- Broadband x-ray spectra
- Fast alpha particles and protons
- 4He off-gas
- Transmutations of various kinds
Part of the motivation for the earlier blog post was to get feedback from people on vortex-l, and as I hoped would happen, Robin, who is on that list, pinpointed several difficulties that needed to be addressed. Here are questions from him (1 and 2) and me (3 and 4) that resulted from the thread:
- How do two deuterons approach the Pd nucleus concurrently, as this seems a very unlikely thing to occur?
- The d+d fusion cross section becomes negligible below 5 keV. Assuming a loss of 400 eV per palladium atom that a 20 keV deuteron passes through as a result of interactions with its electrons, the energy of the deuteron will drop below the 5 keV threshold after passing through 38 palladium atoms. In the unlikely event that it hits another deuteron head-on before that, even then a fusion is not assured. So all-in-all the likelihood of a self-sustaining reaction seems small. How can one be obtained under these circumstances?
- The regular branches for d+d fusion are (a) d+d→t+p (50 percent), (b) d+d→3He+n (50 percent) and (c) d+d→4He+ɣ (almost negligible). What causes branches (a) and (b) to be suppressed and branch (c) to become dominant?
- When you have fast particles flying through a deuterated metal lattice, a particle is likely to bump into a deuteron, and it in turn will hit another deuteron. Occasionally a side reaction of branch (b), above, will occur, yielding a significant number of neutrons which would then exit the system. But neutrons are only rarely seen and at levels barely above the sensitivity of the neutron counters. For this reason Peter Hagelstein places a 20 keV upper limit on the energy of the particles in the system. Ron's account involves alpha particles with energies of tens of MeV, so the lack of neutrons from side reactions on an order above that currently seen could be expected, presenting a challenge to be addressed.
The full vortex-l thread can be found here. I am sure that the difficulties go back to my own understanding and have been anticipated by Ron. I will be interested to hear how he addresses them, especially (3).
EDIT: Concerning items (1) and (3), above, Ron addresses these questions in his original physics.SE post:
The fusion of deuterons always happens through unstable intermediate states, and the cross section to alpha particle is only small because of the same non-relativistic issue. To get an alpha, you need to emit a gamma-ray photon, and emissions of photons are suppressed by 1/c factors. When there is a nucleus nearby, it can be kicked electrostatically, and this process is easier than kicking out a photon, because it is nonrelativistic (the same holds for an electron, but with much smaller cross section due to the smaller charge, and there is no reason to suspect concentration of wavefunction around electron density, as there is for a nucleus).
The time-scale for kicking a nucleus is the lifetime of the two-deuteron resonance, which is not very long, in terms of distance, it is about 100 fermis, this is about the same size as the inner shell. If the deuterons are kicking about at random, this coincidence is not significant, but if the deuteron-hole excitations are banded, it is plausible that nearly all the energetic deuteron-deuteron collisions take place very close to a nucleus, as explained above.
There are conservation laws broken when a nucleus is nearby. The nucleus breaks parity, so it might open up a fusion channel, by allowing deuteron pairs to decay to an alpha from a parity odd state. Such a transition would never be observed in a dilute beam fusion, because these fusions happen far away from anything else. This hypothesis is not excluded by alpha particle spectroscopy (there are a lot of relevant levels of different parities), but it is not predicted either.This only hints at an answer to question (1), by saying that the banded state makes it "plausible" that the energetic deuterons will encounter one another near a palladium nucleus.