The half-life of a radioisotope refers to the time taken for half of the nuclei present in any given 
     sample to decay. Since the activity of a radioactive source is proportional to the number of 
     undecayed nuclei, therefore the half-life can be also defined as the time taken for the  [activity]
     of it to fall to half of its initial value. For example, if the half-life of iodine-131 is 8 days, and 
     that a particular specimen of iodine-131 has 64 grams of the isotope on day 0, only 32 grams 
     of iodine-131 will be left on day 8, 16 grams on day 16, 8 grams on day 32 and so on. 
     Therefore, an equation for calculating the half-lives of any radioisotopes can be derived:

Na = No x (0.5) a

      Where No is the initial activity/ amount of radioisotope, Na is the final activity and a is the 
      number of half-lives.

      Since nuclear reactions are independent of external conditions such as temperature and 
      pressure, the half-life of a particular radioisotope is specific. This property makes it 
      convenient for us to find out how long a particular radioisotope will remain radioactive, 
      and to identify unknown radioisotope by virtue of its unique half-life.

      If we plot the number of undecayed nuclei, or the activity of a radioisotope, against time, 
      a decay curve can be obtained:

Typical decay curve 

      Note that the curve is tends to constant as time tends to infinity. This corresponds to the 
      amount of background radiation. If the background count rate is subtracted from the 
      recorded count rate, a corrected count rate will be obtained.


 *back*                                                                                        (C) 1999 ThinkQuest Team 27954