Half-Life

• Half-Life


  How quickly unstable nuclei decay, measured using activity and half-life
• Radioactivity is a Totally Random Process
• Radioactive substances give out radiation from the nuclei of their atoms
• Measuring radioactivity


  Use a Geiger Muller tube and counter to record the count-rate (number of radiation counts reaching it per second)
• Radioactive decay is entirely random, so you can't predict exactly which nucleus will decay next or when
• Half-life


  The time it takes for the amount of radiation emitted by a source to halve
• Activity


  The rate at which a radioactive source decays, measured in becquerels (Bq, where 1 Bq is 1 decay per second)
• Each time a radioactive nucleus decays


  The activity as a whole will decrease
• For some isotopes it takes just a few hours before nearly all the unstable nuclei have decayed, whilst others last for millions of years
• The activity never reaches zero, which is why we have to use the idea of half-life to measure how quickly the activity drops off
• Half-life


  The time taken for the number of radioactive nuclei in an isotope to halve
• Half-life


  The time taken for the activity, and so count-rate, to halve
• Short half-life


  • Activity falls quickly, because the nuclei are very unstable and rapidly decay
  • Sources can be dangerous due to high initial radiation, but quickly become safe
• Long half-life


  • Activity falls more slowly because most of the nuclei don't decay for a long time
  • Source just sits there, releasing small amounts of radiation for a long time, which can be dangerous
• Calculating half-life from an activity-time graph

  Find the time interval on the x-axis corresponding to a halving of the activity on the y-axis
• Calculating final activity as a percentage of initial activity after two half-lives

  • Initial activity 640 Bq

  After 1 half-life: 640/2 = 320 Bq

  After 2 half-lives: 320/2 = 160 Bq

  Final activity as % of initial: (160/640) x 100 = 25%