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Tetraneutron Discovered
Caid Raspa writes "According to this
Press Release the French have (accidentally) produced six nuclei of tetraneutron (nucleus with four neutrons and no protons). Theoreticians have previously thought that tetraneutron does not exist. As there is no electric charge in these nuclei, they allow better studies of the nuclear forces. The scientific article is also available at
arXiv.org."
A single neutron, not bound to a proton, is not stable against decaying into a proton. (It's oh-so-slighlty more massive than the proton.) Half-life is on the order of minutes (it's been years since I did nuclear research in undergrad and the exact #'s are escaping me). In a nucleus it's stablizied because if it decayed, the electrical repulsion between the old protons and new proton would be to great, so it's more stable if it remains a neutron. Presumably, these neutrons would also decay, and you might expect (if the 4 nucleons remained together) to see it decay to 4He (ie 2 protons and 2 neutrons)--I don't know if Hydrogen 4 has any stability, but I don't think so.
We've got two lives, one we're given, and the other one we make. --Mary Chapin Carpenter
Erm, well you won't have any luck paging Forward. He died about 2-3 months ago from a Brain tumor/cancer.
You're right about balancing the gravitational binding energy of a pile of neutrons with the nuclear binding energy of the same pile of neutrons. As you add more neutrons, you get to a point where the mass of the neutrons makes the gravitational binding energy pass that of the nuclear binding energy. Of course, this number of neutrons is astronomically large (pun intended).
/Radius).
I did this as an undergrad problem in Nuclear physics - take a ball of N neutrons, assume nuclear type densities, and calculate the neutron ball's radius and mass (and thus it's gravitational binding energy = G * M(neutron) * N
When you balance this with the typical binding energy per neutron (erm, cant remember the numbers we used, sorry), you get two simple equations and you solve for N the number of neutrons.
AFAI remembber, you get a radius of 10km and about 2 solar masses - pretty damn good for a back of the envelope calculation!
If I can dig out the old problem sheet, I can post the number later....
Dr Fish
I'm pretty sure that the controversy to which you're referring was related to the beginning of operations at RHIC, the Relativistic Heavy Ion Collider, at Brookhaven National Laboratory in New York, not the LHC, which is at CERN, in Switzerland. People (including some respected scientists) realized that there was the possibility (however slight) that at the energies reached when colliding gold ions together, "strangelets" (clumps of matter containing "strange" quarks, rather than the normal "up" and "down"), could be produced, which could potentially escape, and destroy the entire earth, yadda, yadda, yadda. Another, almost more interesting scenario, is that the collisions would be of sufficiently high energy to trigger a phase shift in teh quantum vacuum energy (kinda like when the very very early universe shifted from pure energy, and forms of matter began to appear). But, like you said, collisions with far higher energies occur every day - cosmic rays bombarding our atmosphere, mostly, so there is nothing to fear.
Um, you really need to work on your argument.
First, penicillin isn't a new form of matter. It might be a new molecule, or one that mankind didn't know about before, but it doesn't rate the "new form of matter" moniker.
Second, just because something exists somewhere in the universe does not mean that it is thus safe or wise to have it here on earth. Black holes are fine, as long as they don't come near. Quasars are fine, as long as they aren't nearby and shining at us. Supernovas? Wonderful, but please keep them many light years away.
Maybe tetraneutron is something that is commonly made when cosmic rays hit our atmosphere, and maybe not. You should be at least a little startled by it, and that it was made _accidentally_.
The concern wan't about black-holes, it was over strange matter. The eating the earth bit is close enough. CERN gathered the worlds greatish physicst to debate the issue, and the results was that "the probabilty is very low". So there you have it.
- Does a tetraneutron spontaneously fly apart?
- Does a tetraneutron undergo beta decay?
The second question doesn't even make sense to ask unless the answer to the first question is no. Until this experiment, nuclear physicists were pretty much convinced that the first answer was yes, which makes the second question nonsensical. Process #1 works via the strong nuclear force, so the time-scale for it to happen is simply the size of the nucleus divided by the typical speed of the neutrons, which is about (10^-15 m)/(10^6 m/s)=10^-21 s. Process #2 works via the weak nuclear force, so the time-scale is much longer --- probably on the same order of magnitude as the beta-decay lifetime of nuclei like 6He, which is maybe 10^-3 s.Since the paper appears to establish that process #1 does not happen, process #2 is what must happen. There is no doubt at all about its being beta-stable --- it's not.
So to answer the original poster's question, here's why people were expecting that the tetraneutron would fly apart. The reason is the Heisenberg uncertainty principle plus the Pauli exclusion principle. If you try to corrall 4 neutrons into a nucleus, their small delta-x requires a large delta-p. That's why they're moving at ~1% of the speed of light. Since they're moving so fast, their attraction might not be enough to hold them together.
So far, this reasoning applies to 4He just as much as it applies to a tetraneutron. So why would 4He be so much more stable? Well, the Pauli exclusion principle says that in a tetraneutron, the first two neutrons can both go in the lowest energy level, with their spins in opposite direction, but the third and fourth have to go in a higher energy level.
The real question is whether the experiment is right or not. Neutron detection is notoriously difficult. In their paper, they go to great lengths to try to show that it wasn't just four neutrons from unrelated events that happened to hit the same detector --- a random coincidence. Their arguments appear convincing, but it's the kind of thing that you could easily get wrong. I'd like to see it reproduced at another lab. If it is correct, then the next step is to start measuring the properties of element zero (zeronium?). What's its lifetime? Its binding energy? Its rms radius? Does it have any bound excited states?
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The best evidence is that the dineutron is unbound. That's why this is an extremely surprising result. The paper does say that calculations can produce a bound tetraneutron, but the problem is that the calculations depend a lot on the parameters you assume for the strong force.
Lastly, what could we expect for 3n, 5n, and 6n? Would the odd number of nucleons make 3n less stable (vs. the strong force) than 4n due to some shell-filling rule? Ditto 5n vs. 6n?
Pairing means that evens are always more bound than odds. I don't think there's a chance in hell that the 3n, etc. are bound.
Would 6n be more or less stable (vs. the strong force) than 4n? (more particles to mutually attract, but more of them in the higher energy shell). :-)
Yeah, interesting question. Since theorists didn't think 4n was bound, I don't think they're ready to predict whether 6n is or not
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