D-D fusion products

1. Sep 27, 2014

TESL@

A basic question for which I could not find any answers on the internet:

What factors do determine whether the fusion results with a He-4 and gamma rays, H-3 and proton, or He-3 and neutron? If it is random, are there any methods to predict the rations of these reactions?

Thank you.

2. Sep 27, 2014

Staff: Mentor

It is random, and quantum mechanics allows to calculate the probabilities (at least in theory).

3. Sep 27, 2014

TESL@

I thought it would be closely related to temperature. So what affects those probabilities?

4. Sep 27, 2014

Staff: Mentor

The kinetic energy of the D-D pair in their center of mass frame (which is related to temperature, of course= will influence the rates at high energies, but apart from that they are fixed, given by the strength of the relevant interactions and the properties of the collision partners and final particles.

5. Sep 27, 2014

mathman

From what I can gather (Wikipedia) there is no He-4 reaction. The H-3 and He-3 reactions are equally likely.

6. Sep 27, 2014

Staff: Mentor

Well, it is an electromagnetic process, so it is significantly weaker. It should still occur sometimes.

7. Sep 27, 2014

SteamKing

Staff Emeritus
In stellar cores, like those of the sun, He-4 only results at the end of a chain of different fusion reactions, starting with two separate H-H fusion chains.
Eventually, you get He3-He3 fusion, which produces He-4 + 2p, where the 2p are then available to start another H-H fusion reaction.

http://en.wikipedia.org/wiki/Nuclear_fusion

Even given the conditions at the sun's core, the rates of these reactions are "notoriously slow" to occur.

8. Sep 27, 2014

Staff Emeritus
He3 + n and t + p should be roughly equal, because of isospin. He4 + gamma should be about 1% of those, because it's electromagnetic.

9. Sep 27, 2014

TESL@

I don't understand why "roughly". Isn't the interaction force between both nucleons equal?

I am working on an inertial electrostatic confinement fusion reactor, that I need more precise percentages in order to observe changes in fusion rate by using a neutron detector. Can you give me a range (...%-...%) or any equations?

Thank you.

10. Sep 28, 2014

SteamKing

Staff Emeritus
If we knew all the answers, we'd have working fusion reactors and flying cars by now. (Sorry.)

It's one thing to be working on the design of a fusion reactor; it's quite another to be looking for answers to theoretical physics questions on a website, even a reputable one like PF.

Don't you have access to a technical library somewhere, you know where all the papers on the latest research can be accessed?

11. Sep 28, 2014

TESL@

OK you are right. Thank you all by the way.

12. Sep 28, 2014

TESL@

Here I found what I was looking for:

source: http://en.wikipedia.org/wiki/Cold_fusion#Lack_of_expected_reaction_products

Conventional deuteron fusion is a two-step process, in which an unstable high energy intermediary is formed:

D + D → 4He* + 24 MeV
Experiments have observed only three decay pathways for this excited-state nucleus, with the branching ratio showing the probability that any given intermediate follows a particular pathway. The products formed via these decay pathways are:

4He* → n + 3He + 3.3 MeV (ratio=50%)
4He* → p + 3H + 4.0 MeV (ratio=50%)
4He* → 4He + γ + 24 MeV (ratio=10-6)

13. Sep 28, 2014

Staff Emeritus
Yes, but the mass of the decay products is not, so you have phase space effects slightly favoring p+t. These are at the fraction of a percent level, but so is the He4 channel.

14. Sep 29, 2014

e.bar.goum

NNDC to the rescue!

What you can do is to compare the relative probabilities ("cross sections") of each of the reactions.

In general, you can look at Sigma (Evaluated Nuclear Data File retrieval and plotting) from NNDC for a lot of cross sections, but that doesn't give me the d-d fusion probability, it seems. A different database - EXFOR (Experimental Nuclear Reaction Data) gives me a whole lot of hits for that reaction. You'll have to decide what energy range you're interested in though.

If you're interested in temperature averaged cross sections (energy is not the same as temperature, after all), S-factors are also available in EXFOR for a lot or reactions.

15. Oct 1, 2014

TESL@

Thank you e.bar.goum!