Proton–lithium-7 fusion - energy of the two He nuclei?

[Sven Andersson]

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

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

 

[mheslep]

I can't quickly find a cross section for that proton lithium7 reaction, but I believe its not feasible because of very low cross section compared to the more likely fast neutron fusion with lithium 7, which produces yet another neutron: n+7Li -> 3T + 4He + n - 2.5MeV.

As far as gammas go, in the light element fusion reactions in which they occur (e.g. p+D->3He+gamma), their energy is a small minority of the total energy which is dominated by the kinetic energy of the products.

 

[e.bar.goum]

There will be very little in gamma rays. The energy will go into the alphas, shared equally in the center of mass frame. In the lab frame, there will be a distribution of energies with angles. It's fairly trivial to calculate this - just use energy/momentum conservation.

 

[mfb]

As far as gammas go, in the light element fusion reactions in which they occur (e.g. p+D->3He+gamma), their energy is a small minority of the total energy which is dominated by the kinetic energy of the products.

If the reaction produces only one nucleus, nearly the whole energy goes to the photon. The nucleus just takes the recoil, but it does not get much energy as it is heavy.

 

[mheslep]

If the reaction produces only one nucleus, nearly the whole energy goes to the photon. The nucleus just takes the recoil, but it does not get much energy as it is heavy.

Why no gamma photon then with D+T to alpha + n, with all the energy (17mev) in the KE of the products?

 

[mfb]

Two nuclei (a neutron counts as nucleus here) that can both take kinetic energy and fly off in different directions to conserve momentum. The neutron will get most of the energy as it is lighter.
The larger the mass difference, the more pronounced the energy difference gets. And photons are massless.

 

[Alexis Komarov]

According to the paper published by Japanese, 7Li-p fusion has high neutron yield, 2.94*10^-2 vs. much lower ~7*10^-4 neutron yields for other reactions. So, having higher neutron yield and lower cross-section, 7Li-p reaction is less attractive vs. the alternatives.

http://ci.nii.ac.jp/els/110003827252.pdf?id=ART0005015547&type=pdf&lang=en&host=cinii&order_no=&ppv_type=0&lang_sw=&no=1433835136&cp=