Double electron capture is a

decay mode Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

of an atomic nucleus. For a nuclide (''A'', ''Z'') with a number of

nucleon In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number (nucleon number). Until the 1960s, nucleons were ...

s ''A'' and

atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...

''Z'', double electron capture is only possible if the mass of the nuclide (''A'', ''Z''−2) is lower. In this mode of decay, two of the orbital electrons are captured via the

weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction ...

by two protons in the nucleus, forming two neutrons (Two neutrinos are emitted in the process). Since the protons are changed to neutrons, the number of neutrons increases by two, while the number of protons ''Z'' decreases by two, and the atomic mass number ''A'' remains unchanged. As a result, by reducing the atomic number by two, double electron capture transforms the

nuclide A nuclide (or nucleide, from nucleus, also known as nuclear species) is a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state. The word ''nuclide'' was coined by Truman ...

into a different element. Example: :

Rarity

In most cases this decay mode is masked by other, more probable modes involving fewer particles, such as single electron capture. When all other modes are “forbidden” (strongly suppressed) double electron capture becomes the main mode of decay. There exist 34 naturally occurring nuclei that are believed to undergo double electron capture, but the process has been confirmed by observation in the decay of only three nuclides: , , and . One reason is that the probability of double electron capture is stupendously small; the

half-lives Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...

for this mode lie well above 10 years. A second reason is that the only detectable particles created in this process are

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30 ...

s and

Auger electron The Auger effect or Auger−Meitner effect is a physical phenomenon in which the filling of an inner-shell vacancy of an atom is accompanied by the emission of an electron from the same atom. When a core electron is removed, leaving a vacancy, a ...

s that are emitted by the excited atomic shell. In the range of their energies (~1–10 keV), the background is usually high. Thus, the experimental detection of double electron capture is more difficult than that for

double beta decay In nuclear physics, double beta decay is a type of radioactive decay in which two neutrons are simultaneously transformed into two protons, or vice versa, inside an atomic nucleus. As in single beta decay, this process allows the atom to move clos ...

. Double electron capture can be accompanied by the excitation of the daughter nucleus. Its de-excitation, in turn, is accompanied by an emission of photons with energies of hundreds of keV.

Modes with positron emission

If the mass difference between the mother and daughter atoms is more than two masses of an electron (1.022

MeV In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacuum. ...

), the energy released in the process is enough to allow another mode of decay, called electron capture with positron emission. It occurs along with double electron capture, their

branching ratio In particle physics and nuclear physics, the branching fraction (or branching ratio) for a decay is the fraction of particles which decay by an individual decay mode or with respect to the total number of particles which decay. It applies to eithe ...

depending on nuclear properties. When the mass difference is more than four electron masses (2.044 MeV), the third mode, called double positron decay, is allowed. Only six naturally occurring nuclides can decay via these three modes simultaneously.

Neutrinoless double electron capture

The above-described process with the capture of two electrons and emission of two neutrinos (two-neutrino double electron capture) is allowed by the Standard Model of particle physics: No conservation laws (including

lepton number In particle physics, lepton number (historically also called lepton charge) is a conserved quantum number representing the difference between the number of leptons and the number of antileptons in an elementary particle reaction. Lepton number ...

conservation) are violated. However, if the lepton number is not conserved, or the neutrino is its own antiparticle, another kind of process can occur: the so-called neutrinoless double electron capture. In this case, two electrons are captured by nucleus, but neutrinos are not emitted. The energy released in this process is carried away by an internal

bremsstrahlung ''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...

gamma quantum. Example: : This mode of decay has never been observed experimentally, and would contradict the Standard Model if it were observed.

References

External links

* {{Nuclear_processes Nuclear physics Radioactivity

Rarity

Modes with positron emission

Neutrinoless double electron capture

See also

References

External links