# Why is it that only hydrogen isotopes are used for Fusion bombs?

1. May 12, 2012

### oktovan

Is there any other elements that can be used for fusion?

2. May 12, 2012

### PhysicoRaj

Hydrogen fusion occurs at a relatively low temperature than any other element. Hence hydrogen and it's isotopes are used. The temperature goes on increasing with the increase in mass of the nucleus.

3. May 12, 2012

### flatmaster

4. May 12, 2012

### mathman

An example related to this question is what happens inside stars. The H to He transformation (in several steps) is the source of stellar radiation until the H is almost all gone. Then the star interior gets much hotter and further fusion reactions, involving He and successive elements (ends at iron), take place.

5. May 12, 2012

### mesa

I thought they used Li6 and deuterium since Li6 is cheaper than tritium? Could be wrong, haven't read much about this stuff in years lol.

6. May 12, 2012

### Drakkith

Staff Emeritus
The easiest material to get to fusion is tritium and deuterium. Lithium is used because during the multi-stage process of detonation, it is turned into tritium.

7. May 13, 2012

### mesa

So the Li6 gets hit by a neutron and breaks into a tritium and two deuterium isotpoes or some other arrangement of the nuetrons and protons?

8. May 13, 2012

### Drakkith

Staff Emeritus
Lithium 6 absorbs a neutron and splits into an alpha particle and a tritium nucleus. The tritium is then fused with deuterium that is present in the lithium-deuteride mix.

9. May 13, 2012

### mathman

Another practical consideration is that Tritium is radioactive with a half life of ~ 12 years, not good for long term storage.

10. May 13, 2012

### Dickfore

The Coulomb repulsion barrier height goes as the product of the atomic numbers of the nuclei $Z^2$.

11. May 17, 2012

Yes, in stars, all elements are used in a variety of fusion reactions to create all elements up to Iron. In supernovas, temperatures are very briefly high enough to create the other heavier elements above iron so, all of the atoms that make up you and the world around you were created in a fusion process at the heart of a star or supernova.

On earth, with human technology and scale, fusion is only practical for Hydrogen and certain forms of helium as the pressures and temperatures required are near impossible for humans to create.

To put things in perspective, it requires the energy of a fission atomic bomb to start a hydrogen (Deuterium - Tritium) fusion process in the heart of a hydrogen bomb. Even with all of this power, the fusion reaction is difficult to achieve as the xray pressure from the fission reaction must be focused on the Hydrogen before blowing it apart. Fusion with anything other then Hydrogen/Helium at human scales is very difficult to achieve.

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12. May 17, 2012

Don't forget about the fabled helium-3 reaction....

13. May 18, 2012

### Staff: Mentor

Drakkith's statement is true, as D+T (one proton each) is easier than any reaction with helium (2 protons) or even heavier nuclei. The advantage of helium-3 is the possibility to have a fusion reaction without (free) neutrons.

14. May 21, 2012

I agree the statement is correct. The possiblity of having a reaction without free neutrons I believe is critical to the success of implementing fusion power stations as a reactor that destroys it containment vessel would be unpractical. I just thought I would mention the reaction as I feel its very relavent to the difficulties associated with fusion and fusion in industry.

15. May 30, 2012

### tasp77

You also have less energy released per pair of nuclei fused as you go from hydrogen to iron. Even if it turned out to be 'relatively' easy to fuse neon into calcium (to cite a fanciful example), the expected energy pay off for the reaction would be low.

16. May 30, 2012

### Drakkith

Staff Emeritus
I don't disagree, but consider the following. Per reaction the decay of Uranium by fission releases FAR more energy than the fusion of Hydrogen. (17 MeV in fusion compared with 200+ MeV in fission of Uranium) I don't know the amount of energy released by fusing neon into calcium but I expect that it might release a substantial amount of energy. (Just less energy per nucleon than hydrogen, similar the the Uranium)

17. May 31, 2012

### shreyakmath

Thats because it requires less energy to fuse 2 hydrogen molecules than fusing molecules of higher mass.
In the core of red giants, carbon is formed by fusion of helium which requires much more energy. So, hydrogen can be fused easily to produce lots of energy.