2.8 physics

• Radioactive decay


  Random in nature, not possible to predict when a specific individual atom will decay
• Half-life


  The time it takes for the number of nuclei of a radioactive element to halve or the time it takes for the count-rate of radioactive radiation to halve
• Half-life is unique to each radioactive element and can range from a few minutes to millions of years
• Many repeat readings over a long period of time are required to calculate an accurate estimate of half-life
• Determining half-life


  1. Graphs of count-rate against time can be used

  2. Every time the count-rate halves, a half-life has passed
• Common radioactive half-lives


  • Uranium-235: 704 million years
  • Plutonium-239: 24,110 years
  • Carbon-14: 5,730 years
  • Technetium-99m: 6 hours
  • Polonium-218: 3 minutes
• Uses of radioactive materials


  • Measuring thickness
  • Sterilisation
  • Smoke alarms
  • Medical tracers
  • Cancer treatment
  • Carbon dating
• Measuring thickness


  Beams of beta radiation can be used to monitor thickness, any variation in thickness will mean the level of radiation picked up will differ
• Sterilisation


  Gamma radiation can be used to kill microbes and sterilise medical equipment
• Smoke alarms


  Alpha radiation is used, it ionises the air allowing an electrical current to flow between two electrodes, if smoke is present it will block this current, break the circuit and set off the alarm
• Medical tracers


  Radioactive materials can be used inside the body to help image organs or monitor fluid flows, Technetium-99m is a gamma source with a half-life of just 6 hours, it is injected into blood vessels to check for arterial blocks or internal bleeds
• Cancer treatment


  • Gamma radiation is directed towards cancerous cells inside the body to kill them and prevent them from spreading, long half-life sources are used so they are powerful enough and don't need to be regularly changed
  • Alpha radiation is used to kill cancerous cells inside the body by directly injecting into the tumour, the strong ionising power of alpha will kill the cell but its weak penetrating power means it will not transfer through the body to other healthy cells
• Carbon dating


  Radioactive carbon-14 is used to date organic ancient artefacts, living plants and animals absorb carbon-14 when alive, but stop when they die, therefore isotope decay takes place which follows a typical decay curve for carbon-14, this can be used to work out how many half-lives have passed since the animal/plant from which the artefact was made died, providing an estimate of how old the object is