Proton—lithium-7 fusion - energy of He nuclei?

[Sven Andersson]

In articles on aneutronic fusion, the reaction Proton–lithium-7 fusion is mentioned as a possibility. Now I wonder; what is the energy of the 2 He nuclei? How much is gamma rays?

And another question; how high voltage would it take to stop those two He nuclei completely?


Proton–lithium-7 fusion 1p + 7Li → 2 4He + 17.2 MeV

Sven

[Richard Hull]

I know you are just interested and have no way to ever do this yourself. It is just not good at all. It has a low cross section and as such is out of the running for any fusion we will ever do.

check out the highly negative review of aneutronic fusion at wiki.

http://en.wikipedia.org/wiki/Aneutronic_fusion

It is a dream not to be realized.

We can't even do the easiest fusion of all to any advantage.....D + T!!!

Let's try something that has a far lower possibility of working.......Sounds good to me.

Richard Hull

[Sven Andersson]

Yes, I know it's a reaction with a very low cross section. Still want to know. Where can I read about it? I want the details on the energy of the gamma rays and the energy of the two He nuclei. And how to calculate "stopping voltage". Thanks in advance if you can share any useful relevant knowledge on this!

I know you are just interested and have no way to ever do this yourself. It is just not good at all. It has a low cross section and as such is out of the running for any fusion we will ever do.

check out the highly negative review of aneutronic fusion at wiki.

http://en.wikipedia.org/wiki/Aneutronic_fusion

It is a dream not to be realized.

We can't even do the easiest fusion of all to any advantage.....D + T!!!

Let's try something that has a far lower possibility of working.......Sounds good to me.

Richard Hull

[Dan Tibbets]

I am less pessimistic than Richard Hull. Aneutronic fusion is possible. Admittedly, the probability is arguably remote, but several approaches claim potential, at least based on theory. These include the Polywell, FRC, and Dense Plasma Focus. The proton -Li7 fuel choice is probably the worst of three possibilities. D-He3 is easiest (relatively speaking), the problem is that He3 is very rare on Earth. Also, while it fits the definition of being an aneutronic reaction, (Less than 1% of the energy in the form of neutrons), it still makes a lot of neutrons. Proton - Boron11 is the best fuel , it is easier than p-Li7. Also, I think, the amount of neutrons produced in side reactions is very small compared to the other two likely fuel combinations.

Fusion of D-T is likely,in my worthy opinion , to reach Q>1 in a few years- JET. The problems, even with net power gain fusion via Tokamaks though is profound. The physics may work, the engineering will be profoundly difficult, and the cost per KWH, if finally successful, will be very expensive. D-D fusion is much more attractive, if it can be achieved, it might actually be economically feasible. Only DT fusion is potentially profitable in Tokamaks for several physics reasons, but in other machines this may not be the case. Note that D-D fusion can somewhat approach D-T in yield and effective probability. This is because the D-D reaction produces Tritium and Helium3 as products. These, at least the Tritium, can be fed back into the reactor so that ~ half of the total fusion power may actually be coming from this D-T fuel . Also, the neutrons can contribute to some energy yield boost by producing radioactive heat by reacting with Boron10 in a external blanket to produce excited Boron11, which breaks down into several products, releasing more energy (I forget the actual products- alphas and or tritium?). In this sense a D-D reactor would not have to quite reach breakeven, only come close, the so called 1/2 cat. reactions would supply the boost to reach potentially useful net positive fusion. Note that neutron capture in an external Boron 10 blanket yields more energy. This is much simpler than an internal liquid lithium blanket in a Tokamak. There is no need to be 100% efficient in capturing/ converting the neutrons to tritium. You do not need to make as much tritium fuel as you consume in a D-T reactor. The produced tritium in a D-D 1/2 cat reactor boosts the output but is not required in 1:1 ratios for use.

There are data tables describing specific reactions and yields. Simply assuming 17 MeV of energy shared between two alphas equally, would imply that each would have to traverse a ~ -8.5 million volt potential to give up all of their kinetic energy.This potential could be distributed across a number of grids- eg 10 grids , each at -850 KV.

[EDIT] Once the neutron is thermalized (gives up it's heat) it combines with boron10 to form an excited boron11, which quickly breaks down into an alpha, Li7, and gamma ray, releasing an additional 1.47 MeV of energy in the gamma ray and ~ 1.3 MeV in the kinetic energy of the li7 and alpha particles.
http://web.mit.edu/nrl/www/bnct/info/de ... ption.html


Dan Tibbets

[Sven Andersson]

Simply assuming 17 MeV of energy shared between two alphas equally

There will be formed a highly excited Be-8 nucleus, which gives up some of it's energy as gamma rays and then decays into two He-4 nuclei. If anybody knows how much is gamma rays, please respond or point me to a source where I can read about it.

[Rich Feldman]

That reaction was first reported by Cockroft and Walton in 1932, and earned them a Nobel Prize.

As for partition of the energy between alpha particles and gammas: C&W did measure alpha energy, and I think made no mention of gammas in the original paper.

it seems not unlikely that the lithium isotope of mass 7 occasionally captures a proton and the resulting nucleus of mass 8 breaks into two a-particles, each of mass four and each with an energy of about eight million electron volts. The evolution of energy on this view is about sixteen million electron volts per disintegration, agreeing approximately with that to be expected from the decrease of atomic mass involved in such a disintegration.

http://www.nature.com/physics/looking-b ... index.html

Their experimental goal was met. For the first time, atomic transmutation not dependent on natural radioactivity. They were not just out to bombard stuff with protons and see what happened.
Those interested in history of science might enjoy the background story in speech by the Nobel Prize presenter: http://www.nobelprize.org/nobel_prizes/ ... press.html
and this 75th-anniversary article in CERN newspaper:
http://cerncourier.com/cws/article/cern/31864

Cockcroft realized in 1928, before anyone else, the implications of Gamow's tunnelling theory, namely that an energy of 300 keV might be sufficient for protons to penetrate a nucleus of boron, and even less for lithium.

The hurdle is lower still for targets of hydrogen, much to the delight of amateur fusioneers. -ed.