The Cosmic Story of Carbon-14

“Life exists in the universe only because the carbon atom possesses certain exceptional properties.” -James Jeans

Here on Earth, every living thing is based around four fundamental, elemental building blocks of life: hydrogen, oxygen, nitrogen and, perhaps most importantly, carbon.

DNA + Nanotubes

Image Credit: Robert Johnson / University of Pennsylvania.

From diamonds to nanotubes to DNA, carbon is indispensable for constructing practically all of the most intricate structures we know of. Most of the carbon in our world comes from long-dead stars, in the form of Carbon-12: carbon atoms containing six neutrons in their nucleus. About 1.1% of all carbon is Carbon-13, with one extra neutron. But there is another form of carbon that, while not at all abundant, is definitely worth talking about.

Carbon Isotopes

Image credit: Press & Silver.

Carbon-14, or carbon atoms with eight neutrons in their nuclei, is unstable, and is so rare that only one-in-a-trillion carbon atoms are carbon-14. With a half-life of just over 5,000 years, any Carbon-14 atoms that were created in stars, billions of years ago, have long since decayed away into nitrogen atoms.

Carbon-14 Decay

Image credit: Steve Gagnon at Jefferson Lab.

But there are small, but not quite negligible amounts of carbon-14 present in all the organic life that we know, including in our own bodies. The way it gets here is, literally, cosmic.

Cosmic Rays

Image credit: Simon Swordy (U. Chicago), NASA.

From across the galaxy and across the Universe, from stars (including our Sun), pulsars, black holes and more, space is flooded with high-energy particles known as cosmic rays. Most frequently, cosmic rays are protons, but a handful are heavier ions and a few are even humble electrons. But once they interact with the atmosphere, look out!

Cosmic ray shower

Image credit: University of New Hampshire.

They produce showers of subatomic particles of many different types, including — for our purposes — the all important neutron. The reason neutrons are so important is because our atmosphere is 78% nitrogen, which you may remember as the thing that carbon-14 decays into.

Well, if carbon-14 can decay into nitrogen-14 and other stuff, then we can create carbon-14 by combining nitrogen-14 with the proper stuff. In this case, that happens to be a neutron, which allows us to do this:

Carbon-14 creation

Image credit: Wikimedia Commons, users NikNaks, Spacexplosion, and Sgbeer.

Once you create carbon-14, it behaves just like any other atom of carbon, readily forming CO2 (a.k.a., carbon dioxide) and mixing throughout the atmosphere and oceans, easily making its way into living organisms into a well-understood equilibrium. As far as we can tell, the levels of carbon-14 throughout the world have remained roughly constant throughout the past few millenia, so that when an organism dies and the carbon-14 decays, we can measure how long ago it became deceased by measuring the ratio of carbon-14 to its normal carbon-12.

The only major fluctuation we know of occurred when we began detonating nuclear weapons in the open air, back in the mid-20th Century. If you ever wondered why nuclear tests are now performed underground, this is why.

Radiocarbon spike

Image credit: Wikimedia Commons, user Hokanomono.

So you can imagine it came as a shock when, just yesterday, nature released a paper showing a big, short-lived spike in carbon-14 levels way back in the 8th Century! By looking at the tree rings of ancient Japanese Cedars, you can see a rise in the concentration of carbon-14 that starts in the 770s, peaks in the 780s and then falls off.

Carbon-14 spike

Image credit: Fusa Miyake, Kentaro Nagaya, Kimiaki Masuda & Toshio Nakamura, 2012.

What does this correspond to, in terms of creating this carbon-14? Well, there were no nearby supernovae that happened at that time, so that’s out. There’s no evidence of an unusually large solar flare or any other bizarre solar activity, so that can’t be the culprit, either. What this appears to correspond to, at least at this preliminary stage, is an increase in cosmic rays during the year 774-775.

Spike in Cosmic Rays

Image credit: Fusa Miyake, Kentaro Nagaya, Kimiaki Masuda & Toshio Nakamura, 2012.

