2.1.5 Particles, Antiparticles & Photons

• The universe is made up of matter particles (protons, neutrons, electrons etc.)
• All matter particles have antimatter counterparts
  • Antimatter particles are identical to their matter counterpart but with the opposite charge

• This means if a particle is positive, its antimatter particle is negative and vice versa
• Common matter-antimatter pairs are shown in the diagram below:

Matter-Antimatter Table

• Apart from electrons, the corresponding antiparticle pair has the same name with the prefix ‘anti-’ and a line above the corresponding matter particle symbol
• A neutral particle, such as a neutron or neutrino, is its own antiparticle

• Although antimatter particles have the opposite charges to their matter counterparts, they still have identical mass and rest mass-energy
  • The rest mass-energy of a particle is the energy equivalent to the mass of the particle at rest

• The datasheet provides the masses in kg and rest-mass energies in MeV for a proton, neutron, electron and neutrino
• These masses are identical for their corresponding antiparticles (antiproton, antineutron, positron and antineutrino respectively)

Mass & Rest Mass Energy Table

Annihilation

• When a particle meets its antiparticle pair, 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

• The minimum energy of one photon after annihilation is the total rest mass energy of one of the particles is:

E_min = hf_min = E

• Where:
  • E_min = minimum energy of one of the photons produced (J)
  • h = Planck's Constant (J s)
  • f_min = minimum frequency of one of the photons produced (Hz)
  • E = rest mass energy of one of the particles (J)

• To conserve momentum, the two photons will move apart in opposite directions
• As with all collisions, the mass and energy is still conserved

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

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
• The minimum energy for a photon to undergo pair production is the total rest mass energy of the particles produced:

E_min = hf_min = 2E

• Where:
  • E_min = minimum energy of the incident photon (J)
  • h = Planck's Constant (J s)
  • f_min = minimum frequency of the photon (Hz)
  • E = rest mass energy of one of the particles (J)

• To conserve momentum, the particle and anti–particle pair move apart in opposite directions
• Remember for both calculations, the frequency f is also defined by the wave equation
• Using this equation, the wavelength λ can also be calculated
  • Remember that v is c (the speed of light) for gamma ray photons