What can stop an alpha particle?

• A. a sheet of glass
• B. a few inches of water
• C. a piece of paper
• D. a metal plate

Answer: (C)

Alpha radiation has very low penetrating power because these particles are heavy and doubly charged helium nuclei. As they pass through matter, they lose energy rapidly by ionizing atoms in their path, so they travel only a short distance before stopping. That’s why a simple, thin barrier is enough to halt them—something as common as a sheet of paper can stop most alpha particles. The other materials mentioned would also stop alpha radiation if thick enough, but paper illustrates the basic idea most clearly: alpha particles don’t go far, and a light barrier is sufficient.