Nuclear and particle physics
Atoms, nuclei and radiation
The α-scattering experiment showed that an atom's mass and positive charge are concentrated in a tiny nucleus. A nuclide is written ( nucleons, protons). Unstable nuclei emit α (a helium nucleus), β⁻ (an electron plus an antineutrino), β⁺ (a positron plus a neutrino) or γ (a photon). Nucleon number and charge are conserved in every decay.
≈ 35 minFundamental particles
Protons and neutrons are made of quarks. There are six flavours: up (), down (), strange (), charm (), top () and bottom (), each with an antiquark of opposite charge. Hadrons are baryons (three quarks) or mesons (quark + antiquark). Leptons, such as electrons and neutrinos, are fundamental. In β⁻ decay a down quark becomes an up quark; in β⁺, up becomes down.
≈ 35 minParticle interactions
Every particle has an antiparticle with the same mass and opposite charge. They can annihilate into photons, and a photon with enough energy can create a particle–antiparticle pair: . Forces act by exchange particles (photon, W bosons, gluons). A reaction can happen only if charge, baryon number and lepton number are all conserved; strangeness is conserved in strong interactions.
≈ 45 minParticle accelerators and detectors
Accelerators give charged particles huge kinetic energies using electric fields. A linac uses drift tubes of increasing length with an alternating p.d.; a cyclotron bends particles in a magnetic field so they cross the same gap repeatedly. In a magnetic field, , so track curvature reveals momentum and charge. Collision energy can create new particles: .
≈ 40 min