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Nuclear fusion products

by vemvare
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vemvare
#1
Feb11-13, 03:19 PM
P: 24
If we tried to make elements higher than Helium in a terrestrial fusion reactor, what elements could we realistically make?

If I've understood it correctly the triple alpha process reaction rates would be irrelevant due to the considerably lower pressure * time product.

But how would Be and Li work? Could they be synthesized in a manner similar to those of the original big bang nucleosynthesis, or would they just fall apart and form 4He?

Another problem is that I've only recently begun to understand what a triple product is but I cant' find them for any of the "stellar" nuclear processes, only typical processes like deuterium-tritium and so on.
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mfb
#2
Feb11-13, 03:28 PM
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Why do you want to create heavier products?

Wikipedia lists D+6Li-fusion, with 7Li and 7Be as possibile results.

In general, you can fuse hydrogen and helium with many "heavy" nuclei and get energy, but those reactions require very high temperatures.
Astronuc
#3
Feb11-13, 03:45 PM
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Quote Quote by vemvare View Post
If we tried to make elements higher than Helium in a terrestrial fusion reactor, what elements could we realistically make?

If I've understood it correctly the triple alpha process reaction rates would be irrelevant due to the considerably lower pressure * time product.

But how would Be and Li work? Could they be synthesized in a manner similar to those of the original big bang nucleosynthesis, or would they just fall apart and form 4He?

Another problem is that I've only recently begun to understand what a triple product is but I cant' find them for any of the "stellar" nuclear processes, only typical processes like deuterium-tritium and so on.
The triple alpha process requires high temperatures and high particle densities, much higher than we can achieve in terrestrial magnetic confinement systems.

http://hyperphysics.phy-astr.gsu.edu...ro/helfus.html

http://csep10.phys.utk.edu/astr162/l...riplealph.html

The process is described as α + α → 8Be, then 8Be + α → 12C + γ. 8Be is unstable, so to produce carbon, an α-particle must interact very shortly after 2 α's form the Be nucleus.

Realistically, we would not use fusion to create elements in any economical sense.

vemvare
#4
Feb11-13, 04:04 PM
P: 24
Nuclear fusion products

Why do you want to create heavier products?
Imagine if you had antiprotons, but wanted something more storable...

As far as I've understood, even the first step, turning 1H+1H--->2H + neutrino and positron, wouldn't happen under "terrestrial" circumstances. I missed this one in the OP.
Drakkith
#5
Feb11-13, 04:14 PM
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Quote Quote by vemvare View Post
Imagine if you had antiprotons, but wanted something more storable...
What do antiprotons have to do with this?
vemvare
#6
Feb11-13, 04:34 PM
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Quote Quote by Drakkith View Post
What do antiprotons have to do with this?
That they should "work" the same way as ordinary protons, that meaning it being equally difficult to react them with each other in a proton-chain type reaction, which is annoying since we're not going to be able to produce heavier elements.

Apparently, heavier anti-nuclei have been detected a couple of times, how did they form? Could we do it on purpose? Perhaps more energy than what is needed to form a simple proton-antiproton-pair and then hoping the resulting hyperon mess "chooses" the right decay path?
Bill_K
#7
Feb11-13, 04:43 PM
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A few anti-helium nuclei have been produced at RHIC resulting from collisions of gold ions. No naturally occurring ones have yet been found.
mfb
#8
Feb11-13, 05:11 PM
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Quote Quote by vemvare View Post
Imagine if you had antiprotons, but wanted something more storable...
Anti-hydrogen should be fine. Any fusion process with reasonable technology would be so inefficient (you lose protons to the reactor walls and so on) that it is not worth the effort. And there is no good way to go from helium to heavier nuclei.

Apparently, heavier anti-nuclei have been detected a couple of times, how did they form?
Collisions of high-energetic quarks release so much energy that many anti-baryons are formed. Sometimes, 2 to 4 fly in the same direction with the same velocity, and can form a nucleus. More would be possible, but that is extremely unlikely.
They are too high-energetic to capture them.
vemvare
#9
Feb12-13, 08:05 AM
P: 24
But how are the neutrons formed and absorbed into the antihelium? Neutron-antineutron pairs? β+-decay? Hyperons? I just don't understand why the particles would "stick together"
Astronuc
#10
Feb12-13, 08:29 AM
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Quote Quote by vemvare View Post
But how are the neutrons formed and absorbed into the antihelium? Neutron-antineutron pairs? β+-decay? Hyperons? I just don't understand why the particles would "stick together"
Anti-neutrons have been produced by interaction of anti-protons with matter.

Antineutrons produced by 440-Mev antiprotons incident upon Pb, C, and CH2 targets have been observed. The antineutrons were detected by their energy release upon annihilation. Charge-exchange cross sections for antiprotons in the three targets have been calculated. The results show that the effective charge-exchange cross section per proton of the target nucleus decreases rapidly with increasing Z.
Antineutron Production by Charge Exchange
J. Button, T. Elioff, E. Segrč, H. M. Steiner, R. Weingart, C. Wiegand, and T. Ypsilantis
Phys. Rev. 108, 1557–1561 (1957)

However, it would be virtually impossible to trap/collect anti-neutrons since they would tend to annihilate with protons or neutrons in matter with which they would interact.

p+p fusion in terrestrial systems would be impractical given the low cross-section of the reaction. It works in stellar cores because of the high density and temperature in stellar cores.

