Alpha particles and electrons (deflected by a magnetic field) from a thorium rod in a cloud chamberNuclear decay (Radioactive decay) occurs when an unstable atom losses energy by emitting ionizing radiation. Radioactive decay is a random process at the level of single atoms, in that, according to quantum theory, it is impossible to predict when a particular atom will decay. Alpha decay is one of many types of radioactive decay.

Alpha decay is a type of radioactive decay in which a alpha radiation (alpha particle) is emitted from an atomic nucleus. Alpha radiation consist of alpha particles, that are energetic nuclei of helium. The production of alpha particles is termed alpha decay. Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. Alpha particles are relatively large and carry a double positive charge.

Key characteristics of alpha particles are summarized in few following points:

  • Alpha particles are energetic nuclei of heliumand they are relatively heavy and carry a double positive charge.
  • Typical alpha particle have kinetic energy about 5 MeV. This is due to the nature of alpha decay.
  • Pure alpha decay is very rare, alpha decay is frequently accompanied by gamma radiation.
  • Alpha particles interact with matter primarily through coulomb forces (ionization and excitation of matter) between their positive charge and the negative charge of the electrons from atomic orbitals.
  • Alpha particles heavily ionize matter and they quickly lose their kinetic energy. Therefore alpha particles have very short ranges. On the other hand they deposit all their energies along their short paths.
  • For example, the ranges of a 5 MeV alpha particle (most have such initial energy) are approximately only 0,002 cm in aluminium alloy or approximately 3.5 cm in air.
  • The stopping power is well described by the Bethe formula.
  • The Bragg curve is typical for alpha particles and for other heavy charged particles and describes energy loss of ionizing radiation during travel through matter.

Shielding of Alpha and Beta RadiationThe stopping power of most materials is very high for alpha particles and for heavy charged particles. Therefore alpha particles have very short ranges. For example, the ranges of a 5 MeV alpha particle (most have such initial energy) are approximately only 0,002 cm in aluminium alloy or approximately 3.5 cm in air. Most alpha particles can be stopped by a thin piece of paper. Even the dead cells in the outer layer of human skin provides adequate shielding because alpha particles can’t penetrate it. 

See also: Interaction of Heavy Charged Particles with Matter