Nuclear Friday: The Three Carbons

Carbon, atomic number 6, has three naturally occurring isotopes with atomic masses of 12, 13, and 14. The relative abundance of these three isotopes in a sample of material can reveal many secrets. We can tell how fusion works in stars from carbon isotopes. Carbon can reveal the source and age of organic molecules. Carbon can tell us when someone was born relative to the era of above ground nuclear testing, and where the increasing levels of carbon dioxide in the atmosphere are coming from.

Scientists are able to separate and measure the isotopes of carbon using a mass spectrometer. In mass spectrometry, a sample of material is heated to a vapor that is so hot that all chemical bonds are broken and the resulting atoms are partially stripped of electrons. The resulting charged ions are then accelerated with an electric field and then deflected with a magnetic field. Because the ions in a sample have the same charge, but slightly different masses, the field will deflect the ions in by slightly different amounts. A detector can then measure the relative abundance of the isotopes in question.

Here’s a picture of how it works:

carbmassspec

This looks fairly simple, and for a lighter element like carbon, it is. In the case of carbon that difference in the mass of one or two neutrons compared to the mass of the whole ion is proportionally bigger than the mass difference between uranium 235 and 238. Mass spectrometers that can accurately distinguish between heavy nuclei require some careful engineering. Think of it this way: It’s easier to tell the difference between a 12 pound cat and a thirteen pound cat than a 235 pound wrestler and a 238 pound wrestler.

This is why the Y-12 uranium facility at Oak Ridge was so inefficient. It was attempting to take a mass spectrometer, a useful measuring device only just recently discovered, and turn it into an industrial machine. We know better now and can measure the relative abundance of elements and their isotopes to a high degree of accuracy.

And with carbon, there are still other ways of measuring them. Nuclear magnetic resonance can easily tell the difference between carbon 13 and carbon 12 or 14. It’s less good at telling the difference between 12 and 14. However, carbon 14 is radioactive and has a very distinctive energy signature to its beta decay. Carbon 12 and carbon 13 are not radioactive. This ability to distinguish between 12 and 13 with magnetic resonance has eased research into atmospheric chemistry and is important in the study of climate change as we shall see in a moment.

Carbon 12 is the most abundant form of carbon here on Earth and throughout the observable universe. Almost all carbon 12 is made from helium nuclei through the Triple Alpha Process. Our Sun will begin this process of fusing three helium nuclei into carbon in a few billion years, once levels of free protons and deuterium nuclei become so depleted that the current forms of fusion will not keep the sun from collapsing. But when that collapse does occur, pressure and temperatures will be high enough to sustain the triple alpha process. The intense radiation of this change will completely destroy the inner planets. As our sun runs out of helium a few billion years later, it will become a white hot chunk of mostly carbon 12.

Carbon 12 is why our Sun and stars a little lighter than it have their characteristic glow and color. Stars just 30% heavier shine differently. This due in part that they have small amounts of carbon in their cores. It’s not a lot, and it’s not from triple alpha fusion, but from the CNO Cycle. Modeling these processes in theory and matching them to spectral evidence helps explain where carbon comes from and why over 98% of it is carbon 12.

Carbon 14 it turns out is not made by any fusion process and is much more abundant here on Earth than most places in our solar system. Earth has a lot, Venus much less, mars and mercury almost none. This is because Earth’s atmosphere is the correct composition for making carbon 14 through a process called Cosmic ray spallation. This form of “micro fission” was not understood until the 1970’s. Carbon 14 was a mystery before that, although it was known that the amount of carbon 14 had been constant, with one exception, over a long period of time. This suggested that carbon 14 was being replenished from somewhere, but no one was quite sure where. In spallation a neutron from a cosmic ray interaction with nitrogen 14 replaces a proton. This makes the nitrogen dip down one place in the periodic table to the carbon position while still having the mass of the original nitrogen atom. Carbon 14 is unstable and undergoes beta decay to become nitrogen 14 again. The half life of this decay cycle is around 5700 years.

Carbon 14 is absorbed into the biosphere by photosynthesis in almost exactly the same ratio that it occurs in the atmosphere. Remember that carbon 12 and 14 are hard to tell apart by nuclear magnetic resonance? It turns out that the two main types of photosynthesis can’t tell them apart later. All carbon in the biosphere is taken from the atmosphere by photosynthesis, so while an organism is alive, the ratio of carbon 12 to 14 is the same as the atmospheric constant. Once an organism dies, carbon is no longer cycled through it, and over time radioactive decay will deplete the amount of carbon 14 in the organic matter in a predictable and measurable way. This allows scientists to determine the age of an organic sample. Radiocarbon dating is accurate through a period up to 60,000 years into the past. After that much elapsed time, carbon 14 levels are too low to measure accurately.

