Radioactive Decay + half-life

• Activity

  The overall rate of decay of all the isotopes in a sample, measured in becquerels (decays per second)
• Half-life

  The time taken for the number of radioactive nuclei in a sample to half, or for the activity to half
• Radioactive decay

  1. Unstable isotope emits radiation (alpha, beta, gamma) to become more stable

  2. Decay process is random, cannot predict when individual isotope will decay

  3. With a large sample, overall rate of decay (activity) can be measured

  4. As sample decays, number of unstable nuclei and activity decrease, but rate of decrease also falls
• Fewer radioactive nuclei

  Lower activity
• Geiger-Muller tube

  • Measures count rate of radiation, used to estimate activity
• Calculating half-life

  1. Find time for activity to halve (e.g. 600 to 300 becquerels)

  2. Or find time for number of radioactive nuclei to halve
• Example: Radioactive sample with 3 million nuclei and 40 hour half-life, after 5 days (120 hours) there will be 375,000 nuclei remaining