In Libbys radiocarbon dating technique， the weak radioactive emissions （放射） from his decay process are counted by instruments such as a radiation detector and counter. he decay rate is used to determine the proportion of C-14 atoms in the sample being dated.

Carbon-14 is produced in the Earths atmosphere when nitrogen （氮）-14， or N-14，

interacts with cosmic rays （宇宙射线）。 Scientists believe since the Earth was formed， the mount of nitrogen in the atmosphere has remained constant. Consequently， C-14 formation is thought to occur at a constant rate. Now the ratio of C-14 to other carbon toms in the atmosphere is known. Most scientists agree that this ratio is useful for dating items back to at least 50，000 years.

All life on Earth is made of organic molecules （分子） that contain carbon atoms coming from the atmosphere. So all living things have about the same ratio of C-14

atoms to other carbon atoms in their tissues （组织）。 Once an organism （有机体） dies it tops taking in carbon in any form， and the C-14 already present begins to decay. Over time the amount of C-14 in the material decreases， and the ratio of C-14 to other carbon toms goes down. In terms of radiocarbon dating， the fewer C-14 atoms in a sample， the older that sample is.

16　Nowadays many scientists depend on radiocarbon for dating age-old objects

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