Which type of radiation is least penetrating and can typically be stopped by a sheet of paper?

• A. Alpha particles
• B. Beta particles
• C. Gamma rays
• D. Neutron particles

Answer: (A)

Alpha particles are indeed the least penetrating type of radiation and can typically be stopped by a sheet of paper. This characteristic is due to their larger mass and positive charge, which makes them interact more strongly with matter. When alpha particles encounter materials, they tend to lose their energy quickly, as they collide with atoms in the material.

Because of this low penetration ability, alpha radiation poses minimal external hazard; however, it can be harmful if alpha-emitting substances are ingested or inhaled, as they can damage internal tissues directly. In comparison, beta particles, gamma rays, and neutron particles have greater penetration abilities. Beta particles can penetrate several millimeters into materials, gamma rays can pass through the human body, requiring dense shielding like lead or thick concrete to protect against them, and neutrons can penetrate even more deeply due to their neutral charge. Thus, the unique properties of alpha particles explain why they are effectively stopped by something as simple as a sheet of paper.