Radioactive decay is a random process. A block of radioactive material will contain many trillions of nuclei and not all nuclei are likely to decay at the same time so it is impossible to tell when a particular nucleus will decay. It is not possible to say which particular nucleus will decay next, but given that there are so many of them, it is possible to say that a certain number will decay in a certain time. Scientists cannot tell when a particular nucleus will decay, but they can use statistical methods to tell when half the unstable nuclei in a sample will have decayed. This is called the half-life. The illustration below shows how a radioactive sample is decaying over time. From the start of timing it takes two days for the count to halve from 80 down to
So, we use the time in which half of any of these unstable nuclei will decay. The half-life of a radioactive isotope is the time taken for half the unstable nuclei in a sample to decay.
Different isotopes have different half-lives. Plutonium has a half-life of 24, years but plutonium has a half-life of only The half-life of a particular isotope is unaffected by chemical reactions or physical changes.
For example, radioactive decay does not slow down if a radioactive substance is put in a fridge. Half-life can be used to work out the age of fossils or wooden objects. Living things absorb carbon dioxide and other carbon compounds.
How Carbon-14 Dating Works
Some of the carbon atoms are carbon, which is a radioactive isotope of carbon. Carbon has a half-life of about 5, years.
When a living thing dies, it stops absorbing carbon This means the amount of carbon will decrease over time. The amount left can be compared to currently living organisms and an approximate age given for the fossil.
For example, if an ancient dead tree contains half the expected amount of carbon, it must have died about 5, years ago.
How does carbon dating work bbc bitesize
It takes another two days for the count rate to halve again, this time from 40 to Note that this second two days does not see the count drop to zero, only that it halves again. A third, two day period from four days to six days see the count rate halving again from 20 down to This process continues and although the count rate might get very small, it does not drop to zero completely. The half-life of radioactive carbon is 5, years. If a sample of a tree for example contains 64 grams g of radioactive carbon after 5, years it will contain 32 g, after another 5, years that will have halved again to 16 g.S085LS20 Radioactivity
It should also be possible to state how much of a sample remains or what the activity or count should become after a given length of time. This could be stated as a fraction, decimal or ratio.
For example the amount of a sample remaining after four half-lives could be expressed as: This could then be incorporated into other data. So if the half-life is two days, four half-lives is 8 days. The half-life of cobalt is 5 years.
If there are g of cobalt in a sample, how much will be left after 15 years? As a ratio of what was present originally compared to what was left, this would be