Half-Life

• The half-life of a particular isotope is defined as the time taken for half the nuclei of that isotope in any sample to decay
• The rate at which the activity of a sample decreases is measured in terms of half-life

  • This is the time it takes for the activity of a sample to fall to half its original level
• This is the time it takes for the activity of the sample to decrease from 100 % to 50 %

  • It is the same length of time as it would take to decrease from 50 % activity to 25 % activity
• Different isotopes have different half-lives and half-lives can vary from a fraction of a second to billions of years in length

  • The half-life is constant for a particular isotope
• Half-life can be determined from an activity–time graph
• The graph shows how the activity of a radioactive sample changes over time. Each time the original activity halves, another half-life has passed

  A) dropped

  B) half

  C) dropped

  D) quarter

  E) 1 half life

  F) 2 half lives

  
• Half-life can also be represented on a table

  • As the number of the half-life increases, the proportion of the isotope remaining halves
• Table showing the number of half-lives to the proportion of isotope remaining

  
• To find the time for the half-life find half of the activity first

  A) initial

  B) dropped

  C) half

  D) dropped

  E) quarter

  F) 1 half life

  G) 2 half lives

  
• To remove background radiation from the decay curve:

  • Start by measuring the background radiation (with no sources present) – this is called the background count
  • Then carry out the experiment
  • Subtract the background count from each reading, to provide a corrected count
  • The corrected count is your best estimate of the radiation emitted from the source and should be used to measure its half-life
• When measuring radioactive emissions, some of the detected radiation will be background

  A) background radiation

  
• When looking for the corresponding time for the activity, it is good practice to draw a line on the graph  with your ruler like is done in the mark scheme of the worked example

  • This ensures you're reading the most accurate value possible.