B Fusion and Alpha Particle Emission: Neutron vs Proton

[Aidan Davis]

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?

 

[mfb]

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.

 

[PSRB191921]

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

 

[mfb]

Ah, good point. The energies are lower. Wikipedia has a list.

 

[Aidan Davis]

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.

 

[PSRB191921]

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

 

[snorkack]

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)?

 

[Aidan Davis]

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.