Why are neutrons given off during fusion?

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Offline thedoc

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Why are neutrons given off during fusion?
« on: 09/12/2014 11:30:01 »
Wilf James  asked the Naked Scientists:
Why are liberated neutrons the only means to detect that fusion has
taken place?

I have only recently heard about the "Collapsing Bubble" idea put
forward by Rusi Taleyarkhan and I remember clearly the furore produced
by the Fleischman and Pons reports. Both ideas were crushed (apparently) because either there were not enough neutrons liberated or that the neutrons appeared at the wrong time. I'm not a nuclear physicist but I thought that I had understood the
principle of nuclear fusion.

I have always understood that if two deuterium atoms combine to produce
one helium atom there is only one thing produced apart from the helium -
energy. Two atoms each consisting of one proton and one neutron are combined to
produce one atom consisting of two protons and two neutrons.

Where do the spare neutrons come from that have been used to decide if
either of the two sorts of experiments are valid? Forgive me if I think of the reaction as being like a chemical reaction where the number of elementary particles that existed before a reaction will be the same number existing after the reaction has taken place. 2(1P 1N) = 1(2P 2N)   energy

If the true fusion reaction works like the equation then it would be the
ultimate clean source of energy. I have not heard anything regarding the two sorts of experiment about the lack of or excess of various sorts of quarks.

All those who have judged the experiments have based their judgements on
the neutrons that were given off. Where would these neutrons have come from if there were no spare neutrons input into the experiments? (I have disregarded any experiments involving tritium where spare neutrons are to be expected.)

With regard to the report I saw about the Fleischman and Pons work, one thing jarred about the way that their work was supposedly repeated. Fleischman and Pons used potassium hydroxide as an electrolyte. The "repeated" experiments used containers open to the atmosphere. Potassium hydroxide is an excellent agent for removing carbon dioxide from the atmosphere. The alleged report on Fleischman and Pons's work said that after exhaustive tests the only product found was potassium carbonate.

What do you think?
« Last Edit: 09/12/2014 11:30:01 by _system »


Offline alancalverd

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Re: Why are neutrons given off during fusion?
« Reply #1 on: 09/12/2014 12:41:20 »
As I understand it, P & F claimed to have detected both neutrons and tritium (a low-energy beta emitter, so very easy to detect) and an anomalous, cyclic increase in temperature. Most current fusion experiments seem to involve tritium which will result in a "spare" neutron.   
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Offline evan_au

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Re: Why are neutrons given off during fusion?
« Reply #2 on: 11/12/2014 10:20:38 »
Current fusion experiments are aimed at a fuel mixture of Deuterium and Tritium, which fuses to produce He + a neutron:
2H + 3H → 4He + 0n
This releases a lot of energy, and requires the lowest temperatures and pressures to initiate the nuclear reaction*.
If this were a chemistry class, the Deuterium+Deuterium reaction sounds like a perfectly balanced equation: 2H + 2H = 4He

However, the problem is that the reaction energy must be released by some mechanism.
  • If it were released as an almighty gamma ray (around 14MeV), we could absorb that in the reactor vessel and use the energy to produce electricity.
  • But this gamma ray has just the right energy to split 4He into 2H+2H (as well as a number of other reactions - pair production requires only 1MeV)
  • But this gamma ray production does not occur, because (in my simple understanding) the energy and momentum carried away by the photon and the recoil of the He nucleus would not balance across the equation.
So the reaction paths that do occur don't release as much energy, and require the release of a proton or a neutron to carry away the energy and momentum of the Deuterium reaction:
2H + 2H → 3H + 1H
2H + 2H → 3He + 1n

In chemical reactions, there are lots of other nearby reactants to carry away any excess energy and momentum.
However in nuclear physics, the range of the strong force is so small, and the timescales so short that you can't rely on other reactants to be close enough to carry away the energy and momentum. Each and every isolated atomic reaction must be balanced (including spin, mass/energy and momentum).

While an energetic proton is quickly stopped due to its positive charge, the uncharged neutrons pass through the reactants, and should be detectable outside the reaction cell - if fusion is occurring. This lack of neutrons cast doubt on whether cold fusion was occurring.

*Muon-catalysed fusion does occur at a much lower temperature, but it takes too much energy to produce the Muons, so it is thought that this reaction could never reach the break-even point. (Could the cold fusion results have been triggered by mesons from cosmic ray showers?)
« Last Edit: 11/12/2014 16:43:53 by evan_au »


Offline evan_au

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Re: Why are neutrons given off during fusion?
« Reply #3 on: 11/12/2014 16:52:12 »
Quote from: evan_au
If this were a chemistry class
Sorry if this comment offended everyone who has attended a chemistry class.

After a number of fraudulent demonstrations of turning lead into gold, transmutation has got a bad reputation with both Chemists and Royalty.

Perhaps I should have described the "expected" deuterium fusion reaction as: np + np → n2p2 + ΔE
This looks a bit more like a balanced reaction (the way I learned it in chemistry class)...