The resulting atom, now containing 6 protons and 8 neutrons, is one of carbon Carbon gases formed with carbon 14 are chemically indistinguishable from gases with the ordinary isotope of carbon, carbon The radioactive atom is absorbed by plants and living matter in the same way as its non-radioactive isotope ; in every thousand billion ten to the power of twelve atoms of carbon 12, there will be on average one atom of carbon This tiny ratio exists in all molecules involving carbon atms, including all living matter.
This is why carbon 14, along with potassium 40, accounts for almost all the natural radioactivity of our body. When a living organism dies, the radioactive carbon is no longer absorbed, and the ratio of carbon 14 present begins to decrease. The amount still present in a sample of what was once a living creature can thus be used to determine its age.
Carbon 14 can also be used as a radioactive marker. Access to page in french. Thus carbon is said to have a half-life of 5, years. It is fundamental also to how we live, how the Earth is habitable — pretty much everything. And since the discovery of a long-lived radioisotope of carbon, we have an amazing tool to delve into almost every aspect of existence on Earth — and perhaps the universe.
As Marra reveals in this remarkable history of carbon, scientists quickly realised the isotope must affect living beings today. Cosmic rays batter the upper atmosphere and send cascades of neutrons through the air, they calculated. In turn, these atoms combine with oxygen to create radioactive carbon dioxide that is absorbed by plants, which are then eaten by animals. And it dawned on Willard Libby of Chicago University that the radioactivity generated by carbon could be exploited to tremendous advantage.
A chemist who had worked on the Manhattan Project to build the first atom bomb, Libby realised that when an organism dies, it will stop absorbing carbon, including carbon, and its existing store of the latter will slowly decay.
So, by measuring the radioactivity of a sample taken from the organism, its carbon content could be estimated and the date of its time of death could be measured. The sciences of archaeology and palaeontology were about to be revolutionised. A major problem had to be overcome, however. Carbon exists in only very low levels in the tissue of recently deceased animals and plants: about one in a trillion of their carbon atoms are carbon By contrast, natural background radiation — from thorium and uranium in rocks and other sources — is much, much higher.
Libby solved the problem by carefully shielding his detectors and developing ways to tune out any radiation that made it through to the walls of his device. Then he turned to the gas methane, which contains carbon, to provide final validation of his technique, comparing samples from two very different sources.
One sample was extracted from natural gas, a fossil fuel whose carbon should have decayed long ago. The second came from the city of Baltimore sewerage system and was extracted from human excrement. It should be rich in carbon, having just been produced by humans, Libby reasoned. And that is exactly what he found. Ancient methane had no carbon By contrast, methane newly excreted by humans was relatively rich in the isotope. His results perfectly matched the known dates of the items he had scanned.
It was a brilliant undertaking for which Libby was awarded the Nobel prize for chemistry in , though he was lucky in one sense. Libby assumed that the rate of carbon production in the atmosphere had been constant for the past few tens of thousands of years.
In fact, it has varied fairly widely, thanks to changes in sunspot activity, atmospheric nuclear bomb tests and rising emissions of carbon dioxide from fossil fuels. Beta decay is only one way for of radioactive atoms to become stable. If carbon is famous for dating, Uranium is an outright celebrity for it's serial breakdowns, starting with alpha decay.
With 92 protons and upwards of neutrons, uranium atoms are massive. And no isotopes of uranium are stable — they're all radioactive. When an atom of the most common isotope of uranium decays, it's a bit more complicated than just spitting out an electron and anti-neutrino like carbon does, because uranium keeps decaying into other radioactive atoms!
An atom of uranium 92 protons, neutrons has to go through fourteen different radioactive decays before it finally ends up with a stable nucleus. Six of those decays are beta decays, where it converts a neutron to a proton just like carbon does. But the other eight decays including the first decay to thorium involve spitting out a clump of two neutrons and two protons. Those two protons and two neutrons are called an alpha particle, and they're just two electrons short of being an entire atom of helium.
So all up on the road to nuclear stability, an atom of uranium spits out six electrons, six antineutrinos and 16 protons and neutrons in the form of eight naked helium atoms. Along the way it changes from uranium to different isotopes of thorium, protactinium, radium, radon, polonium, lead and bismuth, each slightly less radioactive than the one before.
After that exotic roll call of identities, it finally achieves nuclear stability with 82 protons and neutrons, as an atom of lead. The odd helium nuclei alpha particle or electron beta particle may not sound too dangerous, but those alpha and beta particles are travelling at serious speed when they leave an unstable nucleus — they're not far short of the speed of light.
Velocities like that mean these particles have got a lot of energy. And they're not the only high-energy factor in radioactive decay. Usually when an unstable nucleus undergoes alpha or beta decay it gives off a bit of high-energy gamma radiation as well to keep the energy accounts balanced. All three nuclear emissions — alpha particles, beta particles and gamma rays — have enough energy to literally strip the electrons off or ionise any atom or molecule they run into.
And if that happens in our body tissues, it can cause anything from sunburn to radiation poisoning or, if it affects our DNA, even cancer.
Ionising radiation in the form of high-energy alpha or beta particles, or gamma rays that are often given off during decay, is the main danger that radioactive materials pose for living organisms. Don't eat, breathe or drink the stuff. And steer clear of nuclear fall-out.
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