Half-life

1. Radioactive materials by their very nature, decay via various methods to form other elements, ultimately to form stable non-radioactive isotopes.
2. If you monitor a radioactive substance with a Geiger counter, you will hear "clicks" every time the device detects a particle of radiation. Over time, if you graphed the number of clicks per second you would see a graph resembling something like you see below:
   
3. Depending on how quickly an element decays, the time needed to make the above graph could take anywhere from a fraction of a second to billions of years.
4. To see a visual depiction of how this graph is formed go to: http://www.colorado.edu/physics/2000/isotopes/radioactive_decay3.html
5. One way to describe how fast a radioactive substance decays is to determine how long it will take half of the currently present atoms to decay. Let's look at a particular example - Iodine-131 which is used to diagnose problems with the thyroid gland.
   
   By looking at the graph you can see that half of the Iodine-131 has decayed every 8 days. We then say that Iodine-131 has a "half-life" of 8 days.
   
   

   If you started with 80 grams of radioactive Iodine-131, how long would it take until you had less than 5 grams left?

   
   
6. The shorter the half-life the more unstable the atoms of that isotope are, and the faster they will decay.