Everything about Electron-positron Annihilation totally explained
Electron-positron annihilation occurs when an
electron and a
positron (the electron's
anti-particle) collide. The result of the collision is the conversion of the
electron and
positron and the creation of gamma ray photons or, less often, other particles. The process must satisfy a number of
conservation laws, including:
As with any two charged objects, electrons and positrons may also interact with each other without annihilating, in general by
elastic scattering.
Low energy case
There are only a very limited set of possibilities for the final state. The most likely is the creation of two or more
gamma ray photons. Conservation of energy and linear momentum forbid the creation of only one photon. In the most common case, two photons are created, each with energy equal to the
rest energy of the
electron or
positron (511
keV)
. A convenient
frame of reference is that in which the system has
no net linear momentum before the annihilation; thus, after collision, the gamma rays are emitted in opposite directions. It is also common for three to be created, since in some angular momentum states, this is necessary to conserve
C parity. It is also possible to create any larger number of photons, but the probability becomes lower with each additional photon because these more complex processes have lower
quantum mechanical amplitudes.
Since
neutrinos also have a smaller mass than electrons, it's also possible — but exceedingly unlikely — for the annihilation to produce one or more neutrino/
antineutrino pairs. The same would be true for any other particles, which are as light, as long as they share at least one
fundamental interaction with electrons and no conservation laws forbid it. However, no other such particles are known.
High energy case
If the electron and/or positron have appreciable
kinetic energies, other heavier particles can also be produced (for example
D mesons), since there's enough kinetic energy in the relative velocities to provide the
rest energies of those particles. It is still possible to produce photons and other light particles, but that'll emerge with higher energies.
At energies near and beyond the mass of the carriers of the
weak force, the W and Z bosons, the strength of the weak force becomes comparable with
electromagnetism.
This means that it becomes much easier to produce particles such as neutrinos that interact only weakly.
The heaviest particle pairs yet produced by electron-positron annihilation in
particle accelerators are
/ pairs. The heaviest single particle is the
Z boson. The driving motivation for constructing the
International Linear Collider is to produce
Higgs bosons in this way.
Practical uses
This process is the physical phenomenon relied on as the basis of
PET imaging.
Also used as a method of measuring the
Fermi surface and
Band structure in metals.
Further Information
Get more info on 'Electron-positron Annihilation'.
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