Carbon 14 can also sometimes tell something about the source of a material. I recently learned that US regulations require that distilled alcoholic beverages be radioactive. As the video explains, there are industrial processes which can make ethanol. So how do you determine that some cheap vodka was made by fermentation rather than petroleum engineering. Quite simple. Any carbon that comes from oil will have long since lost all its carbon 14 to radioactive decay because it is very old . Alcohol from fermentation will have lots of freshly harvested carbon 14, enough that you don’t need a mass spectrometer to find it. A radiation detector capable of distinguishing specific energy levels of beta decay could easily tell the difference.

Notice that I said that carbon 14 ratios have been fairly constant in the atmosphere with one major exception? Wonder what that is? I think you can already guess that it involves nukes. From the mid-50s to early 70s there was a huge spike in the amount of carbon 14 in the atmosphere with a peak in the early 60s right around the time the Partial Test Ban treaty put an end to almost all atmospheric nuclear testing. This peak was almost double measurements of the carbon 14 ratio in the 1930s and 40s. Levels have dropped rapidly. This dip is not due to radioactive decay but rather absorption into the biosphere and oceans. By 1990 levels returned to close to historic norms. One interesting effect of this is that we can measure when a person was born or got their adult teeth relative to the spike. People born near the spike have a record of the spike in the lenses in their eyes and those who got their adult teeth near the spike can be distinguished from people who grew up earlier or later. The carbon 14 spike will also complicate things for scientists a couple of thousand years form now. Organic samples from the carbon spike era will seem to be only hundreds of years old. I hope that any people around in that era will have good historic records and know about the carbon spike, but imagine if they had to reconstruct the radiological history of previous civilizations from isotope ratios.

Now a doubling of carbon 14 sounds frightening, but it’s not that much of a problem. The most abundant radioactive material in the human body is potassium 40, about 160 grams per person. Carbon 14 is a distant second, even with the measurable temporary increase in levels. Both isotopes expose us to around 4,900 radioactive decay events per second because carbon 14 decays faster than the fairly stable (but also very abundant) potassium 40. Carbon 14 emits beta particles at the low end of the beta scale, while potassium decay is much more energetic. If you are not afraid of bananas, do not be afraid of carbon 14. Yes, there is some level of carbon 14 exposure that could harm you, but anything that could expose you to that much carbon 14 would already have killed you a hundred other ways. And as always, if your body is emitting a strong radiation signature not associated with potassium or carbon: Stop eating those little capsules inside smoke detectors. I know comic books have promised you superpowers, but that’s just pretend. If I can resist, so can you.

What I’ve said about carbon 14 suggests a way of tracking the flow of carbon in and out of the atmosphere. If fossil fuels are carbon 14 depleted, then increased carbon dioxide from human activity should change the balance of carbon isotopes in the atmosphere. Well, indeed it does, but since carbon 14 is only about a trillionth of all the carbon in the atmosphere, oceans, and biosphere, it’s almost impossible to measure the effects of human activity on isotope ratios; excepting, of course, that time when were carelessly testing so many bombs. As it tuns out, Carbon 13 and its distinct magnetic resonance provides important clues about atmospheric chemistry and climate change.

About 1.1% of the carbon on earth is carbon 13, the second stable isotope of carbon forged long ago in supernova explosions. You might expect that as with carbon 14, the ratio of carbon 13 in the biosphere would be the same as the atmosphere. It is not. Plants don’t like carbon 13 and are less likely to capture it by photosynthesis. Plants are biased against it to different degrees. Corn (maize) is less biased against 13 than potatoes or wheat are, but all photosynthetic processes on Earth fall short of the expected ratio of carbon 13 and we can tell from tests of isotope ratios in coal and oil that plants on Earth have always disliked carbon 13 about as much as they do now. Thus, if increasing levels of carbon dioxide are from burning fossil fuels, we should expect a small, but fairly steady increase in carbon 12 relative to carbon 13 in the atmosphere over time. And we do. It’s not volcanoes, it’s us.

The chemistry is exciting and more than I can get into here, but by measuring seasonal variations in carbon 13 we’ve been able to tell that our poor little biosphere is sequestering more carbon than expected, but that net levels are still increasing. I’d like to say thinks to the biosphere for doing its part. I know we’ve hurt you and we appreciate that you’re still pulling for us. And the oceans, they’re sequestering carbon too. There’s a downside to that , though. It’s possible that the oceans may soak up so much carbon that it’s curtains for us all.

The use of nuclear magnetic resonance to track atmospheric changes may prove to save many more lives than it ever has or will through medical MRI scans. NMR analysis of the carbon may be able to tell us whether we are decades or centuries away from a from an ocean acidification event, and current models suggest if that doomsday is near, the biosphere could save us in twenty years if we just went carbon neutral. Of course, we’d still have wars, mass migrations, and economic losses from “regular” global warming which is now underway.

And what sickens me most is that we now have people coming into power who understand nothing of this. The likely nominee for Secretary of Energy understands nothing about this. He’s not stupid, but willfully incurious, and allied with people who think that carbon 14 isn’t real, let alone climate change.

Not a day goes by that I don’t think this can’t be happening.

 

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