# Pair production

## Main Question or Discussion Point

when do high energy photons become a paritcle/antiparticle pair?

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malawi_glenn
Homework Helper
it can become when its energy is above twice the particle mass.

e.g. pair production of electron + positron is possible when the photon has energy above 2*m_e = 1022keV

mathman
it can become when its energy is above twice the particle mass.

e.g. pair production of electron + positron is possible when the photon has energy above 2*m_e = 1022keV
Your description is somewhat incomplete. In order for pair production to take place the photon has to interact with a nucleus so that momentum conservation will hold. Alternatively if two photons collide (such as just after the big bang) pair production will also be possible.

Pair production requires BOTH the photon energy exceeding 1.02 MeV, AND something momentum can be transferred to, like a nucleus. The cross section is proportional to Z2 Ln(E) from 3 MeV to over 1 GeV. (E = hv)

why cant the momentum be transferred to to the particle/antiparticle pair? and what is it transferred to when they collide?

malawi_glenn
Homework Helper
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what about the photons colliding after the big bang?

malawi_glenn
Homework Helper
they can 'collide' with each other, as mathman told you also. Don't you read the answers you've got?

"Alternatively if two photons collide (such as just after the big bang) pair production will also be possible."
"The photon must 'interact' with a nucleus or other heavy, eletrically charged particle, to make pair production possible."
they seem to contradict each other dont they? i was asking about the physics of the first and i would appreciate it if you would not insult me

malawi_glenn
Homework Helper
because in post #5 you are referring to post #4 where the "pair production in matter" was discussed by Bob_S.

i dont understand if they can conserve momentum by colliding why cant they conserve it otherwise without a nucleus?

blechman
you cannot have a massless thing split to two massive things and still conserve momentum. just work it out.

you CAN, on the other hand, have two (massless or massive) things go to two massive things and conserve momentum and energy.

so as to the DIFFERENT processes that you are referring to:

1. photon -> e+ e- : that violates momentum conservation.

2. photon + Nucleus -> e+ e- + nucleus: that is allowed by momentum conservation (think of the nucleus as a "catalyst" that allows the process to occur).

3. photon + photon -> e+ e- : that is also allowed by momentum conservation.

mometum conservation is nothing deep: it's straight out of first-year physics.

ah ok i get it thanks everyone