Alpha Particle
Definition
A heavy, slow chunk shot out of decaying nuclei: 2 protons and 2 neutrons, identical to a helium nucleus. Stopped by paper, but dangerous inside the body.
Symbol: ·
Technical definition
An alpha particle is a helium-4 nucleus (2 protons + 2 neutrons) emitted in alpha decay, carrying +2 charge and roughly 4-9 MeV of energy. Its high mass gives it strong ionizing power but very short range: stopped by skin or paper, though highly damaging if the emitter is ingested.
Example
Uranium-238 decays to thorium-234 by emitting an alpha particle: the first step in its long decay chain.
Common confusion
Alpha particles are the LEAST penetrating radiation but among the most ionizing per distance. Stopped by paper does not mean harmless: inhaled alpha emitters are serious hazards.