Spin-forbidden reactions?

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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RobertMendelsohn
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Spin-forbidden reactions?

Post by RobertMendelsohn »

Consider the fusion reaction:

7-Li + p -> 4-He + 4He + 17MeV

Say you had a source of spin-polarized lithium-7 ions (a simple Stern-Gerlach apparatus would do) and a spin-polarized 1-H target (SGE or a very cold and magnetized surface will do); there are 8 possible spin configurations in the reactant side:

7-Li has a spin of -3/2: 3/2, 1/2, -1/2, -3/2
1-H has a spin of 1/2: 1/2, -1/2

But the product, two alpha particles / helium nuclei, have a spin of 0+ each.
This would seem to indicate that (key assumption) if nuclear spin is conserved (at least on these timeframes), only a reaction between a (-1/2) and (1/2) 7-Li and p OR a (1/2) and (-1/2) 7-Li and p is possible, the other 6 configurations not conserving spin, since they would not sum up to 0.

Is this the case, or is the key assumption (that nuclear spin is conserved) incorrect/misapplied?

This seems to me an interesting area of [theoretical] research, since spin is often overlooked.
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Re: Spin-forbidden reactions?

Post by RobertMendelsohn »

To add: perhaps gamma radiation explain this? Since it has a spin?

Or perhaps, spin is accounted for by meta-stable states, which typically have different spin than their ground-state counterpart?
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Dennis P Brown
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Re: Spin-forbidden reactions?

Post by Dennis P Brown »

If any reaction is permitted, and the starting particles have spin zero then the net spin of all particles after is spin zero (where all daughter particles are summed - so yes, spin is conserved.) Ditto if they have a specific spin. Photons don't change spin systems for particles that have rest mass. Not sure what you are getting at - creating specific spin states is difficult and results in very few particles at enormous energy cost.
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Re: Spin-forbidden reactions?

Post by Richard Hull »

This all goes back to the way nature doesn't allow large casual energy exchanges of a civilization sized energy source outside of "hunter-gathering" by intelligent species. Otherwise, the universe would burn up in a flash due to any number of chance operations, be they chemical or nuclear. Isolated stellar sized fusion events are common on a galactic scale. Colliding galaxies react only on epoch time scales. Classic chemistry is forever limited to electronic, unit electron volt, (eV), level energy exchanges. (Fire, explosives, etc.) Nuclear, as we currently have under complete control is still a hunter-gather Uranium mining operation, and at the ideal, breeder efforts. Fusion as we now explore it as a non-energy producing, controllable system, is also a hunter-gather operation, even if totally successful (take deuterium from extant water, manufacture tritium using uranium fission systems, etc.)

We, as a civilization, are newborns, and only have 50 years of working on a moderately, yet accelerating, nuclear understanding. Tesla conceived of man ultimately connecting to the very wheelwork of the universe for his energy needs. He also realized that if there was no way to connect to this imagined, seemingly infinite source of energy, that man would be doomed to being a hunter-gatherer, thereby limiting his ability to advance beyond a point, as yet to be understood. If hunter gathering is to go beyond earth, it will involve raping asteroids and other rocky planets and moons of their surface mined elements in a distant future. This may include man, like some super spreader event, living on other worlds.

We are still poking at the bear, so to speak, at the nuclear end of things as it seems a possible step forward toward Tesla's dream or at least a stop gap plan for the immediate future to maintain civilization at some survivable level.

The talk of nuclear spins and their control represents how far out on a nuclear limb we are musing for the future. Alas, musing doesn't feed the bulldog. Physical embodiment feeds the bulldog, be it via experiment and subsequent success or new insights gained from failures that point a way forward. Thinking and musing do nothing without a follow up physical experiment.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Spin-forbidden reactions?

Post by Dennis P Brown »

Richard, your answers are always fun, interesting, very informative, and a pleasure to read - I need to start taking notes on how to answer questions with as much flair as you manage.

