Definition of Alpha Decay

This photo shows the alpha particles emitted by alpha decay from a particle of radium salt.

Alpha decay is the spontaneous radioactive decay where an alpha particle is produced. An alpha particle is essentially a helium nucleus or He2+ ion. Although alpha decay presents a significant radiation risk if the radioactive source is inhaled or ingested, alpha particles are too large to penetrate very far through the skin or other solids and require minimal radiation shielding. A sheet of paper, for example, blocks alpha particles.

An atom that undergoes alpha decay will reduce its atomic mass by 4 and become the element two atomic numbers less. The general reaction to alpha decay is

ZXAZ-4YA-2 + 4He2

where X is the parent atom, Y is the daughter atom, Z is the atomic mass of X, A is the atomic number of X.

Examples: 238U92 decays by alpha decay into 234Th90.