Half-life

• Half-life of a radioactive isotope

  The time it takes for the number of nuclei of the isotope in a sample to halve
• Determining the half-life of a radioactive isotope

  Observe the time it takes for the number of undecayed nuclei to halve
• Calculating the decrease in radioactive count rate after a given number of half-lives

  Count rate halves after each half-life
• Radioactive decay is a random process, so scientists cannot predict when a nucleus will decay
• The count rate is the number of decays per second recorded by a detector such as a Geiger counter tube
• Radioactive isotopes with different half-lives

  • Isotope with relatively long half-life
  • Isotope with relatively short half-life
• After each half-life, the count rate will have halved
• As the number of undecayed nuclei decreases


  The count rate decreases
• A radioactive isotope has an initial count rate of 200 counts per second and a half-life of 15 days
• After 45 days (3 half-lives), the count rate will have halved 3 times, resulting in a count rate of 25 counts per second