User:ObsidianOrder/Cold fusion/theory

Theory and mechanisms
Cold fusion reactions of the type reported by P&F have a number of features which are (impossible according to/incompatible with/counter to) conventional fusion theory:

Insufficient energy to bring nuclei together
In hot fusion, colliding deuterium nuclei (D+) require on the order of 100 keV in kinetic energy to overcome the electrostatic repulsion and come close enough that the strong electrostatic force takes over and results in fusion. That corresponds to an effective temperature of hundreds of millions degrees Kelvin. It is unclear how any process can concentrate that much energy in a single deuterium atom in an electochemical cell, or inside the palladium metal lattice, at room temperature, since the average energy of D atoms under those conditions is many orders of magnitude lower.

A more general statement of the theoretical problem is that since the energy and distance scales of chemical (or electrochemical) reactions and nuclear reactions are so disparate, the chemical environment of an atom should have no effect on its nuclear reactions. However, (non-CF/non-controversial) experimental results from the bombardment of deuterium or other targets in metal foils with low-energy D+ ions which suggest that the probability of nuclear reactions is strongly enhanced by the metal lattice environment compared to bombarding the same target outside of the metal, which (cannot be explained by/contradicts the predictions of) conventional theory that the chemical environment of the deuterium should have no effect.

Since excess heat and other anomalous behavior has only been reported in D or H absorbed in a metal lattice, some physicists have tried to formulate a theory that explains how the metal lattice can serve to effectively lower the electrostatic repulsion or help nuclei tunnel through the potential barrier, in a manner similar to high-temperature superconductivity or other macroscopic quantum effects. Several such theories have been proposed, including resonant tunneling, ion band states and Bloch condensates.

Lack of expected isotopic products
In hot fusion the reaction between two deuterium nuclei produces T + p and 3He + n in roughly equal quantities. Although production of low levels of both T and 3He has both been reported in cold fusion  they are not observed in a quantity consistent with the energy output.

It has been proposed that the predominant reaction pathway is D + D -> 4He + gamma, which is a very rare reaction pathway in hot D+D fusion (1 in 104). Production of 4He has been reported in cold fusion reactions in quantities sufficient to account for a large fraction of the energy output, but the corresponding gamma rays are mostly absent. There is no definite theoretical explanation for why a different reaction pathway should predominate in cold fusion, although there is some support for the existence of such an effect from experiments that show a change in reaction branching ratios in deuterated metal foils bombarded with deuterium ions.

Other reactions, such as H + D -> T+gamma, and a number of compound reaction paths such as H + D -> 3He followed by 3He + D -> 5Li -> 4He + p have also been proposed, but the combined probability of compound reactions would ordinarily be expected to be much lower than that of a single reaction.

Lack of energetic protons, neutrons and gamma rays
If cold fusion was D + D fusion with branching ratios equal to those in hot fusion, approximately half of the reaction would follow the D+D->3He + n pathway, and consequently a high level of neutron radiation corresponding to the excess energy output would be expected. Such neutron radiation would be easily measurable and even enough to be dangerous to anyone in the vicinity (?? rad/second for a person 1m away from a cell with 1mW excess power). While that level of radiation is not seen, there are reports of very low neutron emissions, starting with the original P&F announcement (cite Mizuno, T., et al., Neutron Evolution from a Palladium Electrode by Alternate Absorption Treatment of Deuterium and Hydrogen. Jpn. J. Appl. Phys. A, 2001. 40(9A/B): p. L989-L991.). The measurement of low levels of neutrons is difficult due to background radiation and so the validity of these results is disputed (cite Jones, S.E., et al., Search for Neutron, Gamma, and X-Ray Emissions From Pd/LiOD Electrolytic Cells: A Null Result. Trans. Fusion Technol., 1994. 26(4T): p. 143.).

The alternative proposed reaction, D+D->4He+gamma, should produce a high level of 20MeV gamma rays, which would also be very easy to detect. Those are not seen either, although there are reports of low levels of gamma ray emissions (cite Narita, S., et al. Gamma Ray Detection and Surface Analysis on Palladium Electrode in DC Glow-like Discharge Experiment. in Tenth International Conference on Cold Fusion. 2003). Most other proposed pathways also involve the emission of either gamma rays or neutrons.

The only type of energetic particle which has been detected at significant levels is alpha particles (4He2+ nuclei) with energies of ??MeV (cite Szpak, Miley). Conventional fusion theory does not predict the production of alpha particles.

Direct conversion of energy to heat
In hot fusion, a large fraction of the total energy output is carried away as high-energy particles. Since those are mostly absent in reported experimental results, cold fusion would require a highly efficient mechanism to convert fusion energy directly to heat. Several theories of such a mechanism have been proposed, typically by some means of coupling the fusing nuclei to a region of the palladium lattice such as phonons (cite Hagelstein) (similar to the Mossbauer effect, but transferring orders of magnitude more energy). An alternative mechanism for conversion of energy to heat would be through energetic particles with a low mean path such as alpha particles.