Alpha decay

Alpha decay is a form of radioactive decay in which an unstable atomic nucleus emits an alpha particle, a tightly bound cluster consisting of two protons and two neutrons. This process transforms the parent nucleus into a daughter nucleus with its mass number reduced by four and its atomic number reduced by two. Because an alpha particle is identical to a helium-4 nucleus, the decay effectively represents the spontaneous emission of a helium nucleus from within a larger, less stable atom.

Characteristics and Occurrence

Alpha decay occurs predominantly in very heavy nuclides whose binding energy per nucleon has decreased from the maximum found near iron and nickel. As atomic numbers increase, the electromagnetic repulsion between protons grows more strongly than the short-range nuclear force that holds nuclei together. Nuclei with more than about 210 nucleons can therefore gain stability by ejecting an alpha particle, reducing both size and repulsive stress. Although theoretically possible in nuclei only slightly heavier than nickel, alpha decay is in practice observed almost entirely in heavy and very heavy elements. The lightest known alpha emitter is a rare isotope of antimony, while beryllium-8 provides a unique case in which the nucleus disintegrates into two alpha particles.
Alpha particles possess typical kinetic energies of around 5 MeV, corresponding to about five per cent of the speed of light. Their large mass and double positive charge result in rapid energy loss through interactions with matter, limiting their penetration to only a few centimetres of air or a thin layer of solid material. Helium generated by alpha decay of underground uranium- and thorium-bearing minerals accounts for nearly all naturally occurring helium on Earth, later recovered as a by-product during natural gas extraction.

Historical Development

Ernest Rutherford identified alpha particles as a distinct component of radioactive emissions in 1899. By 1907, experimental evidence confirmed that they were helium ions. A major breakthrough followed in 1928 when George Gamow, and independently Ronald Gurney and Edward Condon, applied the newly formulated principles of quantum mechanics to explain how alpha particles escape the nucleus. Their work connected alpha decay with quantum tunnelling and provided a theoretical foundation for the empirically known Geiger–Nuttall relationship linking alpha-particle energy to the half-life of the decay.

Mechanism of Decay

Within an atomic nucleus, the strong nuclear force binds protons and neutrons with great intensity, but its range is extremely short, dropping rapidly beyond a few femtometres. The electromagnetic repulsion between protons, by contrast, has unlimited range. In large nuclei the cumulative repulsive force becomes so great that the strong force barely maintains cohesion.
Although several types of particle emission are theoretically possible, the alpha particle is the preferred cluster because of its unusually high binding energy. Its mass is less than the combined mass of two free protons and two free neutrons, so emission of an alpha particle releases substantial energy. Single-proton or single-neutron emissions typically require additional energy and are therefore far less common.
The total disintegration energy available in an alpha decay is determined by the mass difference between the parent nucleus, the daughter nucleus, and the alpha particle. Most of this energy appears as the kinetic energy of the alpha particle, with only a small fraction imparted to the daughter nucleus as recoil. Even this modest recoil energy is large compared with chemical bond energies, ensuring that the daughter atom is displaced from any molecular environment the parent atom previously occupied.
Alpha spectra, defined by the energies and relative intensities of emitted alpha particles, provide valuable diagnostic information and allow the identification of specific radioisotopes.

Quantum Tunnelling

Classically, an alpha particle is confined within a nuclear potential well whose outer boundary forms a steep electromagnetic barrier, often more than 25 MeV in height. Because emitted alpha particles possess energies far below the barrier height—typically between 4 and 9 MeV—classical physics would forbid their escape.
Quantum mechanics, however, allows particles to tunnel through potential barriers with finite probability. The alpha particle continually collides with the inner face of the barrier as it moves within the nucleus. At each encounter there exists a small but nonzero probability of tunnelling through the barrier and escaping. Given that the alpha particle undergoes an extremely high number of such collisions per second, even an exceedingly small tunnelling probability can produce measurable decay rates, leading to the wide range of half-lives observed among alpha-emitting nuclides.
This quantum tunnelling framework successfully explains the Geiger–Nuttall law, which shows that nuclides emitting more energetic alpha particles decay more rapidly. It also clarified that alpha emission is a natural consequence of the laws of quantum mechanics, not requiring any special instability beyond the intrinsic balance of nuclear forces.

Nuclear Stability and Special Cases

Alpha decay is prominent among nuclides with mass numbers above about 209, many of which are long-lived primordial isotopes. Certain isotopes that are stable with respect to beta decay—and in some cases double beta decay—are predicted to undergo alpha decay with extremely long half-lives, though such decays have not always been observed experimentally.
Mass numbers 5 and 8 represent special cases: nuclides at these masses are intrinsically unstable and break apart almost instantly. Helium-5, lithium-5, and beryllium-8 exist only for extremely brief periods before decaying to lighter configurations, including alpha particles.

Significance and Applications

Understanding alpha decay is fundamental to nuclear physics, radiometric dating, radiation protection, and the study of nuclear structure. Alpha emitters are used in smoke detectors, static eliminators, industrial gauges, and specialised medical therapies. Because alpha radiation is easily absorbed, it poses minimal hazard outside the body but can be dangerous if alpha-emitting materials are inhaled or ingested.