Radioactive decay & half-life

• Background Radiation


  The safe level of radiation that is around us all of the time.
• 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


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


  • Measures count rate of radiation (decays reaching the detector)
  • Used to estimate the activity of a radioactive sample
• Samples with shorter half-lives have higher initial activities and decay more rapidly
• Calculating remaining radioactive nuclei after multiple half-lives


  1. Find number of half-lives elapsed

  2. Halve the initial number of nuclei that many times