Physics 6.6

• Half-life


  The time it takes for the number of radioactive atoms to reduce by half. This process is random.
• Radioactive decay


  1. Cannot tell which individual atoms will decay

  2. After each half-life, half of the atoms have decayed
• Activity


  The number of atoms that disintegrate or decay in one second, measured in becquerels
• Technetium-99


  • Has a half-life of 6 hours
  • After 6 hours, activity is 49.5%
  • After 12 hours, activity is 24.75%
  • After 18 hours, activity is 12.375%
• Finding half-life


  1. Choose activity falls from 80 cpm to 40 cpm

  2. It takes 3 half-lives to fall from 80 cpm to 20 cpm

  3. 1 half-life is every 2 hours
• Radioactive decay modelling


  • Can be modelled by throwing coins or dice
  • Coin flips have a 1:2 probability, dice rolls have a 1:6 probability
  • Combining many groups reduces effects of anomalies
• Carbon dating


  • Living things absorb carbon-14 daily
  • When they die, carbon-14 levels decrease at a constant rate
  • Carbon-14 has a half-life of 5,730 years
  • Can be used to date objects up to 100,000 years old
• Uses of radiation


  • Blood and fluid tracers
  • Smoke alarms
  • Cancer treatment (external and internal)
  • Sanitising medical equipment
• Blood and fluid tracers


  • Use short half-life, highly penetrating gamma sources to locate leaks or blockages
• Smoke alarms


  • Use alpha-emitting americium isotope with long half-life
  • Alpha radiation ionises air, current flows between electrodes
  • Smoke blocks alpha, current drops, alarm sounds
• External cancer treatment


  • Use gamma radiation beams targeted at tumour
  • Individual beams not strong enough to damage healthy cells
  • Gamma source needs long half-life to maintain dose
• Internal cancer treatment


  • Use short half-life alpha sources injected directly into tumour
  • Alpha is highly ionising, kills cancer cells but not penetrating, stays within tumour
• Sanitising medical equipment


  • Gamma radiation kills microbes
  • Also used to extend freshness of fruits and vegetables