# Positron-electron annihilation

## Main Question or Discussion Point

Do the gamma rays (from positron-electron annihilation) must interact to have reversible reaction? I read in my text book that this reaction is reversible? Is that true?
The reaction:

$$^{0}_{-1}e+ ^{0}_{+1}e \rightarrow 2 \gamma$$

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pam
In principle, the reverse is possible. It is just difficult to achieve experimentally.

Does the gamma rays must interact?

mathman
Does the gamma rays must interact?

Yes. Right after the big bang, when the universe was very small and very hot, gamma-gamma reactions were taking place not only for electron-positron pairs, but heavier particle-antiparticle pairs (such as proton-antiproton or neutron-antineutron) also.

but how those gamma rays will interact them selfs when they are going parallel to each other?

electron positron pair

You should visit wiki pedia to check the phenomenon of pair production; also if I am not wrong then it is not necessary to for the production of electron and positron pair that we should annihilate two gamma rays. A single gamma (photon) ray having energy greater then 1.02 Mev can be splitted in to electron positron pair, when ever an attempt to stop it with the help of nuclei of heavy element is made. Moreover two gamma rays moving parallel to each other, I don’t think they will annihilate in to electron positron pair.

malawi_glenn
Homework Helper
Physicsissuef: It does not have to be the same photons which are emitted in the annihilation process of the electron and positron. Any two photons which have enough energy may produce electron - positron pairs.

kosher: There is a difference between gamma-gamma reactions and pair production. Gamma-gamma reactions are a photon interacting with another photon. In pair production, one has a photon of E > 1.022MeV producing a electron - positron pair, this reaction MUST take place near an atomic nucleus or an atomic electron.

Here is http://pic.mkd.net/images/1016371213.jpg" [Broken] from my book. That's why I said they are parallel.

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Anybody knows, please? How will the reverse reaction go?

malawi_glenn
Homework Helper
Anybody knows, please? How will the reverse reaction go?
It depends on how the initial electron-postiron pair annihilates, remember our discussion in the other thread? Initial momentum = final momentum & initial energy = final energy

Yes, but for the reverse reaction they must interact, how will they interact when they are parallel?

malawi_glenn
Homework Helper
Post #7:

It does not have to be the same photons which are emitted in the annihilation process of the electron and positron. Any two photons which have enough energy may produce electron - positron pairs is good enough.

Redbelly98
Staff Emeritus