OCR A Level Physics

Revision Notes

6.10.2 Particle-Antiparticle Pairs

Test Yourself

Annihilation of Matter & Antimatter


  • When a particle meets its antiparticle partner, the two will annihilate
  • Annihilation is:

When a particle meets its equivalent anti–particle they both are destroyed and their mass is converted into energy in the form of two gamma ray photons


When an electron and positron collide, their mass is converted into energy in the form of two photons emitted in opposite directions

Pair Production

  • Pair production is the opposite of annihilation
  • Pair production is:

When a photon interacts with a nucleus or atom and the energy of the photon is used to create a particle–antiparticle pair

  • The presence of a nearby neutron is essential in pair production so that the process conserves both energy and momentum
  • A single photon alone cannot produce a particle–anti-particle pair or the conservation laws would be broken
  • Pair creation is a case of energy being converted into matter

2.2.5 Pair Production

When a photon with enough energy interacts with a nucleus it can produce an electron-positron pair

  • This means the energy of the photon must be above a certain value to provide the total rest mass energy of the particle–antiparticle pair
  • Einstein's famous mass-energy relation showed that energy can be converted into mass, and vice versa
  • It is given by:

ΔE = c2 Δm

  • Where:
    • Δ= rest mass of the particle (kg)
    • c = speed of light (m s–1)
    • ΔE = rest mass energy of the particle (J)

  • Therefore, in order to create a particle & anti-particle pair, the energy carried by a single photon must be at least twice the rest-mass energy required, i.e.

E = 2(c2 Δm)

  • This also means if a particle meets its anti-particle and annihilates, the energy carried away by each of the two photons Ephoton is given by: 

Ephotonhf fraction numerator h c over denominator lambda end fractionc2 Δm

Worked example

Calculate the maximum wavelength of one of the photons produced when a proton and antiproton annihilate each other.

2.1.5 Annihilation Worked Example

Exam Tip

Since the Planck constant is in Joules (J) remember to always convert the rest mass-energy from MeV to J.

You've read 0 of your 0 free revision notes

Get unlimited access

to absolutely everything:

  • Downloadable PDFs
  • Unlimited Revision Notes
  • Topic Questions
  • Past Papers
  • Model Answers
  • Videos (Maths and Science)

Join the 80,663 Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.