Now, since we’ve been watching the skies, we’ve never seen an increase in levels like this, but it’s only recently that our sophistication in measuring the carbon-14 levels in old tree rings like this has allowed us to test this.

The follow-up? Looks like we’re going to have to unearth more old trees that can be radiocarbon-dated back to these years, and see whether they have elevated levels of carbon-14 in them. If not, then it’s conceivable that these trees are just flukes, or that there was a mistake done in the analysis. But that doesn’t seem likely; there is data from North American and European trees that this is consistent with! If this is confirmed, then there was very likely an extremely large increase in cosmic radiation over a very short period of time, the likes of which we’ve never seen or recorded, until now.

What could’ve caused an influx of cosmic rays like this? While there are many possibilities, I wouldn’t count out a relatively nearby, flaring black hole!

Black hole flare

Image credit: NASA / Goddard Space Flight Center.

The Universe may never cease to surprise us, but we may never cease, as long as we exist, to figure out exactly why it does the things that it does. How remarkable is this!

Thanks to Sarah Kavassilis for suggesting this story; it’s a great one!

Comments

  1. #1 Michael Fisher
    Birmingham, UK
    June 4, 2012

    A starship engine revving up in the Edgeworth–Kuiper belt? [just joshing]

  2. #2 OKThen
    Beyond the Milky Way Galaxy
    June 4, 2012

    I did not know the origin or Carbon-14. Very nice explanation.

    Hmm, I don’t think a nearby black hole. It would have to be within 5,000 light years or so.

    But the sun of course is continuous nuclear detonations. But why would it produce an extra amount of carbon-14? My off the wall crazy thought is, remember the shoemaker comet.

    “Comet Shoemaker–Levy 9 (formally designated D/1993 F2) was a comet that broke apart and collided with Jupiter in July 1994… The collision provided new information about Jupiter and highlighted its role in reducing space debris in the inner Solar System… Galileo detected a fireball which reached a peak temperature of about 24,000 K, compared to the typical Jovian cloudtop temperature of about 130 K… Over the next 6 days, 21 distinct impacts were observed, with the largest coming on July 18 at 07:33 UTC when fragment G struck Jupiter. This impact created a giant dark spot over 12,000 km across, and was estimated to have released an energy equivalent to 6,000,000 megatons of TNT (600 times the world’s nuclear arsenal)…” Wikipedia

    OK so that comet striking Jupiter was equivalent to “600 times the world’s nuclear arsenal”. It sounds to me that such an event (assuming it trigger a nuclear event) would result in an increase of carbon-14 showering down upon the earth. After figuring the time of travel of such Jupiter debri to Earth.

    Assuming we found such carbon-14 debri on earth associated with the Shoemaker–Levy 9 comet; then I hypothesize that such a similar comet impact into the sun or Jupiter could account for the carbon-14 event of the 8th century.

    http://www.sciencenews.org/pages/pdfs/data/1995/147-21/14721-19.pdf
    “To test more rigorously whether the gases indeed came from fragments of Shoemaker-Levy 9, Crisp plans to measure the isotopic ratios of oxygen, carbon, and hydrogen, which have characteristic values in comets.” this is all I can find about analyzing the debri, but I suspect somebody has done it and determined if there is nuclear debri.

    OK that’s my 2 cents.

  3. #3 OKThen
    Beyond the Milky Way Galaxy
    June 4, 2012

    my 2nd hypothesis.

    The Tunguska event in Russia in 1908 … “The explosion, having the hypocenter, (60.885833°N, 101.894444°E), is believed to have been caused by the air burst of a large meteoroid or comet fragment at an altitude of 5–10 kilometres (3–6 mi) above the Earth’s surface.. It is the largest impact event in recorded history… Estimates of the energy of the blast range from 5 to as high as 30 megatons of TNT… about 1,000 times more powerful than the atomic bomb dropped on Hiroshima, Japan, and about one-third the power of the Tsar Bomba, the largest nuclear weapon ever detonated. The explosion knocked over an estimated 80 million trees covering 2,150 square kilometres (830 sq mi). It is estimated that the shock wave from the blast would have measured 5.0 on the Richter scale. ” Wikipedia

    Hypothesis 2: Such an event occured in the 8th century.

