radioactive decay, half life, types of radiation

• Activity


  The overall rate of decay of all the isotopes in a sample, measured in becquerels (decays per second)
• Half-life


  The time taken for the number of radioactive nuclei in a sample to half, or for the activity to half
• Radioactive decay


  1. Unstable isotope emits radiation (alpha, beta, gamma) to become more stable

  2. Decay process is random, cannot predict when individual isotope will decay

  3. With a large sample, overall rate of decay (activity) can be measured

  4. As sample decays, number of unstable nuclei and activity decrease
• As number of radioactive nuclei decreases


  The rate of decay (activity) also decreases
• Geiger-Muller tube


  • Measures the count rate of radiation, which is used to estimate the activity
• Radioactive samples


  • Sample 1 with half-life of 2 hours
  • Sample 2 with half-life of 1 hour
• Calculating half-life from a graph


  1. Find the time for activity to halve (e.g. 600 to 300 Bq)

  2. Confirm by finding time for activity to halve again (e.g. 300 to 150 Bq)
• The half-life of a radioactive source is 40 hours, and there are initially 3 million radioactive nuclei in the sample. After 5 days, how many nuclei will remain?
• Solving the half-life problem


  1. Convert 5 days to 120 hours

  2. Divide 120 hours by 40 hour half-life to get 3 half-lives

  3. Half the initial 3 million nuclei 3 times to get 375,000 nuclei remaining
• Isotopes


  Different forms of the same element, with the same number of protons but different numbers of neutrons
• Only one or two of an element's isotopes are stable, while the rest are unstable and undergo radioactive decay</b>
• Radioactive


  Consisting of unstable isotopes that can decay
• Types of nuclear radiation


  • Alpha particles
  • Beta particles
  • Gamma rays
  • Neutrons
• Alpha particles


  • Made up of two protons and two neutrons, the same as a helium nucleus
  • Don't have any electrons, so have an overall charge of 2+
  • Easily stopped by collisions, can only travel a few centimeters in air and are absorbed by a single sheet of paper
  • Strongly ionizing, can easily knock electrons off atoms they collide with
• Beta particles


  • Just electrons
  • Emitted when a neutron in the nucleus decays into a proton and an electron
  • Moderately ionizing and penetrate moderately far, stopped by several meters of air or 5mm of aluminium
• Gamma rays


  • Waves of electromagnetic radiation, not particles
  • Often emitted after alpha or beta radiation to get rid of extra energy
  • Don't have any mass or charge, so pass straight through materials
  • Weakly ionizing, can penetrate far into materials before being stopped, require thick lead or concrete to stop them
• Neutron emission


  • Occurs when a nucleus contains too many neutrons, making it unstable