beta decay

A naturally occurring radioactive decay process in which a neutron in an atomic nucleus spontaneously breaks up into a proton, which remains in the nucleus, and an electron (beta particle), which is emitted. The process is always accompanied by the emission of an antineutrino, and is governed by the weak nuclear force. Strontium-90, for example, is a beta emitter with a half-life of 28·1 years.

Nuclear processes
Radioactive decay processes Alpha decay Beta decay Cluster decay Double beta decay Double electron capture Electron capture Gamma radiation Internal conversion Isomeric transition Neutron emission Positron emission Proton emission Spontaneous fission Nucleosynthesis Neutron Capture The R-process The S-process Proton capture: The P-process The Rp-process Spallation

In nuclear physics, beta decay is a type of radioactive decay in which a beta particle (an electron or a positron) is emitted.

In β− decay, the weak interaction converts a neutron into a proton while emitting an electron and an anti-neutrino:

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In β+ decay, a proton is converted into a neutron, a positron and a neutrino:

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So, unlike beta minus decay, beta plus decay cannot occur in isolation, because the mass of the neutron alone is greater than the mass of the proton.

In all the cases where β+ decay is allowed energetically (and the proton is a part of a nucleus with electron shells), it is accompanied by the electron capture process, when an atomic electron is captured by a nucleus with emission of neutrino:

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If the proton and neutron are part of an atomic nucleus, these decay processes transmute one chemical element into another.

Historically, the study of beta decay provided the first physical evidence of the neutrino. In 1911 Lise Meitner and Otto Hahn performed an experiment that showed that the energies of electrons emitted by beta decay had a continuous rather than discrete spectrum. In 1931 Enrico Fermi renamed Pauli's "neutron" to neutrino, and in 1934 Fermi published a very successful model of beta decay in which neutrinos were produced.

Beta decay does not change the number of nucleons A in the nucleus but changes only its charge Z. Among them, several nuclides (at least one) are beta stable, because they present local minima of the mass excess: if such a nucleus has (A, Z) numbers, the neighbour nuclei (A, Z−1) and (A, Z+1) have higher mass excess and can beta decay into (A, Z), but not vice versa. It should be noted, that a beta-stable nucleus may undergo other kinds of radioactive decay (alpha decay, for example).

Some nuclei can undergo double beta decay (ββ decay) where the charge of the nucleus changes by two units. In most practically interesting cases, single beta decay is energetically forbidden for such nuclei, because when β and ββ decays are both allowed, the probability of β decay is (usually) much higher, preventing investigations of very rare ββ decays. Like single beta decay, double beta decay does not change A; thus, at least one of the nuclides with some given A has to be stable with regard to both single and double beta decay.

Beta decay can be considered as a perturbation as described in quantum mechanics, and thus follows Fermi's Golden Rule.