Which type of radiation is stopped by a sheet of paper?

Alpha radiation is stopped by a sheet of paper. Alpha radiation consists of alpha particles: tightly bound nuclei containing two protons and two neutrons, identical to helium-4 nuclei. They carry a positive charge and interact strongly with matter.

That strong interaction makes alpha radiation highly ionising but poorly penetrating. In ordinary conditions, alpha particles travel only a few centimetres through air and are stopped by the outer dead layer of human skin. A sheet of paper is therefore enough to block the alpha particles emitted by typical radioactive sources.

This does not mean alpha radiation is harmless. Alpha-emitting material can be dangerous if it is inhaled, swallowed, or enters the body through a wound, because the particles then release their energy directly into nearby tissue. The usual comparison is that beta radiation can pass through paper but is reduced by materials such as aluminium, while gamma radiation is much more penetrating and requires dense shielding such as lead or concrete. High-energy alpha particles, including some cosmic-ray helium nuclei, can travel much farther than the familiar textbook examples, but those are not the usual meaning of this quiz question.