Half life

• Activity


  The radiation emitted by a radioactive source decreases with time
• Even though radioactive decay is a random process we can predict how the radiation will change
• We can monitor how the activity changes with time
• Activity (in a radioactive source)

  The number of unstable nuclei that decay each second
• Activity measurement unit


  Measured in becquerels (Bq)
• 1 Bq is equal to 1 decay per second
• The activity of this radioactive source halves every 45 minutes
• The time taken for its activity to go from 600 to 300 counts per minute


  Is 45 minutes
• The time taken for its activity to go from 300 to 150 counts per minute

  Is 45 minutes
• Half-life


  The time it takes for the activity to fall to half its initial value
• Different radioactive isotopes have different half-lives
• Isotopes


  Atoms of the same element with the same number of protons and electrons, but a different number of neutrons
• You can use a graph of activity against time to measure the half-life
• Radioactive decay is a random process
• If you monitor the activity of a radioactive source over time you can plot a graph
• You can use the graph to work out the time taken for the activity to halve
• A radioactive isotope has a half-life of 15 hours
• The initial count rate is 400 counts per second
• Calculate the count rate after 15 hours
• Calculate the count rate after 60 hours
• A student monitored how the activity of a radioactive source changed with time
• Activity in counts per second

  • 160
  • 80
  • 40
  • 20
  • 10
  • 5
• Time in hours


  • 8
  • 12
  • 16
  • 20
• Determine how many hours it took for the activity to go from 160 to 80
• Calculate the half-life of the radioactive source