half-life

• what is half-life ?

  the interval of time required for one-half of the atomic nuclei of a radioactive sample to decay
• what is half-life?

  the time taken for half the radionuclides to decay, this is the same as the time taken for the activity to fall to half the initial value.
• what affects half life?


  the isotope

  independent of temperature

  independent of pressure

  independent of chemical composition/reaction
• calculations using half-life 


  no. of half lives = original activity / current activity

  number of half lives = original mass / current mass

  time taken = number of half lives x time for half life

  net decline = fraction that has decayed

  1/2n (n=number of half lives)
• how does c-14 work in living things?

  c-14 beta decays in living things, its half life is 5730 years

  when dead, c-14 production ceases

  with time, activity due to c-14 reduces

  therefore time since death can be calculated

  c-14 to c-12 ratio is a long term constant

  c-14 is produces in the atmosphere.
• isotopes and their half-lives 


  pa-235, beta, 70seconds

  pu-239, alpha, 24000 years

  ra-226, gamma, 1600 years

  sr-90, beta, 29 years
• radioactive decay is entirely random and can be measured with a geiger-muller tube
• a short half-life means that radioactive activity falls quickly because the nuclei are very unstable and rapidly decay. these are dangerous because they emit a high amount of radiation at the beginning but they quickly become safe.
• a long half-life means that radioactivity falls slowly because the nuclei take longer to decay. this can be dangerous for nearby areas that are exposed to radiation for millions of years.
• to calculate half-life, divide the initial activity by two