    Check to see if there is increased carbon-14 levels in tree ring in russia and elsewhere associated with the 1908 Tunguska event. If so calculate the relative size of the 8th century event versus the Tunguska event.

    OK, please knock down and destroy my two hypotheses or tell me they are reasonable or how to make them more reasonable. or your better hypotheses. Every which way, I learn. thanks.

  4. #4 George Monser
    So. Arizona
    June 4, 2012

    concerning OKThen’s suggestion about comet strikes creating lots of C-14 in the past: A large comet strike would produce huge amounts of X-Ray photons. But an X-Ray photon has only a few thousand electron volts (EV) – That’s many orders of magnitude less than the cosmic rays that routinely transform Nitrogen to C-14. I doubt that zillions of X-Ray photons from a comet strike would create C-14, as does a single cosmic ray packing trillions of EV.

  5. #5 Glen Martin
    Leander, TX
    June 4, 2012

    Cue Young Earth Creationists using this to cast doubts on radioisotopic dating in 3…2…1…

  6. #6 Christopher
    Portland Or
    June 4, 2012

    The energies required to do nuclear fusion/fission are so large that no local events like cometary impacts can create the sources. That’s why something like a black hole is required. There are several large energy sources that are possible but not that many that could flood our atmosphere with nuclear hammers. Remember too that Carbon14 has to be created locally since none survives long enough to traverse interstellar or intergalactic space. And yes, this may force Carbon14 dating revisions and make the use of this kind of dating even more complex.

  7. #7 Artor
    June 4, 2012

    This is really fascinating. I’d wondered where C14 came from, since it has such a (relatively) short half-life. It would have to have some steady, continuous rate of replenishment, but I had no idea what the mechanism might have been. Now that I’ve heard it explained, it makes perfect sense.

  8. #8 Michael Kelsey
    SLAC
    June 4, 2012

    @OKThen: (1) The cosmic rays Ethan described are not themselves, C-14. Rather, they (usually high-energy protons) hit the top of the atmosphere and produce a shower of secondary particles. Some of those secondary particles are neutrons, which in turn collide with nitrogen atoms in the atmosphere to convert the N-14 into C-14 (Ethan’s sixth figure).

    (2) The Tunguska event, just like Shoemaker-Levy hitting Jupiter, was just a really large “chemical” explosion. Nothing nuclear, no high-energy particles or neutrons. Just a really big rock getting really, really hot and going blooey. There have been many wonderful hypotheses about Tunguska (everything from a blob of antimatter to a nuclear-powered alien spacecraft), but the blast pattern and energy release are entirely consistent with a simple meteroroid ablating and vaporising in the atmosphere.

  9. #9 Artor
    June 4, 2012

    OKThen- I like the way you think, but as I understand it, a Tunguska-type event, or a solar comet impact would require something moving at relativistic speeds to produce the energy required to fuse N14 into C14.

    “Give a man a relativistic rock, and he will shatter a planet today. Teach him to do the math himself, and he will shatter planets for the rest of his life.”

  10. #10 Stefano
    June 5, 2012

    Ethan, speaking of blackholes, do you have any insights regarding this news? Sounds quite amazing!
    http://freeinternetpress.com//story/Giant-Black-Hole-Ejected-Out-Of-Home-Galaxy-35063.html

  11. #11 David L
    June 5, 2012

    Hi Ethan

    Can you explain the y axes on your graphs please? Unless these are very local peaks quickly reduced by atmospheric mixing, I don’t see how the levels can fall again with anything other than a half life of C14.

  12. #12 Joffan
    June 5, 2012

    Comet impact on Earth, maybe not…. comet impact on the Sun, disturbing the photosphere to stimulate increased activity there? From a position of complete ignorance, and in the knowledge that small comet impacts don’t have a noticeable effect….

