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Pair Production

If a photon with sufficient energy passes near a nucleus or an electron, it can suddenly change into a particle-antiparticle pair which would then separate from each other.The photon creates a particle and a corresponding antiparticle with a minimum energy given by:

min Energy of photon
= h f min
= 2 E0

where E0 = rest energyThe electron has a rest energy of 0.511MeV therefore the photon needs an energy of at least 2 ×0.511MeV or 1.64×10-13J.A photon with less energy cannot create a positron-electron pair.A cloud chamber is a small transparent container which contains air saturated with vapour at a very low temperature.Ionising radiation leaves a visible trail of droplets.
When a magnetic field is applied to a cloud chamber the particles are deflected. Positive particles are deflected in the opposite direction to negative particles.Slower particles bend more.

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Specification: AQA Physics (7407), AS-level, 3.2.1.3 Particles, antiparticles and photons. Topic: Particles and Radiation.

Key terms in this lesson

Weak nuclear force
The fundamental interaction responsible for beta decay, in which particles interact through the exchange of W bosons.
Pion
A meson, made of a quark-antiquark pair, that may be positively charged, negatively charged or neutral; produced through the strong interaction and decaying through the weak interaction.
Kaon
A strange meson, produced through the strong interaction and decaying through the weak interaction, with a rest mass between that of a pion and a proton.

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