Double electron capture
Encyclopedia
Double electron capture is a decay mode of atomic nucleus
. For a nuclide (A, Z) with number of nucleon
s A and atomic number
Z, double electron capture is only possible if the mass of the nuclide of (A, Z-2) is lower.
In this mode of decay, two of the orbital electron
s are captured by two proton
s in the nucleus, forming two neutron
s. Two neutrino
s are emitted in the process. Since the protons are changed to neutrons, the number of neutrons increases by 2, the number of protons Z decreases by 2, and the atomic mass number
A remains unchanged. By changing the number of protons, double electron capture transforms the nuclide
into a new element
.
Example:
|- style="height:2em;"
| ||+ ||2 ||→ || ||+ ||2
|}
In most cases this decay mode is masked by more probable modes (single electron capture etc.), but when all these modes are forbidden or strongly suppressed, double electron capture becomes the main mode of decay. There exist 35 naturally occurring isotopes that can undergo double electron capture. However, there are no confirmed observations of this process. One reason is that the probability of double electron capture is enormously small (the theoretical predictions of half-lives
for this mode lies well above 1020 years). A second reason is that the only detectable particles created in this process are X-ray
s and Auger electron
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
.
If the mass difference between the mother and daughter atoms is more than two masses of electron (1.022 MeV
), the energy released in the process is enough to allow another mode of decay: electron capture with positron emission. It occurs simultaneously with double electron capture, their branching ratio
depending on nuclear properties. When the mass difference is more than four electron masses (2.044 MeV), the third mode - double positron decay - is allowed. Only 6 naturally occurring nuclides can decay via these three modes simultaneously.
of particle physics
: no conservation laws (including lepton number
conservation) are violated. However, if the lepton number is not conserved, or the neutrino
is its own antiparticle
, another kind of the 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
gamma quantum. This mode of decay has never been observed experimentally, and would contradict the Standard Model
if it were observed.
Example:
|- style="height:2em;"
| ||+ ||2 ||→ ||
|}
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
. For a nuclide (A, Z) with number of nucleon
Nucleon
In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...
s A and atomic number
Atomic number
In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...
Z, double electron capture is only possible if the mass of the nuclide of (A, Z-2) is lower.
In this mode of decay, two of the orbital electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s are captured by two proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s in the nucleus, forming two neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s. Two neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
s are emitted in the process. Since the protons are changed to neutrons, the number of neutrons increases by 2, the number of protons Z decreases by 2, and the atomic mass number
Mass number
The mass number , also called atomic mass number or nucleon number, is the total number of protons and neutrons in an atomic nucleus. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion...
A remains unchanged. By changing the number of protons, double electron capture transforms the nuclide
Nuclide
A nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....
into a new element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
.
Example:
- {| border="0"
|- style="height:2em;"
| ||+ ||2 ||→ || ||+ ||2
|}
In most cases this decay mode is masked by more probable modes (single electron capture etc.), but when all these modes are forbidden or strongly suppressed, double electron capture becomes the main mode of decay. There exist 35 naturally occurring isotopes that can undergo double electron capture. However, there are no confirmed observations of this process. One reason is that the probability of double electron capture is enormously small (the theoretical predictions of half-lives
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
for this mode lies well above 1020 years). A second reason is that the only detectable particles created in this process are X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s and Auger electron
Auger electron
The Auger effect is a physical phenomenon in which the transition of an electron in an atom filling in an inner-shell vacancy causes the emission of another electron. When a core electron is removed, leaving a vacancy, an electron from a higher energy level may fall into the vacancy, resulting in...
s that are emitted by the excited atomic shell. In the range of their energies (~1-10 keV
Kev
Kev can refer to:*Kev Hawkins, a fictional character.*Kevin, a given name occasionally shortened to "Kev".*Kiloelectronvolt, a unit of energy who symbol is "KeV".* Krefelder Eislauf-VereinKEV can refer to:...
), the background is usually high. Thus, the experimental detection of double electron capture is more difficult than that for double beta decay
Double beta decay
Double beta decay is a radioactive decay process where a nucleus releases two beta rays as a single process.In double-beta decay, two neutrons in the nucleus are converted to protons, and two electrons and two electron antineutrinos are emitted...
.
If the mass difference between the mother and daughter atoms is more than two masses of electron (1.022 MeV
MEV
MeV and meV are multiples and submultiples of the electron volt unit referring to 1,000,000 eV and 0.001 eV, respectively.Mev or MEV may refer to:In entertainment:* Musica Elettronica Viva, an Italian musical group...
), the energy released in the process is enough to allow another mode of decay: electron capture with positron emission. It occurs simultaneously with double electron capture, their branching ratio
Branching ratio
In particle physics and nuclear physics, the branching fraction for a decay is the fraction of particles which decay by an individual decay mode with respect to the total number of particles which decay. It is equal to the ratio of the partial decay constant to the overall decay constant...
depending on nuclear properties. When the mass difference is more than four electron masses (2.044 MeV), the third mode - double positron decay - is allowed. Only 6 naturally occurring nuclides can decay via these three modes simultaneously.
Neutrinoless double electron capture
The above described process with capture of two electrons and emission of two neutrinos (two-neutrino double electron capture) is allowed by the Standard ModelStandard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
of particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
: no conservation laws (including lepton number
Lepton number
In particle physics, the lepton number is the number of leptons minus the number of antileptons.In equation form,so all leptons have assigned a value of +1, antileptons −1, and non-leptonic particles 0...
conservation) are violated. However, if the lepton number is not conserved, or the neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
is its own antiparticle
Majorana fermion
In physics, a Majorana fermion is a fermion which is its own anti-particle. The term is used in opposition to Dirac fermion, which describes particles that differ from their antiparticles...
, another kind of the 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
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
gamma quantum. This mode of decay has never been observed experimentally, and would contradict the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
if it were observed.
Example:
- {| border="0"
|- style="height:2em;"
| ||+ ||2 ||→ ||
|}
See also
- double beta decayDouble beta decayDouble beta decay is a radioactive decay process where a nucleus releases two beta rays as a single process.In double-beta decay, two neutrons in the nucleus are converted to protons, and two electrons and two electron antineutrinos are emitted...
- beta decayBeta decayIn nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
- electron captureElectron captureElectron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...
- neutrinoNeutrinoA neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
- particle radiationParticle radiationParticle radiation is the radiation of energy by means of fast-moving subatomic particles. Particle radiation is referred to as a particle beam if the particles are all moving in the same direction, similar to a light beam....
- radioactive isotope