Which type of radiation is least penetrating?

Alpha rays are indeed the least penetrating type of radiation. This is primarily due to their composition; alpha particles consist of two protons and two neutrons, which gives them a relatively large mass and charge. Because of these characteristics, alpha particles interact strongly with matter.

When alpha particles encounter matter, they quickly lose their energy through interactions with electrons and nuclei. This results in a limited ability to penetrate materials. In fact, alpha radiation can be stopped by just a sheet of paper or the outer layer of human skin. This lack of penetrating ability is a significant aspect of alpha radiation; while it can cause significant damage if ingested or inhaled, it poses minimal risk when it comes to external exposure.

In contrast, other types of radiation, such as beta rays, gamma rays, and X-rays, possess greater penetrating power. Beta particles can penetrate more deeply than alpha particles but are still significantly less penetrating than gamma rays and X-rays, which can pass through many materials, including human tissue, making them more hazardous in terms of external exposure.