alpha particle

Positively charged (2+), high-energy particle emitted from the nucleus of a radioactive atom. It is one of the products of the spontaneous disintegration of radioactive elements (see radioactivity) such as radium and thorium, and is identical to the nucleus of a helium atom (⁴He) - that is, it consists of two protons and two neutrons. The process of emission, alpha decay, transforms one element into another, decreasing the atomic number by two and the atomic mass by four. Plutonium-239 (²³⁹Pu) is an example of a material that emits alpha particles.

Because of their large mass, alpha particles have a short range of only a few centimetres in air. They have a low penetrating power and can be stopped by a sheet of paper or aluminium. They have a strongly ionizing effect (see ionizing radiation) on the molecules that they strike, and are therefore capable of damaging living cells. Alpha particles are deflected by magnetic and electric fields.