Particles, Antiparticles & Photons (AQA AS Physics): Revision Note

Antimatter

• 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:

• Apart from electrons, the corresponding antiparticle has:

  • The same name with the prefix ‘anti-’

  • A line above its corresponding matter particle symbol

Properties of antiparticles

• 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:

The photon model

• 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:

$E=hf$

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

$E=\frac{hc}{\lambda }$

• 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

Annihilation & pair production

• 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:

The destruction of a particle-antiparticle pair when they collide and convert their mass into two gamma-ray photons

• The two most common particle-antiparticle pairs that are seen are:

  • Proton-antiproton annihilation

  • Electron-positron annihilation

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

$E_{min}=h{f}_{min}=E$

• 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

• It is defined as:

The creation of a particle-antiparticle pair when a high-energy photon spontaneously converts its energy into mass

• To achieve pair production, a single photon must have enough energy to create both particles

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

$E_{min}=h{f}_{min}=2E$

• 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 the antiparticle move apart in opposite directions