Particles & antiparticles

Question 1

Antimatter

Answer 1

• Normal matter & antimatter meet & destroy each other (radiation)
• Used in brain scans, positrons meet with electrons in brain, radiation picked up, develops brain image
• Positron emission: proton to neutron, unstable nucleus, too many protons
• Neutrino (uncharged) emitted to conserve energy / charge

A A 0
X —> Y + ß + v
Z Z-1 +1

-Do not occur naturally, manufactured, stable isotope in path of photons, some nuclei absorb photons, become unstable

Question 2

Antimatter

Answer 2

• Predicted in 1928, English physicist Paul Dirac
• E=mc² (Mass increases as speed increases)
• Mass of stationary particle, rest mass, corresponds to rest energy

Question 3

Annihilation

Answer 3

• Particle & antiparticle
• Converting total mass to photons
• Antiparticle, same rest mass as particle
• Opposite charge to particle (if has a charge)
• Photons travel at 180° in opposite directions
• Conserve momentum
• 2 photons produced, 1 cannot ensure momentum = 0
• 2hf min (energy of 2 photons) = 2E (rest energy)
• Min energy of each photon, hf min = E

                   Photon
                      ↑
Antiparticle →  ←Particle
                      ↓
                   Photon

Question 4

Pair production

Answer 4

• Photon with sufficient energy passing near nucleus / electron can change to antiparticle, particle pair, separates
• Travel in opposite directions, conserves momentum
• Min energy of photon needed, hf min = 2E (Particle, antiparticle)

           ↗ Particle Photon→
           ↘ Antiparticle

-EG: photon has rest energy of 0.511 MeV
Min photon energy = 2 x 0.511 MeV
-Photon less energy, not create positron & electron