Which radiation type is least penetrating?

Alpha radiation is the least penetrating type of radiation. This is mainly due to the nature of alpha particles, which consist of two protons and two neutrons, making them relatively heavy and positively charged. Because of their size and charge, alpha particles can only travel a short distance in air—typically just a few centimeters—and can be stopped by a sheet of paper or even the outer layer of human skin.

In contrast, beta particles, which are high-speed electrons or positrons, have greater penetration ability and can pass through paper but are stopped by materials like plastic or glass. Gamma rays, being electromagnetic radiation, have even greater penetration capabilities and can pass through the human body and many dense materials, requiring dense shielding like lead or several centimeters of concrete to reduce their intensity. Neutrons, which are neutral particles, can penetrate materials fairly well as well, often requiring substances rich in hydrogen for effective moderation and absorption.

Therefore, alpha radiation's limited penetration power highlights its distinction as the least penetrating type of radiation when compared to beta radiation, gamma rays, and neutrons.