Radioactive Decay: Alpha, Beta, and Gamma Radiation Explained

Marie Curie spent years processing tons of pitchblende ore to isolate tiny quantities of a substance that made photographic plates fog and air glow faintly in the dark. She named the phenomenon radioactivity in 1898, the year she and Pierre discovered polonium and radium. The word was new; the process was ancient. Radioactive nuclei have been decaying since the Earth formed, releasing energy stored in nuclear configurations too unstable to persist indefinitely. Understanding why and how they decay required a century of nuclear physics — and the knowledge changed medicine, energy policy, and weapons technology permanently.

Nuclear Stability and the Drive to Decay

An atomic nucleus contains protons and neutrons packed into a volume approximately 10⁻¹⁵ meters across — 100,000 times smaller than the atom itself. Protons repel each other electrostatically; only the strong nuclear force, which acts between all nucleons (protons and neutrons) at extremely short range, holds them together. Stability requires a balance between these competing forces.

For light elements, stable nuclei have approximately equal numbers of protons (Z) and neutrons (N). Iron-56 (⁵⁶Fe) — with Z = 26 and N = 30 — has the highest binding energy per nucleon of any nucleus and is the most stable. For heavier nuclei, extra neutrons are needed to dilute proton repulsion: lead-208 (²⁰⁸Pb) has 82 protons and 126 neutrons. Beyond bismuth (Z = 83), no nucleus is completely stable — all isotopes of elements 84 and above eventually decay.

The binding energy B of a nucleus can be estimated by the Bethe-Weizsäcker semi-empirical mass formula, which includes terms for volume energy, surface energy, Coulomb repulsion, asymmetry energy, and pairing energy. Nuclei with lower binding energy per nucleon than their decay products will spontaneously decay, releasing the energy difference as kinetic energy of the emitted particles and photons.

Alpha Decay

An alpha particle is a helium-4 nucleus: two protons and two neutrons tightly bound. Alpha emission is common in heavy nuclei (Z > 82) because removing an alpha particle reduces both the atomic number (Z) and the neutron number (N) by 2 each, moving the nucleus toward stability.

General form: ^AZ → ^A−4(Z−2) + ⁴He (α)

Example: ²³⁸U → ²³⁴Th + ⁴He (half-life 4.47 billion years)

Alpha particles carry energies of 4–9 MeV. They are highly ionizing — they deposit all their energy in a very short range (~4 cm in air, ~40 μm in soft tissue). A sheet of paper stops them completely. Despite their low penetrating power, alpha emitters inside the body (inhaled or ingested) deliver intense ionizing radiation directly to nearby cells. Polonium-210, the isotope used to poison Alexander Litvinenko in 2006, is an alpha emitter with a half-life of 138 days.

Beta Decay

Beta decay has two variants, both involving the weak nuclear force changing a neutron into a proton or vice versa.

Beta-minus (β⁻) decay: A neutron converts to a proton, emitting an electron and an antineutrino. This occurs in nuclei with too many neutrons relative to protons. Z increases by 1; A is unchanged.

¹⁴C → ¹⁴N + e⁻ + ν̄_e (half-life 5,730 years — the basis of radiocarbon dating)

Beta-plus (β⁺) decay: A proton converts to a neutron, emitting a positron and a neutrino. Occurs in neutron-deficient nuclei. Z decreases by 1; A is unchanged.

¹⁸F → ¹⁸O + e⁺ + ν_e (half-life 110 minutes — used in PET scans)

Electron capture: A proton captures an inner orbital electron (usually K-shell) and converts to a neutron, emitting a neutrino. Produces the same daughter nucleus as β⁺ decay but without a positron. Common when β⁺ is energetically marginal.

Beta particles have a continuous energy spectrum (the neutrino carries variable amounts of the energy), travel up to several meters in air, and are stopped by a few millimeters of aluminum or several centimeters of plastic.

Gamma Decay

Gamma rays are high-energy photons — electromagnetic radiation from nuclear transitions, not electronic transitions. After alpha or beta decay, the daughter nucleus is often left in an excited nuclear energy state. It drops to the ground state by emitting one or more gamma photons, with energies typically in the range 0.01–10 MeV.

