Question 43.8: The Age of Iceman In 1991, German tourists discovered the we...
The Age of Iceman
In 1991, German tourists discovered the well-preserved remains of a man, now called “Ötzi the Iceman,” trapped in a glacier in the Italian Alps. Radioactive dating with {}^{14} C revealed that this person was alive approximately 5 300 years ago. Why did scientists date a sample of Ötzi using {}^{14} C rather than {}^{11} C, which is a beta emitter having a half-life of 20.4 min?
Because {}^{14} C has a half-life of 5 730 yr, the fraction of {}^{14} C nuclei remaining after thousands of years is high enough to allow accurate measurements of changes in the sample’s activity. Because {}^{11} C has a very short half-life, it is not useful; its activity decreases to a vanishingly small value over the age of the sample, making it impossible to detect.
An isotope used to date a sample must be present in a known amount in the sample when it is formed. As a general rule, the isotope chosen to date a sample should also have a half-life that is on the same order of magnitude as the age of the sample. If the half-life is much less than the age of the sample, there won’t be enough activity left to measure because almost all the original radioactive nuclei will have decayed. If the half-life is much greater than the age of the sample, the amount of decay that has taken place since the sample died will be too small to measure. For example, if you have a specimen estimated to have died 50 years ago, neither {}^{14} C (5 730 yr) nor {}^{11} C (20 min) is suitable. If you know your sample contains hydrogen, however, you can measure the activity of {}^{3} H (tritium), a beta emitter that has a half-life of 12.3 yr.