Deuterium fusion into Helium-4


by qraal
Tags: deuterium, fusion, helium4
qraal
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#1
May7-09, 03:41 AM
P: 774
Hi All

Just wondering why does deuterium fused with itself not make helium-4 - instead it makes a triton or a helion and a proton or a neutron. These products could then fuse with deuterium to make helium-4.

What stops deuterium from fusing straight into alpha particles? Is the kinetic energy too high for it to stick together as a whole or is it driven by a more exotic process?
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fatra2
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#2
May7-09, 07:20 AM
P: 478
Hi there,

I could be mistaking, but the reason for deuterium not fusing into helium-4 come from the nuclear spin. Deuterium has a nuclear spin of +1, and helium-4 has a spin of 0. Then again, both combination of tritium (+1/2) + proton (+1/2) or helium-3 (+1/2) + neutron (+1/2) fit well.

Cheers
malawi_glenn
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#3
May7-09, 09:49 AM
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but spin 1 coupled to spin 1 can give spin 2,1 or 0 as final state.

clem
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#4
May7-09, 10:00 AM
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Deuterium fusion into Helium-4


In order to conserve both energy and momentum, fusion can only occur with two particles going to two (or more) particles.
Vanadium 50
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#5
May7-09, 11:25 AM
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Quote Quote by qraal View Post
What stops deuterium from fusing straight into alpha particles?
What makes you think the reaction [itex]d+d \rightarrow {\rm{^4He}} + \gamma [/itex] doesn't occur?
QuantumPion
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#6
May7-09, 12:21 PM
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Quote Quote by clem View Post
In order to conserve both energy and momentum, fusion can only occur with two particles going to two (or more) particles.
How do you explain neutron capture then? E.g. Au-197 + n -> Au*-198
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#7
May7-09, 04:18 PM
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Quote Quote by Vanadium 50 View Post
What makes you think the reaction [itex]d+d \rightarrow {\rm{^4He}} + \gamma [/itex] doesn't occur?
Emitting a photon brings in a factor of alpha=1/137 compared topurely strong processes.
clem
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#8
May7-09, 04:22 PM
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Quote Quote by QuantumPion View Post
How do you explain neutron capture then? E.g. Au-197 + n -> Au*-198
The key there is the star. When states of finite width are involved there can be absorption of neutrons with energies within the width of the excited state. That can't happen with
d+d-->He, all of which are stable.
qraal
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#9
May7-09, 04:35 PM
P: 774
So any definite answers? Guys?
Bob S
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#10
May7-09, 05:54 PM
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Muon-catalyzed d-d fusion has been observed in bubble chambers, and does fuse into an alpha particle and a 5,5 MeV gamma ray. See
http://en.wikipedia.org/wiki/Muon-catalyzed_fusion
http://prola.aps.org/abstract/PR/v106/i2/p330_1
hamster143
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#11
May7-09, 08:12 PM
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I believe clem is correct, there's nothing to carry away extra energy unless you go electromagnetic (d + d -> He4 + gamma) and that process is suppressed by the factor of 1/137 compared to d + d -> He3 + n and d + d -> t + p.
qraal
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#12
May7-09, 09:12 PM
P: 774
Well that makes sense. The case of muonium fusion doesn't change that because the muon is another particle in the mix.

What's the relative fusion probabilities of d+d and d+t and d+He3 then? Obviously it depends on the abundance of d, but how much easier or harder are the other two reactions when both the t and He3 are being made from the d+d reaction?

I'm wondering just how hot the core of a brown dwarf star gets in order to burn deuterium, which must be pretty easy to do (inside brown dwarfs I mean) since it's only 1/6000th of the hydrogen available.

A recent paper on the Anthropic Principle (do you want me to drag out a reference?) discussed alternative Universes with higher levels of deuterium. What if a star was mostly deuterium? How light would it be and still achieve fusion?


Quote Quote by hamster143 View Post
I believe clem is correct, there's nothing to carry away extra energy unless you go electromagnetic (d + d -> He4 + gamma) and that process is suppressed by the factor of 1/137 compared to d + d -> He3 + n and d + d -> t + p.
Vanadium 50
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#13
May8-09, 12:23 AM
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Quote Quote by clem View Post
Emitting a photon brings in a factor of alpha=1/137 compared topurely strong processes.
That doesn't prevent the reaction from occurring.
clem
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#14
May8-09, 07:54 AM
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But it is not d+d-->He, which I thought was the original question.


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