Which of these could be stopped by a sheet of paper?

• A. Gamma ray
• B. Beta particle
• C. Alpha particle
• D. Neutron

Answer: (C)

The key idea is how different radiation types interact with matter and how their penetrating power varies. Alpha particles are heavy, doubly charged helium nuclei. Because of that strong electric charge and large mass, they lose energy very quickly as they collide with atoms in a material, giving them a very short range. A single sheet of paper provides enough material to stop them, so they don’t penetrate through.

In contrast, gamma rays are high-energy photons that don’t have mass or charge, so they pass through paper with little attenuation and require much denser shielding like lead or thick concrete. Beta particles are lighter electrons; they are more penetrating than alpha particles, so a sheet of paper might block some very low-energy betas but many will pass through. Neutrons have no charge either and can travel through most ordinary materials; they’re slowed or absorbed only by materials rich in light nuclei (like water or concrete) rather than by paper.

So, the sheet of paper is effective at stopping alpha particles because of their strong interaction with matter and very short range, which is why that type of radiation is the one that can be halted by such a thin barrier.