A theory of nuclear reactions that was long abandoned may not only be right, but may mean that certain kinds of fusion reactors would be more efficient than people hoped.

Henry Weller, a physicist at Duke University, and his colleagues, examined what happens when a hydrogen atom (a proton) is smashed into an atom of boron at high speed. Weller and the team at the Triangle Universities Nuclear Laboratory expected that when hydrogen and boron fused, the result would be a three alpha particles, or helium nuclei. One would be high-energy, moving fast. The other two would be low-energy ones. Experiments done in the 1980s seemed to confirm this result.

But what Weller found was different. The proton hit the boron nucleus and produced two high energy alpha particles and one that was slower. That seemed wrong until the group looked up an old result from the 1930s. At that time scientists at the Cavendish Laboratory in England had found that when a boron nucleus was hit with a proton it produced two high-energy alpha particles, with the third one moving much more slowly.

Weller was at a loss to explain how this result got missed over the 75 years since the original work was done, especially given the experiments in nuclear fusion that have been done since the 1950s.

The part of this work that is more interesting, however, is that it shows that hydrogen-boron fusion may be better at producing electricity than other methods.

Much of the current research into fusion power involves finding ways to contain a plasma of hydrogen, deuterium, or tritium at extremely high temperatures. A side effect of these reactions is producing neutrons, which can then heat water to run turbines. In that sense a fusion reactor would work in a similar way to fission reactors).

The fusion of a boron nucleus with hydrogen doesn't produce neutrons. This means that the reaction doesn't need the same heavy shielding around it to protect people. It also means that the alpha particles can be used to create electricity directly. By shooting the alpha particles through a gauntlet of magnets, one can generate electrons. This is the reverse of the process used in particle accelerators. Instead of sending energy from a magnet to a subatomic particle, the particle (an alpha particle in this case) is pumping energy into the magnets.

Weller found that since the boron fusion process produces twice as many alpha particles moving at high speed, and thus would produce twice the energy. That makes a boron-proton reactor that much more feasible.

Of course, such a reactor hasn't been built yet. The temperatures required for the sustained fusion of boron with hydrogen are in the tens of billions of degrees, and nobody has managed to build a device that can confine hot plasma at that temperature and achieve self-sustaining fusion. But it does make exploring boron fusion more interesting.

Below: an animation showing a proton (in black) hitting a boron nucleus, with five protons and six neutrons. Courtesy Focus Fusion Society.

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