Beta Decay
Beta decay is a type of radioactive decay in which a neutron inside an unstable nucleus converts into a proton (beta-minus decay) or a proton converts into a neutron (beta-plus decay), emitting a beta particle (an electron or positron respectively) together with a neutrino or antineutrino, mediated by the weak nuclear force.
Curious
While alpha decay involves ejecting a whole cluster of nucleons, beta decay works differently: inside the nucleus, an individual neutron transforms directly into a proton (or, less commonly, a proton transforms into a neutron), changing the nucleus's identity without changing its total number of nucleons. This transformation is accompanied by the emission of a beta particle — a high-speed electron in the more common case (beta-minus decay) or a positron, the electron's antimatter counterpart, in the rarer case (beta-plus decay).
Unlike alpha particles, beta particles are much lighter and more penetrating — they can pass through paper but are typically stopped by a few millimetres of aluminium or plastic, placing them between alpha and gamma radiation in penetrating power.
Exploring
| Type | Nuclear change | Emitted particles |
|---|---|---|
| Beta-minus (β−) | n → p, Z increases by 1 | Electron (e−) + antineutrino |
| Beta-plus (β+) | p → n, Z decreases by 1 | Positron (e+) + neutrino |
| Electron capture | p + e− → n, Z decreases by 1 | Neutrino only (competes with β+ decay) |
Worked example: Carbon-14 undergoes beta-minus decay: ¹⁴₆C → ¹⁴₇N + e⁻ + ν̄ₑ. The mass number stays at 14, while the atomic number increases from 6 (carbon) to 7 (nitrogen), consistent with a neutron converting to a proton.
Deep Dive
Beta decay's energy spectrum posed a serious puzzle in the early 20th century: unlike alpha decay (which produces alpha particles with sharply defined, discrete energies), beta particles were observed to emerge with a continuous range of energies, seemingly violating energy conservation. Wolfgang Pauli proposed a radical solution in 1930 — an undetected, nearly massless, electrically neutral particle (which Enrico Fermi later named the "neutrino," Italian for "little neutral one") carrying away the missing energy, momentum, and angular momentum in each decay. The neutrino was not directly experimentally confirmed until 1956, over a quarter-century after Pauli's proposal, by Clyde Cowan and Frederick Reines.
The weak force: Beta decay is mediated by the weak nuclear force, specifically via the exchange of a W boson — a neutron (composed of one up and two down quarks) converts to a proton (two up and one down quark) when one of its down quarks transforms into an up quark, emitting a virtual W⁻ boson that promptly decays into an electron and antineutrino. This quark-level description, developed as part of the Standard Model decades after beta decay was first observed, provides the modern, fundamental explanation for a phenomenon first studied purely empirically.
Fermi's 1933–34 theory of beta decay, though later superseded by the full electroweak theory, was remarkably successful at explaining beta decay's observed properties and represented one of the first attempts to describe a fundamental interaction (the weak force) using quantum field theory methods, directly influencing the later development of the full Standard Model.
Concept Relationship Map
Requires: radioactive decay, weak force [PENDING FULL ENTRY]
Enables: neutrino oscillations [PENDING FULL ENTRY]
Commonly confused with: assuming beta decay changes the mass number (it doesn't — only alpha decay changes both A and Z)
Sources
- Fermi, E. (1934). Il Nuovo Cimento, 11(1), 1–19.
- Cowan, C.L., Reines, F., et al. (1956). "Detection of the Free Neutrino: A Confirmation." Science, 124(3212), 103–104.
- Krane, K.S. (1987). Introductory Nuclear Physics. Wiley, Ch. 9.