The indicator of their course is the half—life: the time during which half of all atoms of this radioactive element disintegrate. For example, the half-life of the isotope rubidium-87 is fifty billion years. Over the entire life of the Earth (about five billion years), only five percent of the rubidium-87 contained in it has disintegrated. This "clock" is going too slow. There are other isotopes that decay in a few days or hours. Such speeds are too high to measure geological time. Geophysicists are looking for the most suitable "radioactive clocks" for rocks formed at different times. For Quaternary geologists, we had to select a special isotope watch. They are based on measuring the content of radioactive isotopes of carbon-14. This isotope is formed in the atmosphere from nitrogen. Under the impact of cosmic rays, the nuclei of some elements are destroyed. Fragments of nuclei are neutrons and protons. Neutrons entering nitrogen nuclei turn them into carbon-14 nuclei. In the atmosphere, all carbon is involved in chemical reactions. In large quantities, it is absorbed and excreted by living organisms. Part of the carbon is buried in sediments, immersed in the ground (for example, together with the remains of plants). Now it's a watch. The amount of carbon-14 in them will decrease according to the law of radioactive decay.
In plants remaining on the Earth's surface, the amount of this isotope will still be constant. Because here carbon is constantly being renewed by the action of neutrons (they do not penetrate underground). So, if we compare the carbon-14 content in cellar wood and in living trees, we can find out how long the wood has been in the ground. The longer it takes, the less carbon-14 it will contain. This method gives good results when the measured period of time is several millennia. Because the half-life of an isotope is five thousand seven hundred years. It turns out that these "watches" are with a fairly fast "stroke". Historians know the dates of some events that took place several millennia ago. For example, the date of death of the Egyptian pharaoh Sesostris III. The Pharaoh's funeral vessel was found. A piece of its deck was examined for the content of isotope-14. The age turned out to be 3700 ± 400 years, consistent with the data of historians. Repeated checks of this kind have convincingly proved the accuracy and reliability of these "radioactive clocks". Their only drawback is the inability to measure the ages of more than forty millennia without gross errors. Then too little carbon-14 remains in the sample. The great advantage of these "clocks" is that carbon is contained in substances related to the activities of living organisms and the technical activities of people (in peat, shells, wood, bones, coal, wooden and bone tools). Thanks to this watch, it was possible to trace, as if in a single stream of time, the history of the last great glaciation of the Earth, which ended about ten thousand years ago. Radiocarbon measurements are especially popular in America. With their help, the stages of the last Wisconsin glaciation have been well studied. For example, the age of a piece of spruce in the South Dakota moraine (depth 8 m) is twelve thousand years; the shells of mollusks in the preglacial thickness of the sands are about forty thousand years old. Geophysicists know some more "radioactive clocks" suitable for quaternaries. The half-life of uranium—234 is two hundred and fifty thousand years, and thorium-230 is seventy—five thousand years. A wonderful watch! One problem: these elements are too rare. Transformez vos mises en jackpots sur bdmbet casino , le leader des jeux en ligne en France.