Antiparticles, photons and annihilationAQA A-Level Physics: Revision notes

For every type of particle there is a corresponding antiparticle. An antiparticle has the same mass (and so the same rest energy) as its particle but opposite charge. Particles with no charge, such as the neutron and neutrino, have antiparticles with no charge too, but they are still different particles.

The antiparticles you need to know are:

• electron e⁻ → positron e⁺
• proton p → antiproton $\bar{p}$
• neutron n → antineutron $\bar{n}$
• neutrino $\nu_e$ → antineutrino $\bar{\nu}_e$

When a particle and its antiparticle meet they annihilate.

Rest energy is the energy equivalent of a particle's mass, and for particles it is quoted in MeV. A particle and its antiparticle always have the same rest energy.

• electron and positron: 0.511 MeV, charge −1e and +1e
• proton and antiproton: about 938 MeV, charge +1e and −1e
• neutron and antineutron: about 940 MeV, both uncharged
• neutrino and antineutrino: negligible rest energy, both uncharged

To convert, 1 MeV = 1.60 × 10⁻¹³ J. You are not required to use E = mc² in calculations: rest energies are given or are in the data booklet.

Electromagnetic radiation travels as discrete packets of energy called photons. The energy of one photon is

$E = hf = \dfrac{hc}{\lambda}$

where $h = 6.63 \times 10^{-34}$ J s is the Planck constant, $f$ is the frequency in Hz, $c = 3.00 \times 10^{8}$ m s⁻¹ and $\lambda$ is the wavelength in m.

A higher frequency (shorter wavelength) means more energy per photon. Gamma-ray photons carry energies of the order of MeV.

In annihilation a particle and its antiparticle meet and their mass is converted into energy, carried away as photons.

For an electron and a positron at rest, the total rest energy is 2 × 0.511 = 1.022 MeV. At least two photons are produced, travelling in opposite directions so that momentum is conserved. Each has energy 0.511 MeV (plus half of any kinetic energy the particles had).

Worked example. Each photon has E = 0.511 MeV = 8.18 × 10⁻¹⁴ J. Frequency $f = E/h = 8.18 \times 10^{-14} / 6.63 \times 10^{-34} = 1.23 \times 10^{20}$ Hz.

Pair production is the reverse: a photon with enough energy disappears (usually near a nucleus) and produces a particle and its antiparticle.

The photon energy must be at least twice the rest energy of one particle: 1.022 MeV for an electron and a positron, 1876 MeV for a proton and an antiproton. Any extra energy becomes the kinetic energy of the pair.

Charge is conserved: the photon is neutral, so the two particles carry opposite charges.