Fusion Terminology: Is it Fission or Fusion?

[Khashishi]

Ok, I am a fusion physicist, but I still don't know what the strict criterion is for calling a nuclear reaction a fusion reaction. Yes, it's a collision between two nuclei that results in a bigger nucleus. But, then $^{11}B + ^{1}H \rightarrow 3 \alpha$ is usually called a fusion reaction. Isn't it more like fission?

Is it called fusion because most of the energy released comes from the low binding energy of hydrogen-1?

 

[HallsofIvy]

$^{11}B+ ^1H\rightarrow 3\alpha$ starts with two particles (on the left) and ends with one particle (on the right). Isn't that what "fusion" means?

 

[Bandersnatch]

$^{11}B+ ^1H\rightarrow 3\alpha$ starts with two particles (on the left) and ends with one particle (on the right). Isn't that what "fusion" means?

$^{11}B+ ^1H\rightarrow \alpha + \alpha + \alpha$

How about now?

 

[mfb]

@Bandersnatch: :D

That reaction could be both, fusion and fission.
I think (something with protons)+(something with protons) -> (something) is fusion. It might be followed by a fission reaction.

 

[the_wolfman]

I agree that the p+B reaction doesn't fit into the typical cookie cutter textbook definition of "fusion." But IMO its the best label.

Its helpful to at the binding energies per amu

H-1 is 0. MeV/amu
He-4 is 7.07 MeV/amu
B-11 is 6.93 MeV/amu

Notice that B-11 and He-4 have roughly the same binding energy per amu. This means that most of the energy (7.1 out of 8.7 Mev) released is coming from the conversion of hydrogen to helium. Thus from an energetic standpoint I argue that this is a fusion reaction.

Also the technology required to extract power from this reaction is similar to other fusion reactors but very different from a typical fission reactor. Thus I also argue that from a practical standpoint it useful to categorize p+B with other fusion reactions.

Note that p+B is not unique. If you look up the CNO cycle you'll see that there are several intermediate fusion reactions that involve H reacting with an medium sized nucleus resulting in helium and a smaller product nucleus. Example (H+N-15 = He-4 + C-12).

 

[physwizard]

$^{11}B+ ^1H\rightarrow \alpha + \alpha + \alpha$

How about now?

good one!