♡ Topic 6_Radioactivity ♡

• Activity of an unstable nucleus is the rate at which a source of unstable nuclei decays
• Unit of radioactive activity is Becquerel (Bq)
• Count-rate is the number of radioactive decays per second for a radioactive source
• Example of a detector that may be used to measure count-rate is Geiger-Muller tube
• Nature of radioactive decay:

  • Random
  • Which nuclei decays and when is determined only by chance
  • It is impossible to predict which nuclei will decay and when
• Half-life of a radioactive isotope:

  • The time it takes for the number of unstable nuclei in a substance to halve
  • The time it takes for the count rate from a sample to fall to half its initial level
• Advantages of nuclear power for generating electricity:

  • Nuclear fuels do not produce carbon dioxide
  • The fuel is readily available, reducing strain on fossil fuel supplies
  • Less nuclear fuel is needed to produce the same amount of energy as burning fossil fuel
  • Does not contribute to global warming
• Disadvantages of using nuclear power to generate electricity:

  • Unpopular - the public perceives nuclear power as very dangerous
  • Security risks as radioactive substances can be useful for terrorists
  • Expensive to commission and decommission stations, which may be bad for a country's economy
  • Radioactive waste can be difficult to dispose of and remains radioactive for many years, posing dangers to humans and the environment
  • Risk of nuclear accidents
• Nuclear fission is the splitting of large, unstable nuclei to form smaller, more stable nuclei along with the emission of spare neutrons
• To induce fission, the unstable nuclei must absorb a neutron. Spontaneous fission, where no neutron absorption occurs, is rare
• In a fission reaction, alongside two smaller nuclei, two or three neutrons, gamma rays, and energy are emitted
• A common fissile nuclei is Uranium-235
• Three main components of the core of a nuclear reactor:

  1. Fuel rods

  2. Control rods

  3. Moderator
• During a chain reaction in a nuclear reactor, an unstable nucleus absorbs a neutron, undergoes fission, releases 2 or 3 further neutrons, and induces more fission, resulting in a chain reaction
• The consequence of an uncontrolled chain reaction is that the rate of fission events becomes too high, producing too much energy, which can lead to a nuclear explosion
• The chain reaction in a fission reactor is kept under control by positioning control rods between the fuel rods. The rate of fission is controlled by moving these rods up and down, where the lower the rods are inserted, the slower the rate of fission
• The role of the moderator in a nuclear reactor is to slow down the neutrons so they are traveling at speeds that allow them to be absorbed by fissile nuclei and cause fission
• Electricity is produced in a nuclear power station by the reactions releasing thermal energy, which is used to boil water and produce steam. The steam then turns a turbine which starts the generator
• Nuclear fusion is when two light nuclei join to produce a heavier nucleus and release energy
• Two isotopes of hydrogen commonly used in nuclear fusion are Deuterium and Tritium
• Nuclear fusion releases more energy than nuclear fission
• Generating energy through nuclear fusion is difficult because it requires very high temperatures, which in itself requires large quantities of energy. Currently, the production of fusion results in a net loss of energy
• An example of where fusion occurs is in the sun, as stars use fusion as their energy source
• Nuclear fusion requires a lot of energy because both nuclei are positive, requiring a significant amount of energy to overcome the electrostatic repulsion between them
• Four types of nuclear radiation:

  • Alpha particles
  • Beta particles
  • Gamma rays
  • Neutrons
• Background radiation:

  • Radiation that is always present
  • Present in very small amounts and not harmful
• Four sources of background radiation:

  • Rocks
  • Cosmic rays from space
  • Nuclear weapon testing
  • Nuclear accidents
• Methods to measure and detect background radiation:

  • Photographic film
  • Geiger-Muller counter
• Alpha particles:

  • Consist of two protons and two neutrons
  • Same as a helium nucleus
  • Range through air is a few centimetres (2-10cm)
• Beta radiation can be blocked by:

  • A thin sheet of aluminium
  • Several metres of air
• Gamma radiation can be blocked by:

  • Several centimetres of lead
  • A few metres of concrete
• Alpha radiation is the most ionising
• Gamma emission does not affect the mass or charge of an atom
• Gamma radiation is the least ionising
• Plum-pudding model of the atom:

  • Sphere of positive charge with electrons distributed evenly
• Prior to the discovery of the electron, the atom was believed to be indivisible
• Rutherford's alpha-Scattering experiment:

  • Alpha particles fired at a thin sheet of gold foil
  • Conclusions: most of an atom is empty space, nucleus has a positive charge, most mass is in the nucleus
• Beta plus decay:

  • Proton turns into a neutron and a positron
• Beta minus decay:

  • Neutron changes into a proton and an electron
• When alpha decay occurs:

  • Atomic number decreases by 2
  • Mass number decreases by 4
  • A new element is formed