Michael Scott Poe, age 12, of National City Cal for his question:
Why is carbon 14 used to tell the age of things?
Carbon 14 is the dramatic step brother of ordinary carbon, which is carbon 12. Ordinary carbon is present in all plant and animal cells and necessary to all living things. It is present in the air as carbon dioxide. The normal carbon atom has six protons and six neutrons. The six protons give it the atomic nmanber 6, which belongs to carbon and no other element. The six protons and six neutrons give it the atomic weight of 12. About one carbon atom in a trillion, however, has eight neutrons with its six protons. This rare atom is carbon 14.
Wherever we find ordinary carbon, we also find a few atoms of rare carbon ltd. And carbon 14, also called radiocarbon, is radioactive. A radioactive atom is unstable and is in the process of breaking down into something more simple. Some of these substances are dangerous to handle. When you learn that you eat radiocarbon with your food and breathe it from the air you may b e scared. But living creatures have been doing this since the earth began and until a few years ago, nobody suspected it. Radiocarbon was taken up with ordinary carbon by plants and animals living millions of years ago. Traces of it were built into every cell, every bone, every leaf and every splinter of wood. Wherever there was carbon, there were traces of radiocarbon. n
All radioactive substances decay, or break downy at a certain rate. so fast and no faster. In exactly 5,600 years, the original radiocarbon in a fossil will be cut to half. In another 5,600 years, this remainder will be cut in half. We say that 5,600 years is the half life of radiocarbon. Meantime the ordinary carbon in the fossil specimen does not change. A delicate instrument called the ion counter records the amount of radiocarbon in a specimen.
When we know this, we; know how long the radioactive substance has been decaying and this gives us the age of the specimen.
Radiocarbon in the air forms when a high speed neutron crashes into an atom of nitrogen. The nitrogen gains an extra neutron, but loses one of its protons, which makes it a top heavy atom of carbon. This radiocarbon is being made constantly and may be caused by cosmic rays. As old radiocarbon decays, more is formed. In living things, the ratio of carbon and radiocarbon remains fairly constant.
When the life process stops, no more fresh carbon is taken in by the plant or animal, The normal carbon in the tissues remains the same. The radiocarbon begins to decay. The older the fossil, the less radiocarbon is recorded by the ion counter. Radiocarbon can be used to date a fossil up to 44,000 years. In fossils older than this, the radioactivity is too faint to be reliable.