In regards to spin states it is important to know that 'tunneling' has really nothing to do with that quantum property. Since fusors depend on tunneling, I suspect that this might be their interest in spin aspects of fusion fuels. Just as a FYI for everyone expecting tunneling rates to change due to spin states is not supported by known physics. While not your question I'm just saying that is a tree best left unbarked up at.

As for unlimited energy - it is claimed that the Casimir effect could, in principle, supply unlimited energy from the 'vacuum' - if one follows Field Theory and its implications to their logical conclusion. While metal plates on galactic sizes might be needed, in principle, if (and this is the issue for me) - if the vacuum energy exists in all of space (which I have strong reason to doubt), then one could get free, inexhaustible energy from the vacuum. Doubt Tesla had advanced that far into any aspect of quantum to think of this possible methodology. One must be careful when discussing Tesla since, not unlike the totally unhinged thinking of Q-Anon folks, many strange ideas have strung up about Tesla*. So, maybe someone thinks he has/did tap into this 'vast' cosmic force ;)

*However, it is absolutely true he invented the first radio system that worked and deserved credit for that device. Strangely, that fact ultimately led to his ruin but that is a long story and irrelevant to this post.
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Re: Spin-forbidden reactions?

Post by RobertMendelsohn »

It seems as though the answer to my question is here:

http://large.stanford.edu/courses/2011/ph241/ray1/

It appears that spin dependence is a real thing and is known experimentally to improve fusion cross sections, by an experimentally considerable factor.
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Dennis P Brown
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Re: Spin-forbidden reactions?

Post by Dennis P Brown »

Interesting student paper submitted for a undergraduate course. I will add that the composite course paper doesn't say if spin affects tunneling rate (which I do not believe occurs - so for fusors, unlikely to be useful), rather then fusion cross-sections only via the process of overcoming the coulomb barrier that is used by typical fusion experiments (i.e. high energy KE rather than tunneling.)

Aside: that summary article was submitted by a student for a sophomore course - so whether their references support their claims this may or may not be valid (I would think they do but certainly very old references - two are nearly 40 years old!); however, you might want to read those references (and find more recent ones, too) and determine if his statements follow from them before accepting these conclusions as supported fact.

Regardless of this persons claims, the huge energy costs involved in creating specific spin states would make any such process energy prohibitive in all likelihood. That no place doing fusion energy research bothers with spin alignment indicates this is highly likely to be a dead end.
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Re: Spin-forbidden reactions?

Post by RobertMendelsohn »

Yes agreed about a sophomore paper (good work for a sophomore though!); here is a recent paper, indicating this is indeed a theoretically sound if relatively unexplored approach:

https://www.academia.edu/download/43974 ... fc0ycp.pdf

Please correct my understanding, but I thought it did not take a lot of energy to produce particular spin orientations?
I've seen optical pumping, Stern-Gerlach, reflection off of surfaces, magnetic fields + low temperatures all used to produce spin-polarized sources for less than an electron volt per atom. Perhaps I am missing something?

Thank you for an interesting conversation.

-Robert
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Dennis P Brown
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Re: Spin-forbidden reactions?

Post by Dennis P Brown »

Most the methods you suggest can't be used for atomic nuclei spin alignment (you really need to read up on this subject so you understand it better.) The only two that are applicable are ultra-low temperature and magnetic. Both require a great deal of energy. Then there are other issues: for magnetic selection, once the spin state is created it is injected in a plasma but then those spin states can change (and will) after non-fusion ion collisions (which will dominate.) So the yield (which I am not aware of anyone knowing - something you should research) is likely trivial for that application and the energy for the magnetics is high (or extremely costly if SC are used.) As for cooling to create selected spin states, again, very high energy cost to get to the required temps and then only useful for inertial fusion (which has extremely low yields to date.) So while the increased in fusion cross section of '100%' sounds impressive it really isn't considering current low rates of interactions.

