Half Life

• What is the half life of an isotope?

  time taken for the activity or count rate of a sample to decay by half or the time taken for half the nuclei in a sample to decay
• So if 80 atoms falls to 20 over 10 minutes, what's the half life?

  80/2 = 40

  40/2 = 20 - so two half lives in 10 mins

  half-life is 5mins
• A short half-life?

  • source presents less of a risk, as it does not remain strongly radioactive
  • so this means initially it is very radioactive, but quickly dies down
  • so presents less of a long-term risk
• A long half-life?

  • source remains weakly radioactive for a long period of time
  • weak activity means it won't be harmful to anyone
• Americium has a long half-life of 432 years - smoke alarms

  • it is an alpha emitter, and used in smoke alarms
  • emitted into the air around the alarm, and does not reach far because alpha is weakly penetrating
  • if smoke reaches the alarm, the amount of alpha particles in the surrounding air drops
  • this causes the alarm to sound