Radioactive Decay and Half Life

• What is the purpose of radioactive decay?

  To become more stable
• What is a radioactive material?

  Materials that contain unstable isotopes
• How can we predict decay in a large sample of isotopes?

  By measuring the overall activity
• How is activity measured?

  In becquerels
• What does one becquerel represent?

  One decay per second
• What is half-life?

  Time taken for nuclei or activity to halve
• If a sample has an activity of 600 decays per second, how many isotopes are decaying each second?

  600 isotopes
• How does the decay process appear over time?

  It becomes slower as particles decay
• Why does the activity of a sample decrease over time?

  Fewer particles remain to decay
• What is the relationship between half-life and activity?

  • Half-life is the time for nuclei to halve
  • Activity halves as the number of nuclei halves
  • Both concepts are correlated
• How is the decay process visually represented?

  By a graph plotting activity against time
• What does the curve on the decay graph indicate?

  Activity declines at a decreasing rate
• How do you calculate half-life from a graph?

  Find time for activity to halve
• If the activity drops from 600 to 300, how long did it take?

  About two hours
• How many half-lives occur in 120 hours if the half-life is 40 hours?

  Three half-lives
• If you start with 3 million nuclei, how many remain after three half-lives?

  375,000 nuclei
• What device is used to measure activity in radioactive samples?

  Geiger-Muller tube
• What does a Geiger-Muller tube record?

  Decays that reach it each second
• What is the count rate in relation to activity?

  Count rate estimates the activity
• How do you determine the remaining nuclei after multiple half-lives?

  1. Calculate total time in hours
  2. Divide by half-life to find half-lives
  3. Start with initial nuclei count
  4. Halve the count for each half-life