Neutron Production and Capture: Can Neutrons from Deuterium be Used for Fusion?

[Smacal1072]

The binding energy for nucleon in a light atom (like deuterium) is lower than the binding energy for nucleon in a heavier, more stable element like iron.

Suppose you remove the neutron from the light atom, and allow it to conjoin (by neutron capture) with an element that had an atomic number near that of iron. Since the nucleon goes from a state of low binding energy to high binding energy, would this reaction be exothermic?

I think you see where I'm going with this. Any comments?

 

[bcrowell]

The binding energy for nucleon in a light atom (like deuterium) is lower than the binding energy for nucleon in a heavier, more stable element like iron.

Suppose you remove the neutron from the light atom, and allow it to conjoin (by neutron capture) with an element that had an atomic number near that of iron. Since the nucleon goes from a state of low binding energy to high binding energy, would this reaction be exothermic?

This depends on the isotopes. If you take a neutron out of 4He and put it in, say, 70Fe, that will be endothermic, since 70Fe is already very neutron-rich.

In general, the answer to your question depends on (a) which side of the line of stability the light nucleus is on, and (b) which side the heavy nucleus is on.

 

[Smacal1072]

That's a good point. I guess what I'm curious about is: Why don't we pursue some exothermic nuclear reactions (like the transferring a neutron from deuterium to iron 56) in our search for nuclear energy? Is the cross-section too small to separate neutrons from the lower energy nucleus efficiently? Or is the energy gain not enough?

 

[bcrowell]

That's a good point. I guess what I'm curious about is: Why don't we pursue some exothermic nuclear reactions (like the transferring a neutron from deuterium to iron 56) in our search for nuclear energy? Is the cross-section too small to separate neutrons from the lower energy nucleus efficiently? Or is the energy gain not enough?

What method do you have in mind to separate the neutrons from the 2H?

You could generate neutrons in a fission reactor. A certain amount of the energy in fission reactors does come out in the form of neutrons, and all of these neutrons end up doing some kind of exothermic reaction, possibly contributing a little to the thermal energy produced, although I wouldn't think it was very much.

 

[Smacal1072]

What method do you have in mind to separate the neutrons from the 2H?

I've been reading a little on this - you can use a small neutron generator which accelerates deuterons into a deuterium target. You get a few DD fusion reactions, which produce a neutron. Also, you could use a Farnsworth fusor. They work on a similar principle.

I get the impression though that these reactions have very low cross-sections and wouldn't produce the number of neutrons necessary to generate any meaningful neutron flux. Still, it's pretty tempting. If there were a more efficient way to separate neutrons from deuterium...