Neutronium is the fictional material made of protons and neutrons, so it shouldn't be able to absorb photons unless I’m forgetting something.

So working on that assumption alpha and beta radiation wouldn't be able to pass through the material. But then i remembered gamma rays, no electrons in the neutronium so they would pass right through. Then I remembered Dirac’s lake, when photons equal to the energy of two electrons passes close to the nucleus of a heavy atom it creates an electron and a positron.

Now I ask what's the probability the photons will split into electrons and positrons?

And, Could all the electrons that were ripped off the atoms (that made up the neutronium) be confined in a magnetic bottle wall to absorb the photons. This would obviously take more electrons than provided by the neutronium wall needed.


Sorry just needed to ask it's been annoying me for a while now, damn Master of Orion!

Gamma rays are typical for nuclear transitions, not atomic transitions.
So, a material made of nucleons would not be necessarily transparent to gamma photons.

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"...because the logic of the lines traced from reality is as poor of aesthetic value as it is strict in consistency. " - Paolo Bozzi (Naive Physics - free translation)

I think that is also the concept behind the research for a gamma ray laser using nucleonic isomers.

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J Pax

You are forgetting several things.

First, neutronium is not fictional -- that's what neutron stars are made of. The "fictional" part is that in many SF stories it is used as a solid construction material, while in reality it is a superfluid.

Second, neutronium is not "made of protons and neutrons". It consists mostly of neutrons, with small sprinkling of protons AND electrons (so it's electrically neutral).

Third, neutrons packed to nuclear density DO absorb electromagnetic radiation, and not just gamma rays. My understanding is that the proton/electron "impurity" makes neutronium actually shiny -- it REFLECTS much of the light, -- while pure neutronium would be completely black, but by no means transparent.

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i dont want to hijack the thread, but i think Ilya gave the definitive answer on the nature of neutronium. I have always wondered tho, why the universe isnt made up of giant lumps of it. If neutrons exert the strong nuclear force which is what holds atomic nuclei together (protons also exert it) and they have no charge, so do not exert any electromagnetic force. surely a ball of nuetrons would be infinitely stable.

the strong nuclear force doesnt act at any apreciable distance, so it couldnt attract nuetrons together into giant balls, unless the ball was big enough for gravity to do the pulling (a neutron star). but why arent atomic nuclei stable with excesses of neutrons attached to them? surely the more neutrons, the merrier, so to speak.

I hope I understand your question clearly…if so, here’s what I have to offer:

Neutrons aren’t stable—free neutrons decay into hydrogen with a half-life of about 10 minutes. But they are stable with specific configurations of protons/neutrons within nuclei—but only in specific ratios. There are a couple of models used to explain the stability ratios, the ‘liquid drop’ model and the ‘nuclear shell’ model—I’m not sure if one is favored over the other these days, or if they’ve both been supplanted by the quark-gluon model.

So in addition to the stable configurations, there are zones of increasing instability that surround this narrow isle of stability, and I think the most common form of nuclear decay among these isotopes is beta decay, which ejects an electron from the core…which in some cases yields a nucleus with a stable configuration of neutrons and protons. In any case, the more neutrons there are in the nucleus beyond this isle of stability, the less effectively the nucleus seems to counteract the natural propensity of neutrons to decay. I think that may be a crude model, since I’ve read that nucleons tend to merge into a quark-gluon ‘soup’ within the nucleus…but maybe someone here can explain the situation in greater detail.

This question makes me wonder if neutrons stars slowly evaporate into hydrogen and free protons & electrons…?

If you want to see what neutronium looks like, just see the Star Trek TOS episode The Doomsday Device. :wink: :wink:

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thanks scourge. perhaps the gravitational pull of a neutron star, such that it can overcome the chandraseker limit, keeps the neutrons from decaying. not sure how really.

cyswxman: so neutronium looks like crinkly blue paper?

I meant fictional as in the SF version, having a ship with some swanky neutronium armour kind of thing. But i didn't know it was a superfluid, guess that would put a cramp on a solid neutronium hull

I guess i need to do some reading up on neutronium then But you've answered my question, now i can get back to master of orion 8)

For a really crude picture, imagine a neutron that decays to a proton by emitting an electron and an antineutrino in free space. The extra particles just go their merry way. But if it were confined with a bunch of protons around, one might easily interact with the emitted particles, and transform to a neutron, maintaining the balance. The protons and neutrons do actually seem to swap identity in a nucleus.

But perhaps I can give a more detailed explanation why a nucleus with a balance of protons and neutrons is more stable than just a big ball of neutrons. One of the issues is that nucleons don't behave like little spheres that you're sticking together.

Scourge mentioned the models of the nucleus, including the nuclear shell model. Both neutrons and protons are fermions, which means that, just like electrons, two of them can't be in the same state at once. So the first two neutrons in a nucleus go into the lowest energy state (one spin up and one spin down, so they are in different total states), but the next one has to go into a higher energy state. These energy states can accomodate varying numbers of particles, depending on their quantum parameters, just like the electron shells, and they're built up in more or less the same way. However, since neutrons and protons are different particles, they have their own separate energy levels (a neutron in a given state is not the same as a proton in the same state, so that doesn't break the rule about two identical particles being in the same state).

So, if you had a collection of four neutrons, the third and fourth would be in a significantly higher energy state than the first two. So much higher, in fact, that the nucleus could drop to a much lower energy level by undergoing a double beta decay, resulting in two protons and two neutrons, all of which could be at the ground level. That's why generally speaking nuclei tend to be most stable when the number of protons and neutrons is about equal.

As we get to the larger nuclei, though, the electric repulsion of the protons becomes more noticable. That's because the nuclear force is short enough range that each particle only interacts with its nearest neighbors, but electromagnetism is long range so each proton interacts with every other proton in the nucleus. So that adds an extra Coulomb term to the energy of the protons, and eventually, this is large enough that it's more energetically favorable to add a neutron to state that would otherwise be higher energy, due to the extra electrical potential energy of the proton.

Eventually, for the really large nuclei, a new particle would be in such a high energy state that its energy would exceed the binding energy of the nucleus. So even if you one came flying in, it would be so energetic that it would immediately escape, and at this point the nucleus is as large as it can get.

Now, for a typical nucleus, gravity is completely neglible. However, gravity is another long range force (again, every particle would interact with every other particle), and it's always attractive. If the atomic number is really, really big (somewhere around 10^56), the gravitational potential energy contribution is enough to once again make the configuration stable.

That means, by the way, that not only is neutronium superfluid, but having any amount of it smaller than about a solar mass is really tricky, since it's not stable in any smaller quantity. And if you have that much, preventing gravity from naturally compressing it into a sphere is going to be difficult, too. You'd have to hypothesize some kind of direct control over the strong force, I'd imagine.

So essentially the electric force of the protons pushes them up the quantum well formed around the atomic core, in the helium atom it doesn't make the slightest difference but when it gets to the scale of a uranium atom with 92 protons in the quantum well the cup is nearly full so to speak.

If I remember my 'cups' right bismuth is the last element that has a naturally occurring non-radioactive isotope with 83 protons and 126 neutrons. Anything above it (unless I’m missing them) is a balancing act on top of the cup to try and keep it stable. And when a slow neutron hits U-235 it unbalances the pile. But we have a nice 3 dimensional cup, which hurts the head to think about

But inside a neutron star gravity gets to a point where it equals the force exerted by the protons stopping the whole thing expanding to massive sizes and all the neutrons decaying into protons and neutrinos.

As it had been explained to me before the forces inside a neutron star can overcome the other forces and force protons and neutrino's together back into a neutron.