# Positron-electron annihilation

1. Apr 17, 2008

### Physicsissuef

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:

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

2. Apr 17, 2008

### pam

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

3. Apr 17, 2008

### Physicsissuef

Does the gamma rays must interact?

4. Apr 17, 2008

### mathman

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.

5. Apr 18, 2008

### Physicsissuef

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

6. Apr 18, 2008

### kosher

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.

7. Apr 18, 2008

### malawi_glenn

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.

8. Apr 18, 2008

### Physicsissuef

Here is the picture from my book. That's why I said they are parallel.

9. Apr 20, 2008

### Physicsissuef

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

10. Apr 20, 2008

### malawi_glenn

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

11. Apr 20, 2008

### Physicsissuef

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

12. Apr 20, 2008

### malawi_glenn

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.

13. Apr 21, 2008

### Redbelly98

Staff Emeritus
That picture is misleading. They should not be parallel.