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TRYING to extract limitless amounts of energy from nuclear fusion has proved
tantalisingly difficult. Scientists are still struggling with huge reactors
capable of containing the temperatures and pressures needed to make nuclei fuse.
But there is another way: persuading a particle called a muon to squeeze
together adjacent nuclei.

Muon-catalysed fusion has faced two big hurdles. Now an international team
has cracked one of these, with a nifty way to bump up the number of nuclear
reactions each muon achieves before it decays. Meanwhile, a group of Japanese
physicists is making progress on the other.

The muon aids fusion by first replacing an electron orbiting around a tritium
nucleus—a heavy isotope of hydrogen—forming muonic tritium. If a
deuterium nucleus is added, this creates a compound molecule. The muon, now
orbiting the whole compound molecule, squeezes the two nuclei closer together
until they fuse and become a helium nucleus, otherwise known as an alpha
particle (see Diagram).

Because the muon does the squeezing, there is no need for high temperatures
and pressures. The researchers are reluctant to use the name “cold fusion” for
their technique. “But it’s colder than anything else going,” says team leader
Glen Marshall of the TRIUMF particle physics lab in Vancouver, British
Columbia.

In order to get a decent yield, the energy of the muonic tritium must be kept
very low, about 1 electronvolt (eV), as it approaches the deuterium nuclei. In
earlier work, researchers had used a scattergun approach, firing particles with
different energies at a deuterium target. But Marshall’s team discovered that
muonic tritium escapes from a tritium-hydrogen mixture when its energy is about
1 eV. “That was a coincidental discovery,” admits Marshall. “We used that fact
to select atoms with the right kind of energy.”

The researchers fired their beam of 1 eV muonic tritium at concentrated
deuterium condensed onto gold foil, and chilled the whole set-up to 3 kelvin.
When they measured the number of fusions per muon by detecting the alpha
particles produced, they clocked up a rate a hundred times as high as the
scattergun approach. “It’s a very interesting and important step,” says Ken
Nagamine at the Japanese Institute of Physics and Chemical Research (RIKEN) in
Wako-shi.

Nagamine’s team is currently working on the other problem dogging muon
fusion. In a reactor core each muon must catalyse about 300 fusions before its
natural lifetime expires. The trouble is, the negatively charged muon tends to
stick to the positive alpha particle at the end of the reaction.

Nagamine says that the muon can break free under the right conditions. “We
have discovered a process of high energy recoil of products which can strip the
muon from the alpha particle.”