half life

• Half-life


  The time it takes for the number of nuclei of a radioactive isotope in a sample to halve
• Radioactive decay


  • Scientists cannot predict when a nucleus will decay
  • Decay is a random process
• Longer half-life


  Radioactive isotope decays relatively slowly
• Shorter half-life


  Radioactive isotope decays relatively quickly
• Half-life


  The time it takes for the count rate or activity from a sample containing the isotope to fall to half its initial level
• Determining half-life from a graph

  1. Start with 1000 undecayed nuclei

  2. Half of 1000 is 500

  3. Find the time it takes for the number of nuclei to fall from 1000 to 500
• Calculating decrease in count rate after a given number of half-lives

  1. Radioactive isotope has a half-life of 15 days

  2. Initial count rate is 200 counts per second

  3. After 1 half-life (15 days), count rate is 100 counts per second

  4. After 2 half-lives (30 days), count rate is 50 counts per second

  5. After 3 half-lives (45 days), count rate is 25 counts per second