Gamma emission does not change Z or A — it changes only the nucleus's internal energy state. Metastable nuclear states — nuclei that persist in excited states long enough to have measurable half-lives — are denoted with the letter m: ^99mTc (technetium-99m) has a half-life of 6.01 hours and decays to ⁹⁹Tc by emitting a 140 keV gamma ray. This makes it ideal for medical imaging — gamma cameras detect the photons from outside the body. Technetium-99m is the most widely used radioisotope in nuclear medicine, used in 30–40 million diagnostic procedures per year worldwide.

Decay Type	Particle Emitted	Change in Z	Change in A	Penetration	Shielding Required
Alpha (α)	4He nucleus	−2	−4	~4 cm air	Paper, skin
Beta-minus (β−)	Electron + antineutrino	+1	0	~1–3 m air; mm-cm in tissue	Plastic, aluminum
Beta-plus (β+)	Positron + neutrino	−1	0	Same as β−; annihilates to 2 × 511 keV γ	Plastic + lead (for annihilation γ)
Gamma (γ)	Photon	0	0	Many meters in air; cm in tissue	Lead, thick concrete
Electron capture	Neutrino only + X-rays	−1	0	Neutrinos undetectable; X-rays: moderate	Light shielding for X-rays

The Half-Life and the Decay Law

Radioactive decay is a quantum mechanical process — any individual nucleus decays at a random time with a fixed probability per unit time (the decay constant λ). The macroscopic result for a large population N(t) is exponential decay:

N(t) = N₀ e^−λt

The half-life t_1/2 = ln(2)/λ is the time for half of any starting quantity to decay. Half-lives span an astonishing range in nature:

• ¹⁰C: t_1/2 = 19.3 seconds (β⁺)
• ¹³¹I: t_1/2 = 8.02 days (β⁻) — used to treat thyroid cancer
• ¹³⁷Cs: t_1/2 = 30.2 years (β⁻) — major contaminant from Chernobyl and Fukushima
• ¹⁴C: t_1/2 = 5,730 years (β⁻) — basis of radiocarbon dating
• ²³⁸U: t_1/2 = 4.47 × 10⁹ years (α) — comparable to Earth's age; still present in abundance
• ²⁰⁹Bi: t_1/2 ≈ 2 × 10¹⁹ years (α) — effectively stable; measured only with sensitive techniques

Decay Chains and Secular Equilibrium

Heavy radioactive isotopes rarely decay to a stable nucleus in one step. Uranium-238 decays through a series of 14 intermediate daughters — including radium, radon, polonium, and lead isotopes — before reaching stable lead-206. Each intermediate has its own half-life and decay mode. When the parent's half-life is much longer than all daughters', the system reaches secular equilibrium: the activity (decays per second) of every daughter equals the activity of the parent.

Radon-222 — a gas produced in the uranium decay chain with a half-life of 3.82 days — accumulates in poorly ventilated basements built over uranium-bearing granite. It is the second leading cause of lung cancer in the United States after cigarette smoking, responsible for approximately 21,000 deaths per year according to the EPA. The radon itself is quickly exhaled; its short-lived daughter isotopes (Po-218, Pb-214, Bi-214, Po-214) attach to dust particles, are inhaled, and deposit alpha and beta radiation directly in lung tissue.

Application	Isotope	Decay Type	Half-life
PET imaging	18F-FDG	β+	110 min
Thyroid cancer treatment	131I	β− + γ	8 days
Nuclear medicine bone scan	99mTc	γ (isomeric transition)	6 hours
Smoke detectors	241Am	α	432 years
RTG spacecraft power (Cassini)	238Pu	α	87.7 years
Archaeological dating	14C	β−	5,730 years

Natural and Artificial Radioactivity

Curie believed radioactivity was an intrinsic atomic property — not induced by external conditions. She was right. Temperature, pressure, chemical state, and ordinary electromagnetic fields have no measurable effect on nuclear decay rates (with exotic exceptions involving extreme electron capture environments). The nucleus is shielded from chemistry by the dense electron cloud.

Artificial radionuclides are produced in nuclear reactors (neutron capture) and particle accelerators (proton bombardment). Glenn Seaborg's group at Berkeley synthesized dozens of transuranic elements from uranium using cyclotron bombardment — work that earned him the 1951 Nobel Prize and eventually extended the periodic table to element 118. The ~3,000 known artificial radionuclides vastly outnumber the ~254 stable isotopes found in nature, and new ones continue to be synthesized for medical, industrial, and fundamental research purposes.