How Fast Is Alpha Particle?

Alpha particles — helium nuclei from radioactive decay — travel at 15,000 km/s, 5% the speed of light. Despite that speed, they can be stopped completely by a single sheet of paper; their large size and double charge means they interact with everything they meet.

Alpha particles are identical to helium nuclei — making radioactive decay a slow but constant source of natural helium gas in the Earth's crust.

• Stopped by a sheet of paper
• Helium nucleus stripped of electrons