Particles, Antiparticles and Annihilation · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Matter meets its mirror

Science · Matter & Materials · ages 17-18
Name ______________________   Date ____________

  1. An electron meeting a positron gives two photons leaving back to back.

    Circle one:   True   False

  2. How does an antiparticle compare with its particle?

    • Same mass, opposite charge
    • Double mass, same charge
    • No mass, double charge

  3. What signals does a PET scanner detect?

    • X-ray shadows of bones
    • The twin gamma rays
    • Magnetic flips of protons

  4. What is the least photon energy that can create a particle pair?

    • Any tiny photon will do
    • Any photon past radio waves
    • At least twice one particle's mass energy

  5. An electron and positron annihilate at rest. Each holds 511 keV of mass energy. Give the total photon energy in keV.

    Answer: ______________

  6. Why does annihilation give two photons rather than one?

    • Photons always travel in pairs
    • One photon cannot balance both
    • Electrons refuse single photons

  7. A photon carrying exactly one particle's mass energy attempts pair production. What happens?

    • It makes the pair slowly
    • It makes one particle only
    • Nothing; too little energy

  8. A photon makes one electron with no positron and nothing else. Is this allowed?

    • Yes, electrons appear freely
    • Yes, if the photon is bright
    • No, charge and lepton fail
LightMySky · lightmysky.comW1-mt_-Ud20HRNJz-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

Matter meets its mirror W1-mt_-Ud20HRNJz-s1

  1. True · The pair converts fully to photons, split two ways to balance momentum.
  2. Same mass, opposite charge · Mirrors match in mass and spin, differing in charge and quantum numbers.
  3. The twin gamma rays · Tracer positrons annihilate with electrons, and each meeting flashes two gammas.
  4. At least twice one particle's mass energy · Two masses must be built, so the photon must fund both through E equals m c squared.
  5. 1022 · Both masses convert fully: 511 plus 511 is 1022 keV.
  6. One photon cannot balance both · A single photon cannot carry the full energy while keeping total momentum zero.
  7. Nothing; too little energy · Two masses need twice the funding, so a single share cannot start the job.
  8. No, charge and lepton fail · The tallies must match both sides, and a lone electron breaks two of them.
Worksheet · LightMySky