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

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Discussion Overview

The discussion revolves around the proton-lithium-7 fusion reaction and its implications for energy output, specifically focusing on the kinetic energy of the resulting helium nuclei and the role of gamma rays in the process. The conversation touches on theoretical aspects of fusion, potential energy yields, and comparisons with other fusion reactions.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the feasibility of the proton-lithium-7 fusion due to its low cross section compared to other reactions, such as fast neutron fusion with lithium-7.
  • Another participant suggests that in the proton-lithium-7 fusion, the energy will primarily go into the kinetic energy of the helium nuclei, with minimal energy allocated to gamma rays.
  • It is noted that in light element fusion reactions, the energy of gamma rays is a small fraction of the total energy, which is dominated by the kinetic energy of the products.
  • One participant discusses the distribution of energies in the lab frame and mentions that energy and momentum conservation can be used to calculate the outcomes of the reaction.
  • There is a query about the absence of gamma photons in the D+T fusion reaction, despite the energy being primarily in the kinetic energy of the products.
  • A participant highlights that the mass difference between the neutron and the helium nuclei affects the distribution of kinetic energy, with the lighter neutron receiving more energy.
  • Reference is made to a paper suggesting that the neutron yield from the lithium-proton fusion is higher than that from other reactions, which raises questions about its attractiveness as a fusion option.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility and energy distribution of the proton-lithium-7 fusion reaction, with no consensus reached on its viability or the role of gamma rays in the energy output.

Contextual Notes

The discussion includes assumptions about the behavior of particles in fusion reactions and the implications of mass differences on energy distribution, which may not be fully resolved or universally accepted.

Sven Andersson
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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
 
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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.
 
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.
 
mheslep said:
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.
 
mfb said:
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?
 
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.
 
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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=
 

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