Particles

• Rutherford's atomic model


  A small, massive nucleus, containing protons and neutrons, with electrons arranged in discrete shells in orbit around the nucleus
• Rutherford's alpha scattering experiment

  1. A stream of alpha particles were fired at a thin layer of gold foil

  2. The angles at which the particles were scattered were recorded

  3. Some passed straight through (proving that most of the atom is empty space)

  4. Some were deflected by large angles (proving the existence of a very small, positive nucleus)
• The closest approach of a scattered particle to a nucleus can be calculated based on the initial kinetic energy of the particle
• Hadrons


  Feel the strong interaction (or the Strong Force), are not fundamental particles (they are made up of quarks), e.g. protons, neutrons
• Leptons


  Interact via the weak interaction and gravity, have no (or almost no) mass, are fundamental particles, e.g. electrons, neutrinos
• Antiparticles


  Have the same mass as their matter counterparts, but the opposite charge
• Antiparticles


  • Antiproton
  • Antineutron
  • Positron
  • Antineutrino
• Pair production


  Matter can be created from energy, where equal amounts of matter and antimatter are created
• Annihilation


  A particle and antiparticle fuse together and produce energy in the form of a gamma ray
• Quarks


  Fundamental particles, not made up of smaller particles, there are up and down quarks (and their anti-quark counterparts)
• Quarks


  • Up
  • Down
  • Anti-up
  • Anti-down
• Proton


  uud, charge +1
• Neutron


  udd, charge 0
• Antiproton


  u u d, charge -1
• Antineutron


  u d d, charge 0
• Dental X-ray tubes


  Used in particle accelerators, electrons are boiled off a hot piece of metal wire and accelerated from a negative plate towards a positive plate
• Linear particle accelerators


  Involve charged plates lined up in a straight path, with charges reversed at time intervals to ensure particles keep accelerating
• Circular accelerators


  Such as cyclotrons, can reach higher speeds because they are continuous, but particle velocity cannot exceed the speed of light
• Electron arrangement


  Electrons are arranged around the atom in discrete energy levels, they can move up/down energy levels by emitting or absorbing photons
• Line spectra

  Provide evidence of discrete electron energy levels, white light has a continuous spectrum but when passed through a cool gas, certain wavelengths are missing
• Absorption spectra


  Occur when light passes through cold gases
• Emission spectra


  Occur when hot gases cool, emitting photons as electrons drop down to lower energy levels
• Fluorescent tubes


  Use the principle of electron energy level transitions, mercury atoms cool and release UV photons, causing fluorescence
• Wave model of the atom

  Models electron shells as standing waves, the wavelength of the electron must form a whole number of waves around the shell/orbital
• Unstable atoms


  Are radioactive, they release radiation to become more stable, due to having too many neutrons, too much mass, too few neutrons, or too much energy
• Types of radiation


  • Alpha (α)
  • Beta minus (β-)
  • Beta plus (β+)
  • Gamma (γ)
• Alpha radiation


  2 protons, 2 neutrons, mass 4, charge +2, highly ionising, weakly penetrating, slow, absorbed by paper or a few cm of air
• Beta minus radiation


  Electron, mass 0, charge -1, weakly ionising, fast, absorbed by a few mm of metal (aluminium)
• Beta plus radiation


  Positron, mass 0, charge +1, annihilated by electrons so has negligible range
• Gamma radiation


  Electromagnetic radiation (gamma waves), mass 0, charge 0, very weakly ionising, very fast (speed of light), absorbed by lead or several feet of concrete, intensity decreases with distance according to the inverse square law
• Risk


  Described in terms of consequence and likelihood
• Mass defect

  The mass of a nucleus is less than the sum of the protons and neutrons which make it up, this discrepancy is the mass defect
• Binding energy

  The energy that holds the nucleus together, calculated from the mass defect using Einstein's equation
• Nuclear fission


  Breaking apart nuclei to release energy, can be spontaneous or induced by neutron absorption, controlled by control rods
• Nuclear fusion


  Joining two smaller nuclei to form one larger one, also releases energy, requires very high temperatures to overcome the electrostatic force between nuclei