| The Method 14C |
It is not so long when an archaeologist stood before the problem of the time filing of important discovery. What it was valid that he uncovered several cultural layers, was sure to determinate its historical sequence and often the long of lasting of individual epochs, when he couldn’t establish it into a real date.
The break was brought by 1940 when Martin D. Kamen from American Oregon discovered Radioactive Carbon 14 C. How is this research connected with an accurate dating in archaeology?
The earth atmosphere is created with a lot of varies elements, include carbon and nitrogen. 7 protons and 7 neutrons create the atom of nitrogen. The carbon has got 3 isotopes (elements with the same number of protons and a different number of neutrons in a atomic core) marked 12 C, 13 C and 14 C where the upper index marks the number of protons and neutrons (nuclear number) in core. The widest kind of Carbon ( about 98,9 %) is 12 C, created 6 protons and 6 neutrons. In rare cases (about 0,00000001 %) carbon has got except common 6 protons an unusual number 8 neutrons. How is it possible ?
Cosmic ray from the Sun bombs our atmosphere and neutrons have been creating during it. In several cases neutron attacks with an atom of nitrogen, knocked out a proton and takes its place. In this case the number of protons reduces on 6, what answers atom of carbon, because it has got in its core just 6 protons. The nitrogen has changed into carbon (the properties of element are just determined with the number of protons), but with the difference that the atom has got 8 neutrons, so about two more than the common carbon. This way it becomes radioactive.
The life on this planet is based on Carbon: Plants absorb carbon from atmosphere (they breathe C02) from plants the carbon gets into bodies of animals and from it to human bodies together with eating vegetables and fruits. Because our atmosphere contains the certain ratio of radioactive 14 C to common 12 C the same ratio is kept in bodies of plants, animals and human. But on one condition only: that the plant or animal is living! Because so radioactive carbon has got its time of breaking, when it becomes changing consequently in common 12 C. Martin D. Kamen discovered that the half – life period of radioactive 14 C is 5 730 + and _ 30 years.
Willard Frank Libby proceeded from this all (1908 – 1980), American chemist in Institute for Nuclear Studies, when in 1947 discovered the method of dating with help 14 C together with his students. That was enough when he compared the intensity of radiation of researched discovery with its temporary one: The oldest the discovery was the less radioactive carbon it contained because the plant or the animal had not accepted radioactive carbon and the original one was changing into non radioactive one and the intensity of radiation went down this way.
Libby used as the detector so called Geiger computer. Problem was in this that Libby needed to measure the very small degree of beta radiation, smaller than the common hum was. He measured about 500 units in a minute, but the radioactive carbon participated in it only with 6 till 7 units. Libby so covered the detector with armour plates and reduced the activity of the detector to 120 units in minutes this way. Later he improved his method this way that he used several Geiger computers connected electronically. Dating of wood, bones, the mortal frames of marine and freshwater and sedimentations contain these sedimentations and even rocks contain carbon (tuff) was on the world.
But:Iin the course of time it was shown that all was not so easy. For example it was learned that the hypothesis that the ratio 14 C / 12 C is in the course of dating years constant is not true. In the end also this had to take in considering but it is not lowering the importance of Libby discovery for which Nobel Prize priced him in 1960.