    I’ve got to cringe on seeing the fit lines through that discontinuous data, though – no way should there be a rising period, it is clearly an step-type change. Maybe that’s the point they’re trying to make, of course.

  13. #13 Eric Lund
    June 5, 2012

    David L @0308: Radioactive decay is not the only way C14 is removed from the atmosphere. It also gets incorporated into plants, and this can be a much shorter timescale. Some of the C14 gets put back through decay of vegetation (or animals eating the vegetation and breathing out some of the C14), but this process is slower than the plant uptake (effectively, the C14 thus released is an average over years to decades, while the uptake rate depends on the instantaneous concentration). Meanwhile, more C14 is created at a rate closer to normal. The axes in the paper by Miyake et al. are fractional deviations from some “normal” concentration.

    My own hypothesis: It is well known that cosmic ray flux is anticorrelated with the solar cycle. Perhaps there was an unusually deep solar minimum in the 770s, so that the cosmic ray flux was unusually high. Test: we just had an unusually deep solar minimum in 2007-2010, so we should be able to tell within a decade if there was a similar increase in C14 uptake by plants.

  14. #14 OKThen
    Back to Earth
    June 5, 2012

    I did understand that the cosmic rays (i.e. protons) collided with the atmosphere etc and N14. Ethan’s explanation was clear to me.

    But I didn’t understand, how far astronomically C14 might travel hence my 5000 light year remark as well I did not understand if C14 might travel relativistically. Apparently not observed and thus not too likely. I suppose any relativistic C14 would collide with something long before it reached Earth. Oh well.

    As well I did not understand; if a large physical impact (and consequent chemical reaction) could ever be able to trigger a nuclear reaction. OK I accept your credibility in this matter. i.e. a very large physical impact or chemical reaction even if 100′s and 1000′s times more explosive than a nuclear weapon will NOT trigger a fission or fusion reaction.

    So I dug a little bit more.
    “At the temperatures and densities in stellar cores the rates of fusion reactions are notoriously slow. For example, at solar core temperature (T ≈ 15 MK) and density (160 g/cm3), the energy release rate is only 276 μW/cm3—about a quarter of the volumetric rate at which a resting human body generates heat.[20] Thus, reproduction of stellar core conditions in a lab for nuclear fusion power production is completely impractical. Because nuclear reaction rates strongly depend on temperature (exp(−E/kT)), achieving reasonable energy production rates in terrestrial fusion reactors requires 10–100 times higher temperatures (compared to stellar interiors): T ≈ 0.1–1.0 GK.” wikipedia. Hot, hot, very hot!!

    So, it looks like even at 15 million K (Kelvin) the temperature of the sun would cause a puny nuclear reaction. So the shoemaker-Levy comet’s ” peak temperature of (of impact) of about 24,000 K… (even) releasing the (explosive) energy equivalent to 6,000,000 megatons of TNT (600 times the world’s nuclear arsenal)” is not enough. Yikes! I did not know.

    Thus large astronomical bodies (comets and planets) colliding will not make a nuclear reaction; because physical comet collisions can not focus enough explosive energy to a small enough space.

    I never thought of it before; so thank folks for the education..

  15. #15 OKThen
    June 5, 2012

    Just to continue with my education in public.

    “Ultra High Energy Cosmic Ray Acceleration in Engine-driven Relativistic Supernovae… The origin of the highest energy cosmic rays remains an enigma. They offer a window to
    new physics, including tests of physical laws relevant to their propagation and interactions, at energies unattainable by terrestrial accelerators. They must be accelerated locally, as otherwise background radiations would severely suppress the flux of protons and nuclei, at energies above the Greisen-Zatsepin-Kuzmin (GZK) limit (∼ 60EeV=6 × 10^19 eV). Nearby Gamma Ray Bursts (GRBs), Hypernovae, Active Galactic Nuclei (AGNs) and their flares, have all been suggested and debated as possible sources.” http://arxiv.org/pdf/1012.0850v1.pdf

