Beta decay (β-decay), the spontaneous transformations of a neutron n into a proton p and of a proton into a neutron inside an atomic nucleus (and also the transformation of a free neutron into a proton), accompanied by the emission of an electron e- or a positron e+ and of an electron antineutrino ~ve or neutrino ve. Two kinds of beta decay are known:
1) β-decay: n→p+e- +~ve, in which a nucleus is formed with a number of protons Z one greater than that of the initial nucleus, for example:
1) β-decay: n→p+e- +~ve, in which a nucleus is formed with a number of protons Z one greater than that of the initial nucleus, for example:
The simplest example of β- -decay is the decay of a free neutron.
2) Positron beta decay (β+ -decay): p→n+e++ve, in which a nucleus is formed with Z one less than that of the initial nucleus, for example:
2) Positron beta decay (β+ -decay): p→n+e++ve, in which a nucleus is formed with Z one less than that of the initial nucleus, for example:
Beta decay also includes the process in which the nucleus of an atom absorbs an electron with the emission of ve (electron capture). In electron capture, as in positron beta decay, one of the protons of the nucleus turns into a neutron: p+e-→n+ve, and the number of protons Z decreases by one, for example:
Related to beta decay are the processes of interaction of neutrinos and antineutrinos with nuclei:
(A — the mass number of the nuclei X).
Beta decay is due to weak interactions. The half-lives T1/2 of β-active nuclei vary from 10-2 s to 1018 years.
Beta decay is observed both in heavy and in light nuclei. The stability of nuclei depends on the ratio of the numbers of protons Z and neutrons N. As Z grows, the energy of the Coulomb repulsion of the protons increases. For this reason in medium and heavy stable nuclei the value of (N—Z) >0 (see Atomic nucleus). Nuclei in which N is greater than is required for their stability are radioactive and may undergo β--decay; nuclei in which N is too small may undergo β+-decay or electron capture. The total energy Εtot released in beta decay is distributed mainly between two particles, for example between e- and ve. A very small fraction of it (-Ε2tot/Mc2, where M is the mass of the nucleus) is carried away by the residual nucleus, which experiences a “recoil” in beta decay. The distribution of the emitted electrons in energy N (Ε) is called the β-spectrum. The general properties of β-spectra — continuity and the presence of a maximum energy Εmax - the upper boundary of the β-spectrum. It was precisely on the basis of these properties of β-spectra that the Swiss physicist W. Pauli predicted the existence of the neutrino in 1930.
The form of the β-spectrum may depend on the states of the initial and of the resulting nuclei (spin, parity and others).
At low energies of the emitted charged particle (e- or e+ ) the form of the β-spectrum is distorted by the influence of the Coulomb interaction between the nucleus and the electron or positron. Beta decay often takes place not only to the ground level but also to excited levels of the final nucleus. If the decay proceeds to several levels, the β-spectrum acquires a complex form.
The theory of beta decay was created in 1934 by the Italian physicist E. Fermi by analogy with electrodynamics, where the emission and absorption of photons is regarded as the result of the interaction of a charge with the electromagnetic field it creates itself (photons arise at the moment of emission). The process of beta decay is regarded as the result of the interaction of a nucleon with the electron-neutrino field: the nucleon passes into another state, emitting e- or e+ and ~ve or ve.