Nuclear decay

Nuclear decay is a spontaneous change in an unstable atomic nucleus into a daughter nucleus, usually with the release of energy. A nucleus is unstable when its combination of protons and neutrons does not provide a sufficiently favorable balance of nuclear attraction, electrostatic repulsion, and quantum structure. The daughter may be a different element, because changing the proton number changes the element's identity. Radioactivity, the observable phenomenon associated with such transformations, was recognized in the late nineteenth century through the work of Henri Becquerel and Marie and Pierre Curie.¹

Individual decay events are intrinsically unpredictable, but large populations follow precise statistical laws. The activity of a sample is the number of decays per unit time and decreases exponentially. Its half-life is the time required for half of a sufficiently large population of parent nuclei to decay; it is not the time at which every nucleus has disappeared. Temperature, pressure, chemical bonding, and physical state generally have negligible effects on ordinary nuclear decay, distinguishing it from most chemical reactions.

Decay conserves electric charge, energy, momentum, and angular momentum, while also obeying nuclear selection rules and conservation laws associated with particles. The emitted energy is shared among radiation, the recoiling daughter nucleus, and sometimes an escaping neutrino.

The principal decay modes differ in which particles leave the nucleus and how the nucleus changes. In alpha decay, a helium-4 nucleus containing two protons and two neutrons is emitted; the parent loses two units of atomic number and four units of mass number. It is common among very heavy nuclides, including some isotopes of uranium and radium. Alpha particles travel only a short distance through matter but can be hazardous when alpha-emitting material is inhaled or swallowed.

In beta decay, the weak interaction converts a neutron into a proton or a proton into a neutron. Beta-minus decay emits an electron and an electron antineutrino; beta-plus decay emits a positron and an electron neutrino. Electron capture is a related process in which the nucleus absorbs an inner atomic electron and emits a neutrino. These transformations change atomic number by one while leaving mass number unchanged.²

Gamma decay releases excess nuclear energy as a high-energy photon, commonly after alpha or beta decay has left the daughter nucleus excited. Gamma emission changes neither atomic number nor mass number. Some excited nuclear states instead persist as metastable isomers before decaying, producing delayed gamma radiation.

Nuclear decay is measured through activity, energy spectra, and elapsed time, with the becquerel representing one decay per second and the curie representing a much larger historical unit. Detectors such as Geiger–Müller tubes, scintillators, semiconductor spectrometers, and proportional counters register radiation, but each has a different efficiency and energy resolution. Half-lives range from fractions of a second to times vastly exceeding the age of human civilization, so an isotope's activity and radiation type must be considered together.

Radioactive decay supports radiometric dating, medical diagnosis, cancer treatment, industrial gauging, sterilization, and scientific tracing. Carbon-14 dating estimates the age of once-living material through the changing ratio of carbon isotopes, whereas potassium–argon and uranium–lead systems apply to older geological materials. In medicine, technetium-99m is widely used for diagnostic imaging, while iodine-131 can both image and treat some thyroid disorders. Nuclear power relies primarily on induced fission rather than spontaneous decay, although radioactive decay supplies heat in radioisotope power systems and contributes to the residual heat of reactor fuel.³

Rare decay channels reveal details of nuclear structure that common decay modes conceal. Some nuclei undergo double beta decay, in which two neutrons transform into two protons; the ordinary two-neutrino form has been observed, while the hypothetical neutrinoless form would violate lepton-number conservation and would show that neutrinos have additional properties beyond the simplest theory.⁴

Quantum tunneling explains why alpha particles can escape a classically confining nuclear potential, and it helps produce the strong relationship between alpha-decay half-life and emission energy known as the Geiger–Nuttall law. Decay chains create families of successive daughter nuclides: uranium-238, for example, ultimately produces stable lead-206 through alpha and beta transformations. Intermediate daughters can be more radiologically significant than the original parent because they may have shorter half-lives or emit penetrating radiation.

Environmental and biological risk depends on dose, radiation quality, route of exposure, and tissue sensitivity, not simply on a material's activity. Shielding is therefore mode-specific: paper can stop many alpha particles, while dense materials and substantial thickness are needed for significant gamma attenuation. Radioactive decay cannot be switched off, but containment, distance, shielding, monitoring, and careful control of radioactive sources reduce exposure.

Activity

The rate at which radioactive nuclei decay, measured in becquerels.

Half-life

The time required for half the nuclei in a statistical population to decay.

Nuclide

A species of atom defined by a particular number of protons and neutrons.

Isotope

One of several nuclides of the same element, sharing proton number but differing in neutron number.

Radioactive chain

A sequence in which a daughter nuclide produced by decay undergoes further decay.