Half Life
If something is radioactive, will it stay radioactive forever?
As time goes by radioactive things emit less and less radiation. The number of unstable atoms is getting less and less as they decay.
| Imagine a dice. To start with it is unstable. Roll it again and again
until you roll a 6, then it becomes stable.
Now imagine you have a million dice. Whenever a dice falls on 6 then it becomes stable. The amount of radiation given off every second is equal to the number of atoms that decay every second. What will happen to this as time goes by? (the daughter atom isn't usually stable but lets keep it simple for now!)
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Below is a graph of the count rate produced by a certain radioisotope against time. It is impossible to say how long it will take to completely run out of radiation. It is useful, however, to know the "half life" of an isotope. This is the time it takes for half of the atoms in a sample to decay. It is also the time for the count rate to halve.
Different isotopes have different half lives. It is something you can look up in a book. Here are a few. Notice that different isotopes can have very different half lives.
| Isotope | Half Life |
| Cobalt 60 | 5.3 years |
| Strontium 90 | 28.8 years |
| Oxygen 15 | 122 seconds |
| Chlorine 45 | 0.4 seconds |
| Carbon 14 | 5.7 thousand years |
| Uranium 238 | 4.5 billion years |
