Which type of radiation has the least penetrating power?

Alpha particles have the least penetrating power among the types of radiation listed. This is primarily because alpha particles are relatively large, consisting of two protons and two neutrons, which gives them a significant mass compared to other forms of radiation. As a result, they possess a strong positive charge and a limited range in matter.

When alpha particles interact with matter, they exhibit a high probability of ionizing nearby atoms, but they lose energy quickly, making them unable to travel far. For example, alpha particles can be stopped by just a few centimeters of air or by a sheet of paper. This limited penetration makes them particularly hazardous when emitted from radioactive sources that enter the body through ingestion or inhalation, but they are generally not a significant threat outside the body due to their inability to penetrate the skin.

In contrast, gamma rays and X-rays are forms of electromagnetic radiation that have far greater penetrating power and can pass through many materials, including human tissue. Beta particles, which are high-energy electrons, also have greater penetration compared to alpha particles, although they can be stopped by plastic or glass. Thus, when considering the ability of different types of radiation to penetrate materials, alpha particles are distinctly at the lower end of the scale.