    “About 89% of cosmic rays are simple protons or hydrogen nuclei, 10% are helium nuclei or alpha particles, and 1% are the nuclei of heavier elements. These nuclei constitute 99% of the cosmic rays. Solitary electrons (much like beta particles, although their ultimate source is unknown) constitute much of the remaining 1%.” wikipedia

    But look at this.
    “The problems of the origin and propagation of the charged cosmic rays (CRs) in the Galaxy are among the major subjects of the modern astrophysics. It is generally accepted that primary CR nuclei such as H, He, C, N and O, are accelerated in supernova remnants (SNRs) via diffusive shock acceleration (DSA) mechanisms, that produce power-law momentum spectra (Drury, 1983). At relativistic energies S ∝ p^−ν
    ∼ E^−ν.” http://arxiv.org/pdf/1203.6094v1.pdf

    So there is Carbon nuclei in cosmic rays and hence Carbon-14 and also relativistic cosmic rays. But rereading Ethan’s post. Hmm, Ethan has a simpler hypothesis. The increase C14 is not from primary cosmic rays but from the atmosphere. OK.

    Now, I better understand Ethan’s post and his hypothesis. I still don’t like the idea of a “relatively nearby, flaring black hole”. Nor do I understand the “many pssibilities” that could cause an “influx of cosmic rays like this”.

    But Eric Lund’s explanation above makes sense to me. thanks for that.

    Hey Ethan, I just noticed the blog comments are no longer numbered. How are we ever going to discuss each others comments when you post a topic that gets hundreds of comments. Oh well.

  16. #16 BenHead
    United States
    June 5, 2012

    Cool mystery, though I’m as interested in the origin of carbon-14 on Earth, which I’d apparently had wrong all these years. I thought it was part of the decay chain of longer-lived radioisotopes in the crust, like radon or, in a roundabout way, helium. One more way to avoid making a fool out of myself in the future, thanks! :D

  17. #17 Mu
    June 5, 2012

    How can we exclude a supernova over the southern hemisphere? I can’t think of any history keeping culture which would have kept records of an event in the 8th century not visible from northern latitudes.

  18. #18 Mike
    June 5, 2012

    @ Michael Fisher

    It’s Reavers. They run without Core Containment. Raiding party must have come by and loose radiation from their core raised the ambient C14 levels . . .

    But, on an actually non-tongue-in-cheek note, another great article, Ethan. Be an interesting mystery to solve.

  19. #19 Eric Lund
    June 5, 2012

    Mu @0703: We can’t rule out a supernova sufficiently close to the South Celestial Pole, but we can rule out anything more than 20 degrees away (and we can probably tighten that bound). There were literate societies at tropical latitudes in Arabia and India, and perhaps the Maya would have noticed something and carved it on a stela. A supernova close enough to produce this effect would be bright enough to be visible during the day, so dark skies are not a requirement. I don’t know of any candidate objects for supernova remnants in that part of the sky, but as Ethan has mentioned in other posts, a pair-instability supernova would leave no remnant.

  20. #20 chelle
    Latveria
    June 5, 2012

    Nuclear power reactors that run on uranium also produces C14

    So maybe a volcano, or a meteorite impact like OKThen suggested, might have brought rich uranium ores closer to the surface and high into the air, and along with some thermal diffusion …
    Eastern Mongolia (Dornod) which isn’t that far from Japan has open cut uranium mines, and along with the wind …

    … or is this too far fetched?

  21. #21 Doug Little
    June 5, 2012

    GRB?

  22. #22 eric
    June 5, 2012

    Chelle – nope, the isotopic ratios of U235 to U238 in the 700s was basically what it it is now: too low for any sort of spontaneous chain reaction in natural U oxides.* However, you might be interested in looking up the Oklo natural reactor (just google it) – what you suggest did happen, but it was a billion years ago when the isotopic ratios were different, not 1,000 years ago.

