alphadecay

Alpha decay, sometimes written alphadecay, is a type of radioactive decay in which an unstable atomic nucleus emits an alpha particle, consisting of two protons and two neutrons (a helium-4 nucleus). This emission reduces the atomic number by 2 and the mass number by 4, transforming the parent into a lighter element (the daughter). For example, uranium-238 decays via alpha emission to thorium-234: 238U → 234Th + 4He. The process conserves nucleon number but changes Z.

Mechanism and energetics: The alpha particle pre-exists as a cluster within the nucleus. Emission occurs via quantum tunneling through the Coulomb barrier between the charged nucleus and the escaping particle. The energy released (the Q-value) equals the mass difference between parent and daughter (including the alpha particle) and is carried by the alpha particle, often with gamma rays if the daughter is left in an excited state. Alpha-decay energies are typically in the range of about 4 to 9 MeV.

Characteristics and detection: Alpha decay is common among heavy nuclei with high atomic numbers, particularly in the actinides and near the lead region. The decay rate spans many orders of magnitude and is often described by the Geiger–Nuttall law, which relates shorter half-lives to higher alpha energies. Alpha particles have high linear energy transfer but very limited penetration in matter; they travel only a few centimeters in air and micrometers in solids, making external shielding less effective than containment against ingestion or inhalation. Detection requires appropriate in-situ or surface detectors that can register the charged particles.

Applications and implications: Alpha decay is key to radiometric dating (e.g., uranium–lead dating) and to studying decay chains in nuclear physics and reactor physics. Safety considerations emphasize containment to prevent inhalation or ingestion of alpha-emitting materials.