alpha particle 
noun Physics
A helium nucleus emitted by some radioactive substances, originally regarded as a ray.Because the alpha particle (the helium nucleus) with two protons and two neutrons is especially cohesive, the release of nuclear material is often in this form....
- The alpha particle is emitted by certain radioactive elements as they decay to a stable element.
- A beta particle is an electron that could make it just a little way into the skin, while an alpha particle is a charged helium nucleus that will bounce right off of you.