Alpha ray

One kind of rays emitted by radioactive substances. Alpha particles are really nuclei of Helium at high energies (typically some MeV).

As long as they approach the body from the exterior, they are quite harmless to humans, because their interaction with matter is so intense that they are usually absorbed in the dead cells of the outer skin. Alpha rays emitted within the body, on the other hand, are dangerous.

Nuclei emitting alpha rays are usually very heavy, like Uranium or Plutonium.