Alpha decay is a radioactive process in which a particle with two neutrons
and two protons is ejected from the nucleus of a radioactive atom. The particle
is identical to the nucleus of a helium atom.
Alpha decay only occurs in very heavy elements such as uranium, thorium
and radium. The nuclei of these atoms are very "neutron rich"
(i.e. have a lot more neutrons in their nucleus than they do protons) which
makes emission of the alpha particle possible.
After an atom ejects an alpha particle, a new parent atom is formed which
has two less neutrons and two less protons. Thus, when Uranium-238 (which
has a Z of 92) decays by alpha emission, Thorium-234 is created (which has
a Z of 90).
Because alpha particles contain two protons, they have a positive charge
of two. Further, alpha particles are very heavy and very energetic compared
to other common types of radiation. These characteristics allow alpha particles
to interact readily with materials they encounter, including air, causing
many ionizations in a very short distance. Typical alpha particles will
travel no more than a few centimeters in air and are stopped by a sheet