Particles, Antiparticles & Photons

Physics: AQA A Level, topic 2.1.8

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.

Test yourself on Particles, Antiparticles & Photons

What does the following symbol represent?
$\overline{n}$
What does the following symbol represent?
$\overline{\nu}$
Which of these particles and antiparticles has the same charge as a proton?
What is the symbol used for Planck's constant?
What is produced when a particle and its corresponding antiparticle annihilate one another?

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