Fusion and Alpha Particle Emission: Neutron vs Proton

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Discussion Overview

The discussion centers on the fusion of deuterons and the subsequent emission of alpha particles, specifically exploring the mechanisms behind neutron versus proton emission. Participants examine the energy dynamics involved in these processes and the probabilistic nature of the emissions, delving into both theoretical and experimental aspects of nuclear fusion.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that when deuterons fuse, they form an excited alpha particle that can emit either a neutron or a proton, questioning the mechanism that favors neutron emission.
  • Another participant claims that both neutron and proton emissions have about a 50% probability, suggesting this is sufficient for generating neutron beams.
  • Concerns are raised about the energy values associated with neutron emission, with one participant calculating a lower energy of 2.45 MeV, which is identified as the binding energy rather than the energy post-emission.
  • There is a discussion about the possible outcomes of deuteron-deuteron (D-D) reactions, with one participant asserting that alpha particles are produced only briefly before decaying, while others clarify the processes involved and their energy implications.
  • Participants discuss the branching ratios of different fusion pathways, indicating a complexity in the processes that lead to neutron or proton emissions.

Areas of Agreement / Disagreement

Participants express differing views on the energy values and the probabilities of neutron versus proton emission, indicating that there is no consensus on these aspects. The discussion remains unresolved regarding the exact mechanisms and probabilities involved in the emissions.

Contextual Notes

Participants reference various energy values and processes, but there are uncertainties regarding the calculations and definitions of binding energy versus emitted energy. The discussion also highlights the complexity of fusion reactions and their outcomes.

Aidan Davis
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When two deuterons fuse, I understand they form an alpha particle in an excited state. Based on mass differences the fusion releases 23.85MeV, which is enough to eject a proton (19.81MeV) or a neutron (20.58MeV) from the alpha particle. It seems to prefer neutron emission, as this fusion is used to make neutron beams. What mechanism is at work to make the alpha particle emit a neutron and not a proton? Does it always emit a neutron in this case, or is it a probabilistic mix of neutron and proton emission?
 
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Both processes have about 50% probability.
It is used to make neutron beams because deuterium is easily available and 50% is enough to get a large number of neutrons. The protons and helium nuclei are filtered out with magnetic fields.
 
Hi
I am surprised by the energy values you give. For example when I make the calculation,, I find neutrons' energy of 2.45 Mev
PSR
 
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PSRB191921 said:
Hi
I am surprised by the energy values you give. For example when I make the calculation,, I find neutrons' energy of 2.45 Mev
PSR
Those are the energies required to break away the particle from the He 4 nucleus, aka their binding energy. Not the energy they will have after they are released. In this case 2.45MeV per neutron is correct.
 
Ok, but I am not sure to understand. In a D-D reaction you can have n+He3 or p+H3. But no alpha particules ...
PSR
 
PSRB191921 said:
Ok, but I am not sure to understand. In a D-D reaction you can have n+He3 or p+H3. But no alpha particules ...
PSR
Very few.
You have a legal process:
1) d+d->α+γ
which releases lots of energy, almost all of it as the energy of γ (the rest is recoil of α). But, since it is an electromagnetic process, its cross-section is low compared to competing strong processes
2) d+d->t+p
3) d+d->3He+n
But since these strong processes do not create the strongly bound α, only much less strongly bound t or 3He, they release much less energy.
What is the order of magnitude for the branching ratio of 1)?
 
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PSRB191921 said:
Ok, but I am not sure to understand. In a D-D reaction you can have n+He3 or p+H3. But no alpha particules ...
PSR
There are alpha particles for a short time after the D-D fusion occurs. They are in an excited state however, so they quickly decay by proton or neutron emission. Or gamma photon emission apparently on occasion.
 

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