Anti-hydrogen could be stored in a magnetic bottle in theory. Those interested in collecting anti-hydrogen consider storing it at cryogenic conditions. It must be isolated from matter, from which storage systems are necessarily constructed. Solid hydrogen would be easier to store than liquid/gaseous he. If one could produce anti-lithium 36Li, that would be better, but that would required some extraordinary engineering.
mfb
#11
Feb12-13, 09:45 AM
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Quote Quote by vemvare View Post
But how are the neutrons formed and absorbed into the antihelium? Neutron-antineutron pairs? β+-decay? Hyperons? I just don't understand why the particles would "stick together"
In high-energy collisions: Antineutron + baryon. It can be a neutron, but does not have to be, as both baryons are created independently after the initial production of quark/antiquark pairs. Apart from the direct production of an antineutron, some short-living antibaryons which decay to an antineutron are possible, too.
vemvare
#12
Feb17-13, 06:26 AM
P: 24
First of all, thanks for all informative replies so far! Now, more questions. If I've understood this correctly if one wants to use diamagnetic repulsion to store antimatter then it is the diamagnetic susceptibility per mass unit for the concerned elements that is interesting.

http://www-d0.fnal.gov/hardware/cal/...lementmagn.pdf

These divided by molar mass, basically.

H2(l) appears to have a quite high value, and if they can levitate a frog (mostly water) in a magnetic field, then any object with a lower diamagnetic value should levitate in the same field, right?

Why is H2(s) never mentioned? I can't find what magnetic properties it has.

There are no differences in orders of magnitude here, so I suspect that the next thing on the wish list would be antimatter of an element that in its diamagnetic stage doesn't have a vapor pressure high enough to cause trouble, like H2(l) apparently could, according to here: http://www.engr.psu.edu/antimatter/papers/anti_prod.pdf

In the same document (p3) there is also a mention of hydrogen not annihilating with He at a very low temperature, meaning the antiatoms could be "gas cooled" as opposed to "electron cooled". Why would they behave like that?
mfb
#13
Feb17-13, 09:46 AM
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Quote Quote by vemvare View Post
In the same document (p3) there is also a mention of hydrogen not annihilating with He at a very low temperature, meaning the antiatoms could be "gas cooled" as opposed to "electron cooled". Why would they behave like that?
I don't see this in the text. It is mentioned that the cross-section for annihilation goes down with temperature. But even then, the few remaining He atoms (remember: This is one of the best vacua ever achieved in labs) cause significant losses.
vemvare
#14
Feb17-13, 05:13 PM
P: 24
Quote Quote by Anti_prod.pdf
However, the sharp turn-over at 170 seconds is unexpected, signaling a rapid reduction in cross section as the
energy of the antiprotons is further reduced. The implications of this could be profound: (1) it may be possible to transport
cold antiprotons at room temperature (poor vacua), resulting in much simpler and less costly systems, and (2) if cross
sections for atomic antihydrogen behave similarly, it may be possible to degrade these atoms by collisions with gas, thus
facilitating their storage and condensation into liquid or solid states.
Quote Quote by Anti_prod.pdf
First, in order to confine antihydrogen atoms
in an Ioffe trap (Fig. 5), the atoms must be cooled to milliKelvin temperatures. This could be done by laser cooling. Or,
if it turns out that these atoms resist annihilation at low energies, then cooling by collisions with residual gas, as is done in
the case of hydrogen, could be tried.
Bolded by me. I connected the two and assumed that they've found that cold enough anti-H were unlikely to annihilate with He, for some strange reason.

I've probably misunderstood it. What "gas" are they referring to? What do they mean by degradation in that context?
mfb
#15
Feb18-13, 02:31 PM
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No vacuum is perfect - you always have some remaining gas atoms inside, usually hydrogen and helium. Those can cool the antiprotons, but they will also annihilate some of them, so you don't want to use them as cooling medium too long.
In short, helium is bad, but not as bad as expected.
vemvare
#16
Feb20-13, 08:39 AM
P: 24
So, the conclusion is that if I want to produce heavier antimatter for easier storage I need to go through the following almost impossible steps, including retaining and cooling the products of each process which is probably as difficult again:

1. Form particle-antiparticle pairs by colliding heavy ions in burst. The bigger the number of particles colliding in a smaller area, the better?

2. A few of the formed nuclei are going to be heavier, the vast majority of those being anti-deuterium.

3. Try to fuse these to form various isotopes of anti-helium

4. Fuse said anti-helium, preferably in bursts like in the dense plasma focus, to form small amounts of anti-lithium, and perhaps beryllium.

Astronuc
#17
Feb20-13, 08:58 AM
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Quote Quote by vemvare View Post
So, the conclusion is that if I want to produce heavier antimatter for easier storage I need to go through the following almost impossible steps, including retaining and cooling the products of each process which is probably as difficult again:

1. Form particle-antiparticle pairs by colliding heavy ions in burst. The bigger the number of particles colliding in a smaller area, the better?

2. A few of the formed nuclei are going to be heavier, the vast majority of those being anti-deuterium.

3. Try to fuse these to form various isotopes of anti-helium

4. Fuse said anti-helium, preferably in bursts like in the dense plasma focus, to form small amounts of anti-lithium, and perhaps beryllium.

Basically yes. Given the trouble we have with just getting fusion to work with normal matter, adding anti-matter to the challenge makes it virtually impossible.


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