6.4 - Radioactive Decay & Half-life

• What happens to unstable isotopes to become more stable?

  They decay by emitting radiation
• What do we call materials that contain unstable isotopes?

  Radioactive materials
• Why can't we predict when a single radioactive isotope will decay?

  The decay process is completely random
• What can we determine from a large sample of radioactive isotopes?

  The activity and half-life of the sample
• How is activity measured?

  In becquerels
• What does one becquerel represent?

  One decay per second
• If a sample has an activity of 600 becquerels, how many isotopes decay each second?


  600 isotopes decay each second
• What is half-life defined as?

  The time for radioactive nuclei to halve
• How can half-life also be defined in terms of activity?

  As the time taken for activity to halve
• If a sample starts with 1 million unstable nuclei, how many will remain after one half-life?

  500,000 unstable nuclei
• What happens to the overall rate of decay as more isotopes decay?

  The overall rate of decay decreases
• Why does the decay graph curve rather than form a straight line?

  The rate of decay decreases over time
• How do you calculate half-life from a decay graph?

  Find the time for activity to halve
• What device is used to find the activity of a radioactive sample?

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

  Decays that reach it each second
• If the half-life of a radioactive source is 40 hours, how many half-lives are in 120 hours?

  Three half-lives
• How many radioactive nuclei remain after three half-lives from 3 million?

  375,000 radioactive nuclei
• What are the key concepts of activity and half-life in radioactive decay?

  • Activity: overall rate of decay
  • Measured in becquerels
  • Half-life: time for nuclei or activity to halve
  • Decay process is random
  • Fewer nuclei lead to lower activity
• How do you calculate the remaining radioactive nuclei after multiple half-lives?

  1. Determine total time in hours
  2. Divide by half-life to find half-lives
  3. Start with initial nuclei
  4. Halve the number of nuclei for each half-life
• Which of the following describes the decay process for a single atom?

  The decay occurs at regular intervals

  The decay is random

  • The decay is random
  • There is no way to know exactly when an individual atom will decay, it occurs randomly. 
• 'Activity' is the overall rate of decay of all of the radioactive isotopes in our sample.
• We measure radioactive activity as the average number of decays per second.

  What are the units of activity?

  Becquerels (Bq)
•  define for the 'half-life' of a radioactive sample?

  • The time taken for the number of radioactive nuclei in a sample to halve

  OR

  • The time taken for the activity to half
• True or false? As a radioactive sample decays over time, its half-life decreases.

  False
• What device is used to record radioactive decays?

  Geiger-Muller tube and counter