Where do all the neutrons go?

I have been reading “Dark Sun”, about the making of the H-Bomb. One of the problems they have trying to build an atomic stock pile and develop for the H-Bomb is that with the few reactors they have they are “neutron limited”, they can either breed plutonium or tritium (For the H-Bomb) but not both.

However, my question has nothing to do with that. 8^) Heavy atoms like Uranium are constantly spitting out neutrons. What happens to them in real life? I assume they start out real fast, some get absorbed into other nuclei, but some must slow down enough and gradually get pulled to the center of the earth by gravity. Is there a shell of “free” neutron’s at the earth’s core left over from all these decays, like the Oklo (?) reactor in Africa?

Or does every neutron eventually get absorbed into a nucleus?

The thought of a neutron “fluid” is pretty cool. Imagine how good a knife that would be if you could make a blade out of it.

Sorry to disappoint you, but free neutrons are unstable. They decay with a half-life of about 15 minutes into a proton and electron. So neutrons will transmute into hydrogen.

Free neutrons are unstable, with a lifetime of about 15 minutes. They break down into a proton, an electron, and an antineutron (n -> p + e[sup]-[/sup] + v)[sup]1[/sup]. Thus, any neutron spit out by a fission or fusion reaction that doesn’t get snarfed by an atom becomes (effectively) a hydrogen atom.

[sup]1[/sup][sub]The “v” should actualy be a Greek “n”, but I don’t believe that that exists in Latin-1 fonts.[/sub]

Where do all the neutrons go, long time passing?
Where do all the neutrons go—when their half-life ends?
Where do all the neutrons go? Gone to protons by beta decay, electrons and antineutrinos, every one.
When will they ever learn—when will they ever learn?

Lumpy, Akatsukami:

Thanks for the quick reply. Strange, I must have heard it before, but it slipped my mind

By the way is that anti-neutron or anti-neutrino?

Correct me if I’m wrong, but don’t you mean “a proton, an electron, and an antineutrino?” I can’t see how the mass balance would work if a proton and an antineutron came from a neutron.


If Pete Seeger weren’t so anti-nuke he would be proud.

I am having a little trouble with the secend verse:

Where have all the protons gone…
We don’t know since we have never seen one decay yet.

[end of song]

Yes, you’re absolutely right. It should be antineutrino, not antineutron. Some days, my proofreading is better than others; today, it’s obviously not very good. :frowning:

Indeed. These were the original lyrics, long since suppressed, for the 1964 anti Barry Goldwater commercial.:smiley:

BTW, Eve’s lyrics correctly identified it as an antineutrino

It’s also possible to reverse the reaction
so that a proton and an electron combine to form a neutron. This occurs naturally in certain massive stars that eventually collapse to form a neutron star. (More about neutron stars here.)

My guess is that if you could form a knife out of neutron star material, it wouldn’t cut so much as disintegrate anything it touched.

If a neutron decays into a proton, electron and an ani-neutrino; in order to re-assemble a neutron, wouldn’t you need all three? Where does a forming neutron star get the anti-neutrino?

You can create a neutrino/anti-neutrino pair; keep the anti-neutrino and cast off the neutrino.

Wouldn’t a blade made of neutron fluid be too heavy to lift?


I love you!

Dr. Matrix:

I am assuming that there is some sort of “neutron glue” that came keep them together at a normal density, but I guess “normal” density for a neutron is very, very dense. However the blade could be very thin.

My normal experience is so predicated on electrical forces that I cannot reset my common sense.

I guess it keeps me from writing clear phrases, too.
I meant to say:
My common sense is so predicated on electric forces that it is difficult for me to reset my intuition to imagine a neutron fluid.

So neutrons are constantly spitting out anti neutrinos. What happens to them in real life? Do they get pulled to the center of the earth by gravity? Is there a shell of “free” anti neutrinos at the earth’s core left over from all these decays?

As any Future Nukular Engineer should know, Neutrinos are almost massless, and have a tiny cross-section for interaction with normal matter. So, they will keep going in whatever direction the started at for many light-years. When the rare Neutrino does interact with matter, this is one possible outcome.

The real question is:

If God can do anything, can He make a knife blade of neutron fluid so heavy that He can’t lift it?

I came in to make a similar joke. Great minds…

And for what it’s worth, the material of a neutron star is a fluid. A superfluid, in fact. So while it holds up very well under pressure, it has no shear strength whatsoever. Trying to make an object of non-spherical shape, like a knife, out of it would result in the knife just falling apart. It’d literally be easier to make your knife out of liquid water, which at least has a little bit of viscosity.

Well, even aside from the fact that at pressures less than those of a neutron star, the stuff would just explode.

How many neutrons have disappeared in 15 years?