When Cosmic Rays fall down from the sky, they produce lots of neutrons. They are "added" to the Earth system, so what is happening to them?

  • Do they decay to protons? It would follow that they then easily catch a free electron and form Hydrogen. Is Earth enriched by more and more Hydrogen over time?
  • Are they captured by nuclei as soon as they become thermalized? So they all go into production of radioactive (cosmogenic) nuclei? What happens to neutrons in the ocean, where no heavy nuclei are, do they form Deuterium water?

What is the dominating process and why?

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    $\begingroup$ Good question for which I know only a partial answer. Once you get a ways underground quite a few go into the creation of cosmogenic nuclei, but you might be surprised by how many of them are captured on light isotopes. Surprisingly I don't know how many simply decay---KamLAND certainly has the potential to answer that question, but I don't know that anyone has done the study. Alas, I'm not currently connected to a experiment where I could do the study. $\endgroup$ – dmckee --- ex-moderator kitten Aug 14 '14 at 23:05

Yes, they're thermalized and captured. Remember that the free neutron lifetime is roughly fifteen minutes, and that thermal neutrons are moving a couple kilometers per second. The chance of going fifteen minutes without a capture interaction is tiny. (Even in so-called "neutron bottle traps," it's hard to get the mean time before neutron disappearance to approach the beta-decay lifetime.) There'll be some free decay, but not much.

The atmosphere has lots of water vapor, and the ground contains lots of water, for capture on hydrogen; the cross sections for capture on carbon and oxygen are pretty low (they're alpha-particle nuclei) but the cross sections on nitrogen (1.9 barn, for thermal neutrons) and silicon (0.17 barn) are comparable to hydrogen (0.33 barn). Iron's in the same territory. You can browse the Neutron Data Booklet as well as I can. Odd-proton nuclei, like copper, are typically unstable after neutron capture and will beta-decay after some time.

Actually what I remember reading is that more cosmic ray neutrons are produced in the ground, by fast muon spallation, than in processes like $\mathrm{^{14}N(p,n)^{14}C}$ in the atmosphere. The first data appeared in JETP, the Soviet journal, and mentioned that the thermal flux at ground level was markedly increased by the presence of a half-meter of snow to act as a moderator.

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    $\begingroup$ The cross-section for thermal neutron capture on C-12 is--surprisingly to me--about 1% of that on H-1. There was enough of it going on in KamLAND that we had to count them. $\endgroup$ – dmckee --- ex-moderator kitten Aug 15 '14 at 1:39
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    $\begingroup$ @dmckee Well, KamLAND was pretty awesome. $\endgroup$ – rob Aug 15 '14 at 1:43

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