12.3 Pair Annihilation And Pair Creation

Question 1

What happens when a particle and anti particle collide?

Answer 1

The mass and energy of the particle and its anti particle are completely converted into the energy of at least two photons. The particle and the anti particle disappear - they annihilate each other.

Question 2

What happens when a proton collides with an anti proton of the same kinetic energy moving in the opposite direction?

Answer 2

It will produce two photons. Because the total initial momentum is zero in this case the final momentum must also be zero so the photons move in opposite directions and have the same wavelength. The energy of the photons comes from the rest entry of each proton and their kinetic energy.

Question 3

What happens during pair creation?

Answer 3

This refers to a single photon producing a particle and anti particle pair. In order for the momentum to be conserved there must be a nucleus nearly and it participates in the reaction.

Question 4

Whta must the energy of a photon be to create two particles?

Answer 4

Leaving aside the nucleus in order to produce a pair of particles each of mass m the energy of the photon must be equal to at least 2mc^2