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

Alpha particles are indeed the type of radiation that is least penetrating and can be effectively stopped by a sheet of paper. Alpha particles consist of two protons and two neutrons, making them relatively large and positively charged. Due to their mass and charge, alpha particles interact strongly with matter, losing energy rapidly as they pass through materials.

When alpha particles encounter a barrier like paper, they do not have enough energy to penetrate it, resulting in their complete absorption. This property is one of the reasons why alpha radiation is generally considered less harmful to humans than other types of radiation, as alpha emitters can often be safely handled unless ingested or inhaled, where they can cause damage to internal tissues.

In contrast, beta particles, being smaller and carrying a single negative charge, can penetrate more deeply and are not stopped by paper. Gamma rays and X-rays are forms of electromagnetic radiation and have very high penetrating power, which allows them to pass through many materials, including paper, making them significantly more hazardous in terms of external exposure.