Particles, Antiparticles & Photons (AQA A Level Physics)

• The Universe consists of matter in the form of particles i.e. protons, neutrons, electrons etc.
• All particles of matter have an antimatter counterpart
• Antimatter particles are identical to their matter counterpart but have an 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 table below:

Matter-Antimatter Table

• Apart from electrons, the corresponding antiparticle has
  • The same name with the prefix ‘anti-’
  • A line above its corresponding matter particle symbol

• Corresponding matter and antimatter particles have 
  • Opposite charges 
  • The same mass
  • The same rest mass-energy

• The rest mass-energy of a particle is the energy equivalent to the mass of the particle when it is at rest
• Some values of common particle masses and rest mass-energies are shown in the table below:

Mass & Rest Mass Energy Table

• Photons are fundamental particles which make up all forms of electromagnetic radiation
• A photon is defined as:

A massless “packet” or a “quantum” of electromagnetic energy

• This means that energy is not transferred continuously but as discrete packets of energy
• In other words, each photon carries a specific amount of energy, or "quanta", and transfers it all in one go, rather than supplying it consistently

Calculating Photon Energy

• The energy of a photon can be calculated using the formula:

• Using the wave equation, photon energy can also be written:

• Where:
  • E = energy of the photon (J)
  • h = Planck's constant (J s)
  • c = the speed of light (m s^-1)
  • f = frequency (Hz)
  • λ = wavelength (m)

• This equation tells us:
  • The higher the frequency of EM radiation, the higher the energy of the photon
  • The energy of a photon is inversely proportional to the wavelength
  • A long-wavelength photon of light has a lower energy than a shorter-wavelength photon

The energy of a photon is linked to the frequency of the light wave by the Planck constant, h

• Two important interactions involving particles, antiparticles and photons are:
  • Annihilation 
  • Pair production

Annihilation

• When a particle meets its corresponding antiparticle, the two will annihilate
• Annihilation is defined as:

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

• The two most common particle-antiparticle pairs that are seen are:
  • Proton-antiproton annihilation
  • Electron-positron annihilation

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:

• Where:
  •  = minimum energy of one of the photons produced (J)
  • = Planck's Constant (J s)
  •  = minimum frequency of one of the photons produced (Hz)
  • = 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, it is defined as:

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

• In order to achieve the creation of a particle-antiparticle pair, a single photon must have enough energy to create both particles

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

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

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

• To conserve momentum, the particle and anti-particle pair move apart in opposite directions