4.2.3 Half-lives & The Random Nature of Radioactive Decay

• Radioactive decay is random.
• What is the half-life of a radioactive isotope?

  The time it takes for the number of nuclei of the isotope in a sample to halve, or the time it takes for the count rate (or activity) from a sample containing the isotope to fall to half its initial level.
• 'Activity' is the overall rate of decay of all of the radioactive isotopes in our sample.
• True or false? As a radioactive sample decays over time, its half life decreases.

  False.
• The graph shows a radioactive decay curve. What is the half-life of the material?

  2s.

  
• The graph shows three different radioactive materials. Which graph shows the radioactive material with the shortest half-life?

  A.

  
• The activity of a radioactive material started at 240 Bq and is now 30 Bq. How many half-lives have elapsed?

  3 half-lives.
• The activity of a radioactive material is 800 Bq and its half-life is 5 hours. What is the activity of the radioactive material after 15 hours?

  100 Bq.
• A radioactive isotope has a half-life of 10 minutes. If the activity was initially 800 Bq, calculate how long it would take for the activity to fall to 50 Bq. 

  40 mins.