Sonoluminescence is a phenomenon in which megahertz sound waves in a liquid can cause a bubble to collapse into a very small dot, creating a high temperature there. Using deuterated acetone as the liquid, some researchers reported detecting fusion neutrons created by the collapsing bubble. However, experts on sonoluminescence, including Seth Putterman of UCLA, were not able to reproduce these results and have categorically stated that this is not a way to produce fusion. It appears that this is an even more extreme farce than cold fusion.

Muon Fusion

This is the original idea on cold fusion, having been disclosed by Luis Alvarez in his Nobel Prize speech in 1968 [49]. Muons are fundamental particles like electrons but 207 times heavier. They are produced in accelerators and live for 2 ms (an eternity!)

before decaying. As you know, elementary particles and photons have a dual nature, sometimes behaving like particles and sometimes like waves. As waves, they have a wavelength, called the deBroglie wavelength, which is inversely pro­portional to their masses. Being some 200 times heavier, muons have wavelengths 200 times shorter. A negative muon can take the place of an electron in an atom, and the “cloud” of negative charge is then 200 times smaller, bringing the nuclei of molecules closer together. The muon-fusion process for DT molecules is shown in Fig. 10.55.

In the first line of that figure, normal D and T atoms with their large electron clouds can combine into a DT molecule, just as two H atoms can form H2. In the second line, a д-meson (muon) replaces the electron in the tritium atom, and the resulting muonic tritium atom has a smaller size. Next, a deuterium nucleus joins the triton inside the muon cloud, forming muonic DT with the two nuclei close together. Normally, the D and the T repel each other with their positive charges and cannot fuse into helium at room temperature. However, in quantum mechanics, particles can tunnel through the Coulomb barrier if it is thin enough. In a muonic DT molecule, this can happen very fast, and the entire process can happen several hundred times during the 2-ps lifetime of the muon. In the last line of Fig. 10.55, DT fusion has occurred, creating the usual products of a neutron and an alpha

particle. What the muon does then is essential. If it flies off, it can catalyze another fusion again and again. However, if it “sticks” to the alpha particle, it is carried off and is lost. The sticking fraction is between 0.4% and 0.8%, and this limits the number of reactions that one expensive muon can catalyze.

Experiments are being done in accelerator laboratories like RIKEN-RAL4 in England and TRIUMPH in Vancouver, Canada. About 120 DT fusions per muon have been observed [28]. At 17.6 MeV per event, this amounts to over 2 GeV of energy. However, it takes 5 GeV to make each muon. There are ways to improve this ratio, by using polarized deuterons, by working at high temperatures, or by making cheaper accelerators. At this stage, the physics of muon fusion is still in its infancy.