# Carbon dating definition

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Ever wondered how scientists know the age of old bones in an ancient site or how old a scrap of linen is?The technique used is called carbon dating and in this lesson we will learn what this is and how it is used. Carbon dating, or radiocarbon dating, is a method used to date materials that once exchanged carbon dioxide with the atmosphere. In the late 1940s, an American physical chemist named Willard Libby first developed a method to measure radioactivity of carbon-14, a radioactive isotope.

Carbon dioxide in the atmosphere contains a constant amount of carbon-14, and as long as an organism is living, the amount of carbon-14 inside it is the same as the atmosphere.However, once the organism dies, the amount of carbon-14 steadily decreases.By measuring the amount of carbon-14 left in the organism, it's possible to work out how old it is.This technique works well for materials up to around 50,000 years old.Each radioactive isotope decays by a fixed amount, and this amount is called the half-life.The half-life is the time required for half of the original sample of radioactive nuclei to decay.

For example, if you start off with 1000 radioactive nuclei with a half-life of 10 days, you would have 500 left after 10 days; you would have 250 left after 20 days (2 half-lives); and so on.The half-life is always the same regardless of how many nuclei you have left, and this very useful property lies at the heart of radiocarbon dating. The graph below shows the decay curve (you may recognize it as an exponential decay) and it shows the amount, or percent, of carbon-14 remaining.You will notice that after around 40,000 years (or 8 half-lives), the amount left is starting to become very small, less than 1%.Scientists often use the value of 10 half-lives to indicate when a radioactive isotope will be gone, or rather when a very negligible amount is still left.This is why radiocarbon dating is only useful for dating objects up to around 50,000 years old (about 10 half-lives).Radioactive carbon-14 is continually formed in the atmosphere by the bombardment of cosmic ray neutrons on nitrogen-14 atoms.