective against gamma radiation due to its low density and atomic number. **D. Plastic** - **Density**: Plastic typically has a density of around 0.9 to 1.5 g/cm³, which is also much lower than lead. - **Atomic Number**: Plastics are made from carbon and hydrogen, which have low atomic numbers. - **Effectiveness**: Like wood and aluminum, plastic is not effective for gamma radiation shielding due to its low density and atomic number. ### Summary of Key Points - **Lead** is the most effective material for shielding against gamma radiation due to its high density and high atomic number. - **Wood**, **aluminum**, and **plastic** are less effective because they have lower densities and atomic numbers, resulting in fewer interactions with gamma rays. - When selecting materials for radiation shielding, prioritize high density and high atomic number for maximum effectiveness. ### Revision Summary - Gamma radiation requires dense, high-Z materials for effective shielding. - **Lead** is the best option due to its high density (11.34 g/cm³) and atomic number (82). - Other materials like wood, aluminum, and plastic are ineffective due to their lower densities and atomic numbers. - Understanding the properties of materials is crucial for effective radiation shielding.