SPEED DATING

摘要

The fact that different nuclei decay at different rates makes some of them tremendously useful in determining the age of various objects. A familiar example is the radiocarbon dating of organic materials. Carbon consists overwhelmingly of the stable isotope carbon-12, but living things contain a small amount of carbon-14. This isotope, with a half-life of 5730 years, is made at a steady rate in Earth's atmosphere in cosmic-ray collisions. It is incorporated into plants via photosynthesis and then into animals when they eat those plants.As soon as the organism dies, this uptake stops. While the carbon-12 remains unchanged, the carbon-14 gradually decays and is not replaced. Thus, if you measure the ratio of carbon-14 to carbon-12, you get a measure of the time since death. Performing radiocarbon dating used to require large samples because there are, even initially, only a small number of carbon-14 nuclei, and those that are there don't decay very rapidly.