P3- Radioactivity

• Radioactive Decay


  The random process involving unstable nuclei emitting radiation to become more stable
• Radiation Dose is measured in Sieverts (Sv).
• James Chadwick


  • Provided evidence to prove the existence of neutrons 20 years after the nucleus was an accepted scientific idea
• Gold Foil Experiment


  Ernest Rutherford

  • most particles went straight through gold foil
  • indicates most of an atom is empty space
• Short half-life


  Source

  • less of a risk
  • does not remain strongly radioactive
• Irradiation


  The process of an object being exposed to nuclear radiation
• Geiger-Muller Tube


  A detector that measures the count-rate of a radioactive sample
• Electron energy levels


  Electrons can transition to a higher energy level

  • through the absorption of electromagnetic radiation

  Transition to a lower energy level

  • through the emission of electromagnetic radiation
• Chain Reaction


  Neutrons released by a fission reaction

  • absorbed by another unstable, large nuclei
  • inducing further fission reactions
• Bohr Model


  Model of the atom that suggested that electrons orbit the nucleus at set distances
• Beta Particle


  A high speed electron that a nucleus emits when a neutron converts into a proton
• Becquerel


  The unit of radioactive activity
• Count-Rate


  The number of decays that a detector measures per second
• Atomic Number


  The number of protons found in an atom of a specific element
• Alpha Particle


  A positively charged particle consisting of two protons and two neutrons
• It is very difficult to sustain the extreme temperatures and pressures required for fusion reactions
• Conditions of fusion


  
  

  Fusion does not happen at low temperatures or pressures

  • due to electrostatic repulsion of protons
  • protons need to get extremely close together

  If they are close enough or travelling fast enough:

  • some can overcome the electrostatic repulsion and fuse
• Nuclear fusion occurs...

  • When small nuclei combine to form a larger one

  (Fission is the opposite)
• Nuclear fusion is the energy source for stars
• Nuclear fusion

  Creation of larger nuclei resulting in a loss of mass from smaller nuclei accompanied by a release of energy
• The products of nuclear fission are radioactive
• Electricity 


  • Generated by using the energy released from the core
  • Transferred by a coolant, pumped through a reactor

  Coolant is pumped to a heat exchanger

  • used to make steam
  • drives a turbine which turns a generator
  • electricity
• Nuclear reactor

  • Fuel made into fuel rods as fission reactions occur
  • Neutrons leave the fuel rods at high speed

  Inside the core of reactor:

  • fuel rods are inserted into holes in moderator
  • slows down neutrons
• The chain reaction of nuclear fission can be controlled if moderator and other materials absorb some of the neutrons
• A control rod is moved upwards to increase the number of neutrons absorbed and decrease rate of fission
• Moderators slow down fast moving neutrons so they have more chance of causing further fissions
• Control rods contain boron or cadmium that absorbs neutrons
• Uncontrolled nuclear chain reaction


  Neutrons released by nuclear fission are absorbed by other Uranium-235 nuclei

  • Become unstable
  • Release more neutrons
  • Turns into an uncontrolled nuclear chain reaction
• The Fission of U-235 


  Produces two daughter nuclei

  • emission of two or more neutrons
  • release of energy

  Both the daughter nuclei and the neutrons store a lot of kinetic energy

  • moving at high speeds
• Sources of energy


  Nuclear reactions

  • fission, fusion and radioactive decay
• Advantages of Nuclear Power


  • Store a lot more energy per kg
  • Don't burn so they do not need air to allow them to release energy
  • Don't produce carbon dioxide
  • Could last a lot longer than other non-renewables
• Disadvantages of Nuclear Power


  Non-renewable (finite)

  Produce radioactive waste which will stay radioactive for years

  • Can be dangerous
• Isotopes in PET scanners

  Must be produced close to the hospital

  Have a very short half life

  • lose their radioactivity quickly
• PET Scanners

  Tracers emit positrons used to detect medical problems

  • positron meets an electron

  Both are destroyed

  • two gamma rays are emitted in opposite directions

  PET scanner detects the gamma radiation
• Internal/External Radiography

  Cancer cells divide more rapidly

  • more susceptible to be killed by radiation

  Internal

  • uses beta emitter placed inside body

  External

  • uses gamma emitter with rays directed at the tumour
• Effects of exposure to radiation

  Irradiated

  • exposed to radiation from nearby materials
  • when they move away this stops

  Contaminated

  • exposed to the radiation as the unstable isotopes in the material decay
  • continue until this is removed (not always possible)
• Radioactive waste

  • Remains hazardous and radioactive for long periods of time
  • Must be secured safely
  • Effects will last longer with longer half lives than materials with shorter half lives
• Precautions with radiation

  • Increasing distance from the source
  • Shielding source
  • Minimising time spent with it
  • Exposure measured with dosimeter badges
• Ionising radiation can also cause tissue damage
• Ionising radiation

  • Small amounts of ionising radiation over long periods of time can damage the DNA inside a cell called a mutation
  • Some mutations can cause the cell to malfunction and may cause cancer