Half life and decay

• Half-life


  The time it takes for the activity of a radioactive source to halve, or the time it takes for half the radioactive nuclei to decay
• The half-life of a radioactive isotope is always the same
• Calculations using half-life


  1. At the start, 100% of the source has not decayed

  2. After 1 half life this will be 50%

  3. After 2 half lives 25%

  4. After 3 half lives 12.5%
• Fission


  Fission reactions take place in nuclear reactors to produce energy. In this reaction a slow-moving neutron is absorbed by a uranium-235 nucleus which then splits into 2 smaller nuclei and releases 2 or 3 fast moving neutrons.
• You can use the half-life of a material to calculate the age of a sample or to predict the amount of a sample that will be left after a certain time
• Control rods


  • They absorb 2 out of the 3 neutrons released from each fission reaction and so will only allow one neutron to cause a reaction
• Moderator


  • It slows the neutrons released to ensure that they can be absorbed by the uranium nuclei to cause another reaction
• Fission

  One bigger nuclei splits up to two smaller nuclei e.g. 1n + uranium - Ba + Kr + 3n
• Uses of radioactive materials

  • Tracer
  • Treating cancer inside the body
  • Treating cancer from outside the body
  • Monitoring the thickness of paper or foil
  • Smoke detectors
• The symbol for a neutron, 1 n, is often used in these equations.
• Fusion


  two small nuclei are fused together to create one larger nuclei. This requires a lot of energy. e.g. in sun (H+H-He)
• One neutron starts the fission reaction and three neutrons are produced.
• In fusion, two hydrogen isotopes (deuterium and tritium) fuse to form a helium nucleus and a neutron.
• Smoke detectors


  • Very ionising to create the current
  • Least penetrating so will be blocked by smoke particles
• Alpha particles are helium nuclei (2 protons + 2 neutrons) with a mass number of 4 and an atomic number of 2.