Half life

• Half-life


  The time it takes for the activity of a radioactive source to halve, or the time it takes for half the radioactive nuclei to decay
• The half-life for the radioactive source shown on the graph is 100 days
• The time it takes for the activity to halve from 800 to 400 counts per minute (cpm) is 100 days, it takes the same time to halve from 400 to 200 cpm and from 200 to 100 cpm
• The half-life of a radioactive isotope is always the same
• Calculations using half-life


  1. At the start, 100% of the source has not decayed

  2. After 1 half life this will be 50%

  3. After 2 half lives 25%

  4. After 3 half lives 12.5%
• You can use the half-life of a material to calculate the age of a sample or to predict the amount of a sample that will be left after a certain time
• Strontium-90 has a half-life of 29 years
• Three half-lives of strontium-90 would be 3 × 29 years = 87 years
• Uses of radioactive materials

  • Tracer
  • Treating cancer inside the body
  • Treating cancer from outside the body
  • Monitoring the thickness of paper or foil
  • Smoke detectors
• Tracer


  • Weakly ionising so it will have less effect on the cells in the body
  • Very penetrating so it can pass through the body to be detected by the camera
• Treating cancer inside the body

  • Strongly ionising so it will kill the cells it is absorbed by
  • Moderately penetrating so it can pass through the whole tumour to kill all the cells without affecting healthy cells
• Treating cancer from outside the body

  • Very penetrating so can pass through the body to reach the cancer cells
  • Weakly ionising so it has less effect on the healthy cells
• Monitoring the thickness of paper or foil

  • Moderately penetrating, can pass through the paper but the count would be affected by the thickness
• Smoke detectors


  • Very ionising to create the current
  • Least penetrating so will be blocked by smoke particles