Pair Production from Gamma Rays

[Buckeye]

Am I right in understanding that Gamma Rays can decay into an electron-positron pair without interacting with any matter as indicated by Bubble Chamber results?

 

[jtbell]

A single isolated photon cannot convert into a particle-antiparticle pair because such a process cannot conserve both energy and momentum. Another particle has to be nearby, usually an atomic nucleus, to take up some energy and momentum. A bubble chamber is far from a vacuum.

Note that going the other way, a particle-antiparticle pair in isolation cannot annihilate into a single photon, for the same reason. You always get at least two photons.

 

[Buckeye]

A single isolated photon cannot convert into a particle-antiparticle pair because such a process cannot conserve both energy and momentum. Another particle has to be nearby, usually an atomic nucleus, to take up some energy and momentum. A bubble chamber is far from a vacuum.
Note that going the other way, a particle-antiparticle pair in isolation cannot annihilate into a single photon, for the same reason. You always get at least two photons.

When an electron-positron collision produces those two photons, do those two photons soon convert back into electron-positron pairs or two electrons?

 

[Buckeye]

JT, one more question if I may.
Assuming the gamma-ray collides with an atom, it seems obvious that the cross-section of the gamma-ray is so small that it must strike the core and in the process generate the positron. The question is: Does the positron come from a gamma-ray-neutron interaction or from a gamm-ray-proton interaction?
Thanks!

 

[jtbell]

When an electron-positron collision produces those two photons, do those two photons soon convert back into electron-positron pairs or two electrons?

In order for a photon to convert into an electron-positron pair, it has to have enough energy to produce the masses of the electron and positron, that is, a bit over 1 MeV. So if you start out with an electron and a positron which have enough kinetic energy, when they annihilate they can produce photons with enough energy to each produce new electron-positron pairs in turn. But if the original electron and positron don't have much KE, then this isn't possible.
You can't just get two electrons because that wouldn't conserve charge.

Does the positron come from a gamma-ray-neutron interaction or from a gamm-ray-proton interaction?

That should be "positron and electron", of course. This is an electromagnetic interaction, so the "spectator" particle has to have electric charge. I don't know enough about the details of pair production to say whether you can associate the process with a particular proton in a nucleus, or whether you must instead consider the nucleus as a whole.
[Note: I'm leaving tomorrow and won't be back until after Christmas. Happy holidays!]

 

[Buckeye]

In order for a photon to convert into an electron-positron pair, it has to have enough energy to produce the masses of the electron and positron, that is, a bit over 1 MeV. So if you start out with an electron and a positron which have enough kinetic energy, when they annihilate they can produce photons with enough energy to each produce new electron-positron pairs in turn. But if the original electron and positron don't have much KE, then this isn't possible.
You can't just get two electrons because that wouldn't conserve charge.
That should be "positron and electron", of course. This is an electromagnetic interaction, so the "spectator" particle has to have electric charge. I don't know enough about the details of pair production to say whether you can associate the process with a particular proton in a nucleus, or whether you must instead consider the nucleus as a whole.
[Note: I'm leaving tomorrow and won't be back until after Christmas. Happy holidays!]

My understanding is that massive particles can not be made from massless particles even if the photon has enough energy to form those massive particles. Is that right?