Could future fusion reactors also serve as laboratories in which to manufacture dark matter? That’s the provocative question behind a new theoretical study by University of Cincinnati physicist Jure Zupan, working with researchers from Fermi National Accelerator Laboratory, MIT and the Technion–Israel Institute of Technology. Their work, published in the Journal of High Energy Physics, explains how a subclass of fusion reactors lined with lithium are theoretically capable of producing axions-imaginary particles that have long been considered among the leading candidates for dark matter.
It forms about 84% of the universe’s matter content and exerts a gravitational influence on galaxies and other cosmic structures; however, it neither absorbs nor emits light. One proposed constituent of this invisible mass is ultra-light pseudoscalar particles called axions. Searches for axions generally target those that might be produced by the Sun or nuclear fission reactors, while those that are possibly produced by fusion have, until now, been overlooked. According to Zupan and colleagues, that is a matter of tradition rather than physics.
The reactor design under consideration is just like that being contemplated for ITER in southern France: deuterium and tritium fuel, burning inside a vessel lined with lithium. In such a system, each fusion reaction liberates 17.6 MeV of energy-3.5 MeV retained in the plasma by a helium nucleus, and 14.1 MeV carried away by a fast neutron. At full scale, a 2,000 MW fusion plant could support a neutron flux near the inner wall of 10¹⁵ neutrons/cm²/s, roughly 100 times higher than in comparable fission reactors. This intense neutron environment is the key to the axion production proposal.
Two different production mechanisms can be distinguished. First, neutron capture: A fast neutron impinging on the reactor wall or the breeding blanket interacts with the Li or Fe nucleus and thereby excited it, which later must rid itself of the energy. Usually this happens through gamma rays. However, if axions or axionlike particles exist, then they could be radiated instead. Moreover, the sensitivity would become highest for Li-based breeding blankets, which already serve to create the scarce tritium fuel by means of neutron-induced nuclear reactions. Second, neutron bremsstrahlung: As neutrons scatter off nuclei and lose energy, they emit “braking radiation.” In some nuclear transitions, this may be an axion rather than a photon.
Theoretical estimates compare axion emission rates with well-characterized gamma-ray emission rates from magnetic dipole transitions in lithium and iron, using measured neutron capture crosssections. Scalar particle production is more difficult to quantify, but dimensional analysis gives plausible emission rates. Crucially, the particle masses can be taken to be sufficiently low that decay into heavier particles is forbidden, and axions can travel far beyond the reactor walls before decaying.
Next comes the problem of detection. The team suggests locating a large heavy-water detector-about 1,000 tonnes-approximately 10 meters from the reactor. Heavy water hosts deuterium nuclei, which can be broken up into a proton and neutron by an axion carrying at least 2.2 MeV. The outgoing neutron can be counted and the proton tracked, leaving a clean experimental signature. Backgrounds coming from the solar neutrinos, which can also break deuterium, would be suppressed by comparing reactor-on and reactor-off periods and by making use of the directional information from the Sun.
Zupan says earlier attempts to model axion production in fusion reactors, including a recurring whiteboard puzzle in The Big Bang Theory , presumed solar-like production mechanisms within the plasma yielding fluxes far too small to detect.“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. However, one can still produce them in reactors using a different set of processes,” he said. By exploiting neutron–wall interactions rather than plasma physics alone, the new proposal sidesteps that limitation.
If implemented, these ITER-class reactors would doubly act as a means to develop clean energy, as well as acting as controlled sources of possible dark matter particles. This idea relies on specific reactor simulations for tracking events involving the slowing and capture of neutrons, and on engineering detector space within the structure of the reactor. Success will open a whole new experimental window into the dark sector, besides astrophysical searches and underground detectors like LUX-ZEPLIN, already probing the sun for axions and other such exotica.
