Beta decay
In nuclear physics, beta decay is a type of
radioactive decay in which a
beta particle is emitted. In the case of electron emission, it is referred to as "beta minus" , while in the case of a positron emission as "beta plus" .
In β− decay, the
weak interaction converts a
neutron into a
proton while emitting an electron and an anti-neutrino:
At the fundamental level , this is due to the conversion of a down quark to an up quark by emission of a
W boson.
In β+ decay, a proton is converted into a neutron, a
positron and a
neutrino:
Encyclopedia
In nuclear physics,
beta decay is a type of
radioactive decay in which a
beta particle is emitted. In the case of electron emission, it is referred to as "beta minus" , while in the case of a positron emission as "beta plus" .
In β
− decay, the
weak interaction converts a
neutron into a
proton while emitting an electron and an anti-neutrino:
- .
At the fundamental level , this is due to the conversion of a down quark to an up quark by emission of a
W boson.
In β
+ decay, a proton is converted into a neutron, a
positron and a
neutrino:
- .
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.
Beta plus decay can only happen inside nuclei when the absolute value of the binding energy of the daughter nucleus is higher than that of the mother nucleus. The difference between these energies goes into the reaction of converting a proton into a neutron, a positron and a neutrino and into the kinetic energy of these particles.
In all the cases where β
+ decay is allowed energetically , it is accompanied by the electron capture process, when an atomic electron is captured by a nucleus with emission of neutrino:
- .
But if the energy difference between initial and final states is low, the electron capture can occur without being accompanied by positron emission.
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. 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. This was in apparent contradiction to the
law of conservation of energy, as it appeared that energy was lost in the beta decay process. A second problem was that the spin of the Nitrogen-14 atom was 1, in contradiction to the
Rutherford prediction of ½. In a famous letter written in 1930
Wolfgang Pauli suggested that in addition to electrons and protons atoms also contained an extremely light neutral particle which he called the neutron. He suggested that this "neutron" was also emitted during beta decay and had simply not yet been observed. 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. Thus the set of all nuclides with the same
A can be introduced; these
isobaric nuclides may turn into each other via beta decay. Among them, several nuclides are beta stable, because they present local minima of the mass excess: if such a nucleus has numbers, the neighbour nuclei and have higher mass excess and can beta decay into , but not vice versa. It should be noted, that a beta-stable nucleus may undergo other kinds of radioactive decay . In nature, most isotopes are beta stable, but a few exceptions exist with half-lives so long that they have not had enough time to decay since the moment of their
nucleosynthesis. One example is
40K, which undergoes all three types of beta decay with half life of 1.277×10
9 years.
Some nuclei can undergo
double beta 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 much higher, preventing investigations of very rare ββ decays. Thus, ββ decay is usually studied only for beta stable nuclei. 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.
See also