    *IIRC from my nuke physcs, with today’s isotopic ratios, nature would have to produce a pure, metallic (natural) U blob 14m on a side to to get any chain reaction going at all, and U doesn’t generally form the metallic form in nature, oxides are more stable.

  23. #23 OKThen
    Oklo in Gabon 2,000,000 BC
    June 5, 2012

    Eric thanks. Pretty neat, what a comet smashing into a planet can’t do; a little groundwater in a uranium can accomplish.

  24. #24 Artor
    June 5, 2012

    OkThen- Thanks for the Oklo tip. I’d never heard of that before. Amazing!

  25. #25 chelle
    Latveria
    June 6, 2012

    eric, I don’t understand your argument very well. A nuclear bomb is set of with explosives to start a chain-reaction. So why wouldn’t a Volcano or a Meteorite impact in a Uranium rich environment, cause fission on a mass scale, there for not a chain reaction, but still a large production of C14 could be possible, no?

  26. #26 OKThen
    Planet Earth
    June 6, 2012

    Chelle
    A nuclear fusion bomb (hydrogen bob) is set off by a nuclear fission bomb (uranium). A physical collision or chemical explosion is not powerful enough to create the energy densities necessary nuclear fusion, i.e. to overcome the electrostatic repulsion of the positively charged nuclei.

    On the other hand nuclear fission only requires a critical mass of U-235 or other suitable radioactive isotope. And radioactive decay compounds exponentially as a critical mass is brought together.

    Check wiki for a more complete skinny.

  27. #27 Mu
    June 6, 2012

    chelle, you need the right elemental AND isotope combination to get a chain reaction. Natural uranium is a mixture of over 99% of uranium 238, and only about 0.7% of easily fissioned uranium 235. This ratio is constant world wide. Only the uranium 235 can spontaneously fission if you get enough of it to form a critical mass (which you’re doing with the explosives in a nuclear bomb), and you can’t get enough of it close together if it’s diluted by a lot a U238. The way you get U238 to fission is by using it as a tamper around a nuclear bomb; read up on the Castle Bravo mishap.

  28. #28 chelle
    Latveria
    June 6, 2012

    @OKThen

    What about this:
    http://en.wikipedia.org/wiki/Gun-type_fission_weapon

    It is inefficient but than again the peak of C14 found in the Japanese Cedars isn’t so far out of the normal.

    @Mu

    Yes for a chain-reaction, but that’s not what I’m suggesting here. Only the production of a lot of C14, and a Volcano outburst or Meteorite impact might cause a lot of individual fission events and hurdle the by-products into the atmosphere.

    btw isotopes that’s why I mentioned thermal diffusion in my first post, and both cases might set up some ‘natural’ diffusion processing activity:
    http://en.wikipedia.org/wiki/Uranium_enrichment#Thermal_diffusion
    http://en.wikipedia.org/wiki/Uranium_enrichment#Aerodynamic_processes

    I rest my case : )

  29. #29 eric
    June 6, 2012

    chelle, several comments:
    (1) the production of C14 requires thermal neutrons (neutrons that have only a few eV of kinetic energy). Uranium in nature produces fast neutrons, with kinetic energies in the MeV range. So a given amount of uranium will not contribute much to C14 production compared to the same amount of cosmic rays hitting the atmosphere.

    (2) I’m unclear what you think a volcano or meteor impact contributes. I thought you were saying that these violent events would produce chain reactions (which create more thermal neutrons) similar to what goes on in a nuclear reactor. But based on your last post, I am no longer sure. Maybe you are saying that volcanos and meteors could bring more uranium ore to the surface of the earth? If so, see (1); normal, run-of-the mill uranium ore produces neutrons that are too high in energy to effectively make C14. They need a moderator to produce thermal neutrons.

