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In the body of every animal or plant, there is a special radioactive material that can be ready for countdown like a chronograph. It sounds amazing, but in fact, this is a normal natural phenomenon, this radioactive material is carbon-14 (C14). The introduction of carbon-14 into animals and plants is a tortuous process called "global carbon cycle", or "biosphere carbon exchange".

There are several carbon isotopes, carbon-12 and carbon-13 are stable, only carbon-14 is radioactive, it decays to half of its original value every 5730 years, which is its "half-life". Cosmic rays are responsible for the formation of carbon-14. In the atmosphere, cosmic ray particles collide many times to form slow neutrons. after they interact with atmospheric nitrogen atoms, the nitrogen atoms release a proton and become carbon-14. Carbon-14 is constantly formed in the atmosphere, and they combine with oxygen to form carbon dioxide. Carbon-14 enters the "global carbon cycle" in the form of carbon dioxide.

When animals and plants breathe, or when animals eat plants, they absorb carbon-14. Carbon-14 is stored in animals and plants and is finally balanced. When animals and plants die, breathing stops, the act of eating plants stops, and carbon exchange with the biosphere stops. As a result, carbon-14 in dead animals and plants is no longer balanced, and its radioactivity begins to decrease, that is, by half every 5730 years, so from the amount of carbon-14 left in the body, it is possible to infer when carbon-14 begins to decrease in dead animals and plants, thus deducing the time of death. However, the stock of carbon-14 in animals and plants is limited, and it can only be estimated to be 60, 000 years in this way.

The method of carbon-14 dating was first discovered by American chemist Willard Libby and others. Libby stayed as a lecturer after receiving his doctorate from the Department of Chemistry at the University of California, Berkeley in 1933.

In 1939, the radiation Laboratory in Berkeley opened a study to find a ubiquitous element with a long enough half-life in organic matter, which is intended to be used in biomedical research. Libby took on the project, and he quickly discovered that the half-life of carbon-14 was longer than previously thought. Then he found that carbon-14 could be produced continuously in the atmosphere and that it was the product of the interaction of slow neutrons in the atmosphere with nitrogen-14. But pundits at the time agreed that carbon-14 was produced by the interaction between deuteron and carbon-13, and the young man's conclusion was not immediately recognized.

In 1945, Libby came to the University of Chicago. The following year, he published a paper suggesting that there may be carbon-14 in the carbon contained in the bodies of animals and plants, and predicted that the element was stored in the body with carbon dioxide after breathing. To prove this idea, Libby's team needed to look for the decay of a large number of dead animals and plants. So they collected a lot of sludge with biogas from the ongoing Baltimore sewage project. The sludge exudes a rotten smell and is hated and puzzled by the people around it.

Willard Libby. It takes a long time to extract. The Libby team extracted methane from the sludge, concentrated the isotopes of the samples, and began a series of tests using the concentrated materials. Their results confirm that methane in biogas sewage does contain carbon-14, while methane in oil does not. It is also methane, and the comparison of the two samples confirms that the carbon-14 in petroleum methane has long since decayed and disappeared because it is more than 60,000 years old. They collected the results of a series of experiments and published them in the 1947 issue of the journal Nature. Libby found the best material for biomedicine through experiments. Carbon-14 is common in organic matter, and their decay time is long enough, because the longer the half-life, the longer the age can be inferred.

To confirm it from another point of view, Libby used the same method to measure wood of a known age. They chose two samples from the coffins of Pharaoh Zossel and Pharaoh Sniflu of the third Dynasty in ancient Egypt. The time of death of the two kinds of wood determined by carbon-14 is 2625 ±75 BC, while according to historical records, the time of death of the two pharaohs is 2800 ±250 BC. On December 23, 1949, Libby's findings were published in the journal Nature. The carbon-14 dating method initiated by Liberia quickly attracted the attention of archaeology and geology and geography because of its simplicity and accuracy.

Using carbon-14 to determine the age of osteogeny, the age of geological formation can also be inferred from the age of organic matter in rock or soil interlayer, and the formation and changes of geology and geomorphology can be known. Half a century of development has proved that the carbon-14 historical dating method initiated by Liberia is of great significance to history, anthropology, geogeography and archaeology, and it has become a reliable and powerful practical technology. For this achievement, Libby won the Nobel Prize in chemistry in 1960.

Source: 365th days in the History of Science author: Wei Fengwen Wu Yi Editor: Zhang Runxin this article comes from the official account of Wechat: Origin Reading (ID:tupydread), author: Wei Fengwen, Wu Yi, Editor: Zhang Runxin

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