Which radiation has the least penetrating power?

Alpha radiation possesses the least penetrating power among the given types of radiation. This is primarily due to its composition and the type of particles involved. Alpha particles consist of two protons and two neutrons, essentially representing a helium nucleus. Because of their relatively large mass and positive charge, they interact strongly with matter and lose energy quickly as they pass through materials.

Alpha particles can be stopped by just a sheet of paper or even the outer layer of human skin. This limited ability to penetrate materials is a result of their significant interaction with electrons in the surrounding environment, which causes them to lose energy rapidly. In contrast, beta radiation consists of lighter particles (electrons or positrons) and has more penetrating power, as it can pass through paper but is typically stopped by a few millimeters of plastic or glass. Gamma radiation and X-rays are forms of electromagnetic radiation that are even more penetrating, able to pass through most materials, requiring heavy lead or concrete for effective shielding.

Understanding the properties of these different forms of radiation helps clarify why alpha radiation is the least penetrating, as it is more effectively absorbed by materials compared to the others.