Which type of radiation can be completely stopped by a sheet of paper?

Alpha particles are composed of two protons and two neutrons, making them relatively large and heavy compared to other types of radiation. This mass gives them a relatively low penetration capability. When emitted, alpha particles can only travel a short distance in air and can be easily stopped by very thin materials, such as a sheet of paper or even the outer layer of human skin.

Because of their positive charge and relatively high mass, alpha particles interact strongly with matter. As they collide with atoms in the medium they are passing through, they lose energy rapidly. This means that even a simple barrier, like a piece of paper, can absorb their energy effectively, preventing them from penetrating further.

In contrast, beta particles, which are high-energy electrons or positrons, can penetrate more than alpha particles but can still be stopped by materials like plastic or glass. Gamma rays and X-rays are forms of electromagnetic radiation and are capable of penetrating even denser materials, requiring thicker shields, such as lead or concrete, to be effectively stopped. Therefore, the effectiveness of a sheet of paper in stopping radiation is specific to alpha particles due to their unique physical properties.