As for energy cost per atom - first, that is an engineering question and I will wave that off but an eV is NOT a unit that directly translates for a clear understanding of energy cost in a real operating device (so do look up the conversion before posting such a unit and convert to watts relative to a real application.) Secondarily, one useful method is to use first principles and considering for a plasma one needs 10^17 particles and for solid state fusion devices one requires 10^23 atoms, those energy costs look to be huge.

Sorry but your link fails so you need to try that again.

Again, I am not aware that any one has used these methods (but clearly are aware of such) and if that is the case, certainly indicates that such a process is not considered useful and for current fusion real world approaches, is a dead end.
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Re: Spin-forbidden reactions?

Post by RobertMendelsohn »

Working link (ostensibly): https://link.springer.com/article/10.11 ... 9614010250
There is a free PDF from Academia.edu via Google Scholar, if you search "Polarized Fusion Engels".

I'd comment that I do not think that spin-polarization is a route to a energy-positive fusion device, and agree that existing devices probably are not amenable to the use of spin-polarization; the cross section for spin relaxation is approximately equal to the collision cross section. It could perhaps be used for BoT types, but my interest in it is more of a understanding-fusion-type nature than "I've solved fusion!" type claim.

Back to the question of nuclear spin polarization, the most common method in the literature appears to use hyperfine transitions after initial electron-polarization sorting in a Stern-Gerlach apparatus, followed in popularity by Lamb-shift techniques; actual use of the low temperatures + large magnetic fields technique appears uncommon. Experimentally, this is quite realizable, and a steady trickle of papers (and a book) have come out in the last 10 years.

Re: eV vs. watts, this is precisely why it's a good unit - it is independent of engineering choices. In the case of the Stern-Gerlach/hyperfine method, as little as 5W radiated has been used for [on the order of] milliampere currents of polarized nuclei. The energy differences in spin states is vanishingly small, inefficiencies tend to come from the equipment itself. I agree that in light of spin relaxation and real fusion reactor dynamics this approach faces trouble; I'll state that I'm more interested in the theoretical aspects as well as what what spin does w.r.t. reaction channels than notions of a fusion breakthrough. Of interest to those with an engineering perspective is the article "Optimization and first tests of the experimental setup to
investigate the double-polarized DD-fusion reactions", Solovev et. al. 2020. Good apparatus details are given.

I'd summarize by saying that the scientific literature shows an experimental dependence of fusion reactions on spin, and the production of spin-polarized nuclei is a ~70-year-old field, and it appears that suppression of reaction pathways via spin-polarization is a realized thing.
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Dennis P Brown
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Re: Spin-forbidden reactions?

Post by Dennis P Brown »

The primary purpose of the Stern-Gerlach apparatus is to select the nuclei not create them (yes, that results in a small number of ions with a specific spin state.) The plasma current in a typical tokamak (that does very, very little fusion energy) is measured in many mega-amps created by huge capacitor banks. So at 5 milli-amps per '5 watts' cost in power would then require a trillion watts of power for the low end value.

In the paper I read on the subject of Lamb-shift techniques relative to polarized D-D beams - it is a device to measure, not create polarization states - so not clear what you are getting at with that technique.

For useful fusion spin selection isn't viable - the paper said at best it could increase the cross-section by 50% to 100% (you do realize how tiny that is compared to what is needed?) Again, that is nothing compared to where fusion in those machines need to be - orders of magnitude increased, not a factor of 1.5 or 2x. Certainly if this process had significant affect on 'tunneling' that would be of great interest for fusors - though, again, energy cost prohibitive.

Do study this subject if nuclear polarization states for determining energy level aspects of nucleons in nuclei are of interest to you (very important for the 'shell' model.) The process has no use in power generation via fusion nor understanding fusion in stellar systems. This thread now has no relevancy to fusors and the issue of efficiency and methodology of spin production is way off topic and really doesn't need further discussion on my part*.

* Not to say it wasn't both interesting and a useful subject to learn about
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