Half-Life

• Half-Life

  • It is impossible to know when a particular unstable nucleus will decay
  • But the rate at which the activity of a sample decreases can be known
  • This is known as the half-life
  • Half-life is defined as:The time it takes for the number of nuclei of a sample of radioactive isotopes to decrease by half
  • In other words, the time it takes for the activity of a sample to fall to half its original level
  • Different isotopes have different half-lives and half-lives can vary from a fraction of a second to billions of years in length
• Using Half-life

  • Scientists can measure the half-lives of different isotopes accurately:
  • This means it would take 704 million years for the activity of a uranium-235 sample to decrease to half its original amount
  • Uranium-235 has a half-life of 704 million years
• Uranium atoms decay whilst the number of lead atoms increases

  
• • Carbon-14 has a half-life of 5700 years
  • So after 5700 years, there would be 50% of the original amount of carbon-14 remaining
  • After two half-lives, or 11 400 years, there would be just 25% of the carbon-14 remaining
  • With each half-life, the amount remaining decreases by half