Particles and Anti-Particles

Every particle has a corresponding antiparticle:

• Proton -> Antiproton
• Neutron -> Antineutron
• Electron -> Positron
• Neutrino -> Antineutrino

• Antimatter has indentical mass but the opposite charge to it's matter counterpart

Example of Positron Emission

• A PET (Positron Emission Tomography) scanner in a hospital uses antimatter electrons (aka positrons)

• Einstein created a famous formula (E = mc^2) which tells us that mass is really a very dense form of energy.
• If you consider a particle at rest then you can ignore all other energies and assume they are zero, therefore the mass alone contributes to the energy of the particle. This is the rest mass.
• E = M0C^2 [M0 is the subscript showing the rest mass]
• 1 MeV = 1.6x10^-13 J

• If matter and antimatter collide then they annihilate; all of their mass turns into photons however energy is conserved
• E =M0C^2 = hf
• Momentum and Matter : Antimatter Ratio are both conserved

Energy In Annihilation

• hf(min) = E0
• The two particles that aniihilate will be identical, as will the two photons. Therefore, the rest energy of the particles will match the minimum energy of the photons.
• The particles will in reality possess some kinetic energy as well so the photon energy would be expected to be higher than this minimum value.

• A high energy photon can turn into a pair of particles (matter and antimatter together).
• The energy in the photon will be double the rest energy of the particles. Any energy left over will be converted into kinetic energy of the particles.
• hf(min) = 2E0

• A high energy photon can turn into a pair of particles (matter and antimatter together).
• The energy in the photon will be double the rest energy of the particles. Any energy left over will be converted into kinetic energy of the particles.
• hf(min) = 2E0

Rest Energy Calculations

• The data sheet contains the masses of the proton, electron and neutron. Calculate their rest energies and hence the minimum frequency of photon you would expect to obtain from annihilation with each.