2.1.8

# Particles, Antiparticles & Photons

Test yourself

## Antimatter

Antimatter is made up of antiparticles. Every particle has an antiparticle. Antiparticles have the same mass and rest energy but are oppositely charged. They are usually labelled with a line over the top of their symbol.

### Proton and antiproton

• The antiparticle of the proton is the antiproton.
• It has the same mass (1.67×10-27 kg) and rest energy (938 MeV) as a proton.
• It has a relative charge of -1, the opposite to a proton.
• Its symbol is $\overline{p}$.

### Electron and positron

• The antiparticle of the electron is the positron.
• It has the same mass (9.11×10-31 kg) and rest energy (0.51 MeV) as an electron.
• It has a relative charge of +1, the opposite to an electron.
• Its symbol is e+.

### Neutron and antineutron

• The antiparticle of the neutron is the antineutron.
• It has the same mass (1.67×10-27 kg) and rest energy (940 MeV) as a neutron.
• It has a relative charge of 0 (the opposite of 0 is also 0).
• Its symbol is $\overline{n}$.

### Neutrino and antineutrino

• The antiparticle of the neutrino is the antineutrino.
• It has a mass of 0, the same as a neutrino.
• It has a relative charge of 0 (the opposite of 0 is also 0).
• Its symbol is $\overline{\nu}_e$.

## Photon Model of Electromagnetic Radiation

We can think of electromagnetic radiation (light) as little packets of energy. We call these packets photons.

### Frequency and energy

• The energy of a photon is directly proportional to the frequency of the radiation.
• $E = hf$
• The letter 'h' is the Planck constant.
• It is a fundamental constant of nature and is equal to 6.63×10-34 Js.

### Energy of different photons

• Gamma photons have the highest frequency and so carry the most energy.
• Radio photons have the lowest frequency and so carry the least energy.

## Annihilation and Pair Production

We know from Einstein's famous equation, E = mc2, that mass and energy are interchangeable. This is shown in annihilation and pair production.

### Annihilation

• When a particle and its corresponding antiparticle collide, they annihilate each other.
• Their masses are converted into pure energy, producing a pair of gamma photons.
• The energy carried away by the gamma photons must equal the total energy of the particles to begin with (kinetic energy plus rest mass energy).
• So each gamma photon must carry away at least the rest mass energy of one particle.

### Pair production

• Pair production is the opposite of annihilation.
• Pair production is when one high energy photon spontaneously turns into a particle-antiparticle pair.
• The energy of the photon must be at least the total rest mass energy of the particle-antiparticle pair it creates.