    (3) a volcano or meteorite would be very unlikely to cause a chain reaction. Our bombs produce chain reactions via implosion: forcing stuff together. Volcanos wouldn’t do that, neither would a meteorite impact (with the exception of the point of impact, for a short period of time). I suppose that if a meteorite very rich in U-235 hit a U-nat deposit on earth, billions of years ago when our deposits were also rich in U-235, you might get a chain reaction. That’s quite a farfetched scenario, however.

    (4) I’m not sure what either of the separation methods you mention have to do with the discussion. They both require very specific and stable engineering circumstances. I believe they also require uranium in a gaseous form, which it doesn’t exist in naturally on earth. And they probably also require that the uranium go through the same process many hundreds of times, to increase enrichment to a reasonable level. Meteorite hits and volcanic explosions don’t, it seems to me, provide any of the needed circumstances.

  30. #30 chelle
    Latveria
    June 6, 2012

    @eric

    (1): “Carbon-14 can also be produced by other neutron reactions, including in particular … with thermal neutrons, and 15N(n,d)14C and 16O(n,3He)14C with fast neutrons.”
    http://en.wikipedia.org/wiki/Carbon-14#Other_carbon-14_sources

    (2): You say that they need a moderator, wouldn’t there be lots of water available for an emerging underwater volcano, or what about a very rainy or snowy season for a Meteorite?

    (4): That’s true it’s just a lot of speculation from my part, but maybe once every 2000 years such a unique circumstances might take place as those trees show us ; )

  31. #31 daedalus2u
    http://daedalus2u.blogspot.com/
    June 10, 2012

    Chelle, no. The degree of enrichment needed for a gun-type atomic explosion is ~90% and use of pure metallic uranium. Metallic uranium doesn’t exist in nature. You need a critical mass of that isotopic composition with is ~50 kg or so. That is why modern nuclear weapons use plutonium which has a critical mass of ~10 kg.

    What is important is the density of fissile atoms. You can increase that density by packing more atoms into a smaller volume, but if there are impurity atoms, either non-fissile isotopes of uranium (U238), or non-fissile atoms like oxygen as in U2O3, the required density of fissile uranium atoms is harder to reach.

    The energy release once there is a critical mass occurs exponentially, with the magnitude of the exponent dependent on the density of fissile atoms. As the energy release increases, the mass gets hotter, and as it gets hotter, it tends to expand. To get a nuclear explosion of high yield, you need a very short time constant for the energy increase. That is why they use pure isotopes and metallic fissile materials. The time constant they get is on the order of a nanosecond. As the fissioning occurs, the energy release rate approximately doubles every time constant. That means that virtually all of the energy is released in the last few doublings, as the fissioning material gets so hot that it flies apart with a time constant of a nanosecond. 0.1 meter in a nanosecond is 100,000 km/sec. That is fast enough that you can’t neglect relativistic effects.

    Moderators are not useful in atomic weapons. Moderators slow neutrons, which makes the time constant longer. That is desirable for reactors where you want a very slow time constant so that the reactor can be controlled, but undesirable when you want very high energy release rate. If the time constant was milliseconds, then the characteristic velocity would be 0.1 meter in 0.001 second, or 100 m/s. If the uranium got hot enough to vaporize, it would expand as a gas faster than that. That is how hot it would get, hot enough to vaporize but then it would expand and the nuclear reactions would stop.

    Thermonuclear warheads generate energy via fusion of D + T. This produces 14 MeV neutrons and these are the neutrons that generate C14. Much of the C14 in the diagram came from the Tsar Bomba which was mostly fusion. It was set off October 1961, but it takes a while for the atmosphere to mix and C14 to get around.

  32. #32 Kim
    http://www.ceticismoaberto.com/fortianismo/3768/o-navio-de-teseu-e-a-impermanncia-do-carbono-14
    June 10, 2012

    A bit off-topic, and late to the thread, but I want to indicate a beautiful post about carbon-14 relating to our own finitude: http://tinyurl.com/carbon14-theseus . I’ve linked to a Google english translation of the original post in portuguese, which you can find clicking in my name. Hope you like it.

