Physics › Particles › Antimatter and photons
Antimatter and photons
Every particle has a mirror twin with the same mass and opposite charge, and the two sides of the mirror can trade places with light. Annihilation and pair production are the exchange rates, and the rest energies set the prices.
Builds on The photoelectric effect.
IN THIS TOPIC
- Compare particles and antiparticles by mass, charge and rest energy in MeV.
- Use the photon model E = hf = hc/λ.
- Describe annihilation and pair production, including the minimum energies involved.
WHAT YOU PROBABLY THINK
Antimatter is science fiction.
The mirror world
For every type of particle there is an antiparticle: identical mass, identical rest energy, and opposite charge. The named pairs AQA expects: the positron for the electron, the antiproton, the antineutron, and the antineutrino for the neutrino.
Rest energy is the energy equivalent of a particle's mass, quoted in MeV and printed for the common particles in the data booklet: 0.511 MeV for the electron and positron, around 938 MeV for the proton. AQA states explicitly that E = mc2 is not required in calculations; the booklet's rest energies do the work instead. Antimatter is also entirely real: positrons are produced in beta-plus decay and put to work every day in hospital PET scanners.
The photon, priced
Electromagnetic radiation travels as photons carrying
which is the currency the mirror world trades in: high frequency, expensive photon.
Annihilation and pair production
When a particle meets its antiparticle they annihilate: both vanish, and their entire energy, rest energy included, leaves as photons. To conserve momentum there must be two photons, emitted in opposite directions, and for a slow electron-positron pair each photon carries at least one rest energy: a minimum of 0.511 MeV apiece.
Pair production is the reverse: a single sufficiently energetic photon converts into a particle-antiparticle pair, typically while passing a nucleus, which takes the recoil that momentum conservation demands. The entry price is both rest energies: at least 2 × 0.511 = 1.022 MeV to make an electron-positron pair, with any surplus becoming the pair's kinetic energy. Below the price, nothing; the mirror charges in full.
THE EXAM BIT
- Antiparticle comparison in one sentence: same mass and rest energy, equal and opposite charge. All three properties are expected.
- Annihilation produces two photons in opposite directions, to conserve momentum. “A photon” singular is the planted error.
- Minimum photon energies: 0.511 MeV each in electron-positron annihilation; at least 1.022 MeV for the photon in pair production. Convert to joules with 1.60 × 10−19 before touching h.
- Pair production needs a nearby nucleus to take recoil; a lone photon in empty space cannot conserve both energy and momentum while producing a pair.
- Rest energies come printed in the data booklet. Quote them from there rather than deriving anything: E = mc2 calculations are explicitly not required.
CHECK YOURSELF
A slow electron and a slow positron annihilate. Find the minimum frequency of each photon produced. (Electron rest energy 0.511 MeV; h = 6.63 × 10−34 J s.)
Show a hint
One rest energy per photon; convert MeV to joules first.
Show the answer
Each photon carries at least one rest energy: 0.511 MeV = 0.511 × 106 × 1.60 × 10−19 = 8.18 × 10−14 J.
Minimum frequency: f = E/h = 8.18 × 10−14 / 6.63 × 10−34 = 1.2 × 1020 Hz.
That is gamma radiation, and detecting the two back-to-back gamma photons is precisely how a PET scanner locates the annihilation.
Same mass, opposite charge, every particle.
Annihilation and pair production trade at the rest-energy price.
No animated video for this topic yet; these notes stand alone. InkPhysics on YouTube.