# AQA AS physics question

Hi all - just a bit confused about an answer in the AQA AS Physics textbook:

'Explain why a photon of energy 2MeV could produce an electron - positron pair, but not a proton - antiproton pair'

and the ANSWER is:

'The rest energy of an electron and a positron is less than 2MeV but not for a proton-antiproton pair, So pair production can happen for an electron and positron'

I don't understand the answer, so pair production can not happen if the rest energy of the particle-antiparticle pair is less than the minimum energy of the carrier photon?????? is this correct?

## Answers and Replies

Nugatory
Mentor
I don't understand the answer, so pair production can not happen if the rest energy of the particle-antiparticle pair is less than the minimum energy of the carrier photon?????? is this correct?

Yes (unless there is some other source of energy coming in as well as the photon). Otherwise, conservation of energy would be violated.

Yes (unless there is some other source of energy coming in as well as the photon). Otherwise, conservation of energy would be violated.

Ok great, thanks for that.

I have another question if you don't mind? (i'm self studying physics so i might have quite a few :( )

The question says: The rest energy of an electron is 0.511 MeV; a positron collides with an energy of 0.158 MeV of energy, calculate the total energy of the positron and the electron' with an answe of 1.180 MeV

I've tried using E=hf but that gives me an answer of around 0.7 MeV?? Sorrry!

For anyone else who may be confused about this:

The rest energy of the positron must be taken into account. As it is the antiparticle of the electron it will have the same rest mass/energy as the electron, which the question tells us is 0.511MeV. The positron has a kinetic energy of 0.158Mev, so adding this to the rest energy of the positron (0.551MeV) gives us an energy of 0.669MeV. The total energy of the positron is 0.669MeV, add this to the energy of the electron (0.511MeV) to give an answer of 1.180MeV.