  33. [...] Starts with a Bang + NRC-Handelsblad, 9 juni 2012. Gerelateerde [...]

  34. #34 Chelle
    Latveria
    June 11, 2012

    @daedalus2u

    I never suggested here a chain-reaction like an a-bomb, I only thought of fission on a large scale. The reason I mentioned a “gun type fission weapon” was because it could cause fission, that’s all. Scroll through the previous comments to catch up.

    Scroll through the previous comments

  35. #35 OKThen
    Planet Earth
    June 11, 2012

    Chelle

    You seem to be suggesting that there is some other type of fission than what daedalus2u explains. There isn’t; that was my misunderstanding too. Reread what daedulus2u explains.

  36. #36 chelle
    Latveria
    June 12, 2012

    @OKThen

    Ok, I might be wrong.

    Anyway, spontaneous fission does happen, and I thought that extreme high pressure and heat might possibly induce it:

    “For naturally occurring thorium, uranium-235, and uranium-238, spontaneous fission does occur rarely, but in the vast majority of the radioactive decay of these atoms, alpha decay or beta decay occurs instead.” – http://en.wikipedia.org/wiki/Spontaneous_fission

    And what if a Volcano or Meteorite impact blows a lot of Uranium (and Nitrogen) high up into the atmosphere, than wouldn’t those ‘normal’ cosmic ray-showers cause some fissioning in those heavy elements, and generate more C14 than normal?

  37. #37 Wow
    June 12, 2012

    “Anyway, spontaneous fission does happen”

    This is called “radioactivity”.

    As to your theory on cosmic rays and volcanic/meteoric uranium:

    a) the fluxes of cosmic rays are generally too low
    b) the energies too high for a high cross-section for capture
    c) the density of the uranium very very low

    for this mechanism to create any measurably different C14 ratios.

    Not to mention that a large event would be visible in geological strata.

  38. #38 chelle
    Latveria
    June 12, 2012

    @Wow

    “Not to mention that a large event would be visible in geological strata.”

    I agree, although one remark: if you check the timeline of volcano’s, there has been a lot of action between 710 and 800 CE with the outburst of the Pago and Dakataua, all part of the Bismarck Volcanic Arc in Papua New Guinea (http://en.wikipedia.org/wiki/Timetable_of_major_worldwide_volcanic_eruptions#Overview_of_Common_Era)

    It is a geographical area straight below Japan, and the C14 peak was in the 8th Century.

  39. #39 Wow
    June 13, 2012

    That’s correct, chelle. But looks to be several orders of magnitude too small to produce the effect.

    All that’s really needed are a lof of slow neutrons. The capture cross section for N14 is much much higher for slow neutrons than fast ones. Okolo has a high concentration of uranium and moderators of the neutrinos and hence it has an “unnaturally enhanced” decay because of this moderation of the energies of the neitrons. Someone would have to come up with a much more effective way of turning that ash into slow neutrons than any naturally occurring process. Just as with Okolo.

    Thing is, I’m not too sure of Ethan’s proposal of a flaring black hole, I’d be more inclined to a nearby nova event. Much less energetic and, if it was highly isentropic, could still be intense enough on the earth to be a cause.

    At the moment it seems to me we have only enough data to preclude options.

    And the anomaly may have to go down as “just one of those things”. I’m OK with that.

  40. #40 Wow
    June 15, 2012

    moderators of the *neutrons*, not neutrinos.

    Duh…

  41. #41 Philip
    South Africa
    June 16, 2012

    Regarding the possible need to recalibrate carbon dating – I don’t think so. Carbon dating has already been extensicely calibrated against dendrochronologocal records – tree rings. Thsi is needed because the C14 ratio in the atmisphere has not been perfectly constant over millenia time scales. There is a “correction” calibration for carbon dating based on the entirely accurate tree ring data. I think this would naturally catch even this rapid variation. Would be worth an expert on this letting us know if there is a particular feature in the calibration data at this time, as one would expect.

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