Pair production creates a particle-antiparticle pair from a photon

Particles, Antiparticles & Photons — AQA A-Level Physics

Key Definition

Pair production — When a photon interacts with a nucleus or atom and its energy is used to create a particle-antiparticle pair.

Pair productionThe creation of a particle-antiparticle pair from a high-energyThe capacity to do work. Measured in joules (J). photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. in the presence of a nucleus. The photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. energyThe capacity to do work. Measured in joules (J). must be at least 2mc². is the reverse of annihilationThe process in which a particle and its corresponding antiparticle collide and convert their combined rest mass into energyThe capacity to do work. Measured in joules (J)., typically producing two gamma photons..
A single photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. must have enough energy to create both particles.
The particle and antiparticle move apart in opposite directions to conserve momentum.
A nearby nucleus is needed to conserve momentum (the nucleus absorbs recoil momentum).

\begin{aligned} E_{\min} &= hf_{\min} \\ &= 2E_0 \end{aligned}

$E_min$: minimum energy of the incident photon (J)
$h$: Planck's constant (J s)
$f_min$: minimum frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). of the photon (Hz)
$E_0$: rest mass-energy of one particle (J)

The factor of 2 is because the photon must create two particles (particle + antiparticle).
For electron-positron pair productionThe creation of a particle-antiparticle pair from a high-energy photon in the presence of a nucleus. The photon energy must be at least 2mc².: $E_{\min} = 2 \times 0.511 = 1.022$ MeV.

Examiner Tips and Tricks

AnnihilationThe process in which a particle and its corresponding antiparticle collide and convert their combined rest mass into energy, typically producing two gamma photons.: minimum energy of each photon = rest mass-energy of ONE particle ($E_0$).
Pair productionThe creation of a particle-antiparticle pair from a high-energy photon in the presence of a nucleus. The photon energy must be at least 2mc².: minimum photon energy = rest mass-energy of TWO particles ($2E_0$).
Don't mix these up.