Why Does He3 Fusion Create He4 + 2 Protons?

[BWV]

Curious, why does He3 fusion (3He + 3He → 4He+ 2p) create He4 + 2 protons instead of Beryllium? (or any other arbitrary rearrangement of the protons & neutrons)

 

[CompuChip]

You shouldn't just look at the number of protons, but also the number of neutrons.
(3, 2)He + (3, 2)He
would have to give (6, 4)Be, which does not appear to be an existing isotope (or may be so unstable that it immediately decays to He(4, 2) + 2 p(1, 1)).

 

[BWV]

You shouldn't just look at the number of protons, but also the number of neutrons.
(3, 2)He + (3, 2)He
would have to give (6, 4)Be, which does not appear to be an existing isotope (or may be so unstable that it immediately decays to He(4, 2) + 2 p(1, 1)).

so is the stability of the isotope the sole determinant? (6,3)Li is stable, so is there some reason you would not get (3, 2)He + (3, 2)He = (6,3)Li+ (0,1) H ? which superficially seems more stable than a combination with 2 free protons

 

[robert2734]

If you create a single particle, you won't conserve momentum and energy. Suppose two nuclei of He3 of equal energy collide head on. Then conservation of momentum demands that the resulting Li6 be at rest. So where does the excess binding energy go?

If the He3 were not of equal energy, we transpose the problem into the center of mass frame. The Li6 must be at rest in the CoM frame of the heliums.

It's an offshoot of the "a stable particle can't decay by endowing it with lots of energy" problem. In some reference frame the stable particle is at rest.