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Radioactive decay processes
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In nuclear physics, beta decay (sometimes called neutron decay) is a type of radioactive decay in which a beta particle (an electron or a positron) is emitted. In the case of electron emission, it is referred to as "beta minus" (β−); in the case of a positron, "beta plus" (β+).
In β− decay, the weak nuclear force converts a neutron into a proton while emitting an electron and an anti- neutrino:
In β+ decay, a proton is converted into a neutron, a positron and a neutrino:
If the proton and neutron are part of an atomic nucleus, these decay processes transmute one chemical element into another. For example:
Historically, the study of beta decay provided the first physical evidence of the neutrino. The energies of electrons emitted by beta decay were observed to be non-discrete (some being more energetic than others). A problem arose in trying to explain what happened to the missing energy if an electron was emitted with less than maximum energy — the law of conservation of energy appeared to be violated. To solve this, Wolfgang Pauli proposed that the "missing" energy was actually carried away by another yet undiscovered particle — the neutrino. This was analysed in more detail by Enrico Fermi.
The Beta decay can be considered as a perturbation as described in quantum mechanics, and thus follow Fermi's Golden Rule.