Science ScienceAlert Fusion Reactors Might Create Dark Matter Particles, Physicists Show

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Reactors designed to produce energy from the fusion of atoms could have an unexpected scientific side benefit.

An international team of researchers has shown that low-mass dark-sector particles, such as the hypothesized axion, might be forged in fusion facilities – not as by-products of the fusion process, but through interactions between high-energy neutrons and the reactor walls.

Their proposal turns what was once thought impossible into a realistic theoretical pathway and a promising step towards future experimental searches.

Related: Korean Fusion Reactor Sets New Record For Sustaining 100 Million Degree Plasma

Dark matter is one of the biggest cosmic question marks, a theoretical solution to an observed conundrum.

The gist of it is that the amount of normal matter in the Universe is far too low to produce the amount of gravity we see. Something out there we have yet to identify is gravitationally binding the Universe together in a vast web, without producing or absorbing any light we can detect, or interacting much at all with anything else beyond gravity.

We call this something dark matter. Scientists calculate that normal matter accounts for only about 16 percent of the matter in the Universe, with the remaining 84 percent consisting of dark matter.

There are many theoretical candidates for its identity, from microscopic black holes to weakly interacting massive particles to ultra-light particles, including axions – one of the leading contenders.

The notion that axions or axion-like particles can emerge from stellar fusion is not a new one, with multiple mechanisms proposed. It stands to reason, therefore, that axions could also emerge in a fusion reactor.

But there’s a big, devastating catch: The amount of axions expected from a star, never mind a much smaller reactor, is far, far too low to be detected.

“After completion of this work we became aware that a similar idea of producing axions in fusion facilities was discussed in episodes SE501-SE503 of the sitcom show The Big Bang Theory,” writes a team led by physicist Jure Zupan of the University of Cincinnati in a new paper.

“Sheldon Cooper and Leonard Hofstadter considered production of axions in plasma, which unfortunately does not lead to a large enough axion flux.”

Rather than looking to the plasma, Zupan and his team considered another approach: the absorption of the enormous flux of high-energy neutrons by lithium in the breeding blanket of a deuterium-tritium fusion reactor.

Here’s how it works. In this kind of fusion reactor, the breeding blanket is a thick layer of lithium-rich material wrapped around the vacuum vessel at the reactor core. The purpose of this is twofold. As the plasma swirls around, it produces a huge flux of very energetic neutrons. These slam into the blanket, which helps convert the kinetic energy they carry into heat for power production.

At the same time, the neutrons are captured by lithium nuclei, which then break apart into helium and tritium. The reactor can use that tritium to fuel itself further. It’s called a breeding blanket because it ‘breeds’ tritium. It’s very nifty.

The interactions with the breeding blanket and the walls of the reactor, the researchers ascertained, may produce other particles, too.

Their mathematical analysis shows that axions or axion-like particles could also emerge in neutron-capture interactions, or from the release of energy as the neutron slows down after scattering off another particle, a phenomenon known as neutron bremsstrahlung.

The theoretical flux of axion-like particles from these processes is much higher than the flux from fusion, and may even reach detectable levels outside the reactor walls, the researchers found. Their work offers a new way to look for solutions to the mysteries of dark matter.

“The Sun is a huge object producing a lot of power. The chance of having new particles produced from the Sun that would stream to Earth is larger than having them produced in fusion reactors using the same processes as in the Sun,” Zupan says.

“However, one can still produce them in reactors using a different set of processes.”

The research has been published in the Journal of High Energy Physics.

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