Alpha Decay
Alpha decay is a type of radioactive decay in which an unstable, typically heavy nucleus spontaneously emits an alpha particle — a helium-4 nucleus, consisting of 2 protons and 2 neutrons — reducing the parent nucleus's atomic number by 2 and mass number by 4, via a quantum tunnelling process through the nuclear potential barrier.
Curious
Heavy nuclei, like uranium or radium, contain a lot of protons packed closely together, all mutually repelling each other electrically while the strong force tries to hold everything together. One way such a nucleus can relieve this strain is by ejecting a small, especially stable cluster of 2 protons and 2 neutrons — identical to a helium-4 nucleus — known as an alpha particle. This transforms the original nucleus into a different, lighter element, two places earlier in the periodic table.
Alpha particles are relatively large and heavy compared to other forms of radiation, and carry a double positive charge, which means they interact strongly with matter and lose their energy quickly — a sheet of paper or even a few centimetres of air is enough to stop most alpha particles entirely, making them the least penetrating of the three classic types of radioactivity, despite carrying substantial energy.
Exploring
| Symbol | Name | Change |
|---|---|---|
| Z | Atomic number of parent | Decreases by 2 |
| A | Mass number of parent | Decreases by 4 |
| ₄₂He | Emitted alpha particle | 2 protons, 2 neutrons |
Worked example: Uranium-238 undergoes alpha decay: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He. The mass number drops from 238 to 234 (238 − 4), and the atomic number drops from 92 (uranium) to 90 (thorium), consistent with the general alpha decay pattern.
Deep Dive
George Gamow's 1928 theory of alpha decay (developed independently around the same time by Ronald Gurney and Edward Condon) was one of the first triumphant applications of quantum mechanics to nuclear physics, and remains a textbook example of quantum tunnelling. Classically, the alpha particle inside the nucleus lacks sufficient energy to overcome the combined strong-force-and-Coulomb potential barrier confining it; quantum mechanically, its wavefunction has a small but nonzero amplitude beyond the barrier, giving it a small probability per unit time of "tunnelling" through and escaping.
The Geiger-Nuttall law: Empirically observed well before Gamow's theoretical explanation, the Geiger-Nuttall law (1911) notes a strong correlation between an alpha emitter's decay energy and its half-life — higher-energy alpha particles correspond to dramatically shorter half-lives, because a higher-energy alpha particle faces an effectively thinner, lower barrier to tunnel through, exponentially increasing the tunnelling probability. This explains why observed alpha-decay half-lives span an astonishing range — from microseconds to over 10¹⁰ years — despite the emitted particles' energies varying by only a relatively modest factor.
Alpha decay releases significant kinetic energy, calculated from the Q-value (the mass difference between the parent nucleus and the combined daughter nucleus plus alpha particle, converted via E = mc²) — this energy is shared between the alpha particle and the recoiling daughter nucleus in inverse proportion to their masses, meaning the much lighter alpha particle carries away the large majority of the released energy.
Concept Relationship Map
Requires: radioactive decay, quantum tunnelling
Enables: Q-value [PENDING FULL ENTRY], radiation shielding applications
Commonly confused with: assuming alpha particles are always more dangerous than other radiation types (their danger depends heavily on whether exposure is internal or external)
Sources
- Gamow, G. (1928). Zeitschrift für Physik, 51(3–4), 204–212.
- Geiger, H., Nuttall, J.M. (1911). "The Ranges of the α Particles from Various Radioactive Substances." Philosophical Magazine, 22(130), 613–621.
- Krane, K.S. (1987). Introductory Nuclear Physics. Wiley, Ch. 8.