Might the secret to dark matter be lurking in the very devices engineered to fuel our destiny? Physicists have now sketched out a theoretical route to creating axions-theoretical particles that have long been thought to constitute dark matter-within deuterium-tritium fusion reactors lined with lithium, an idea that wittily solves a scientific puzzle once lampooned in “The Big Bang Theory”.
In the fifth season of the sitcom, fictional physicists Sheldon Cooper and Leonard Hofstadter struggled with axion production in a fusion device and were defeated-a defeat commemorated by a forlorn doodle on a whiteboard. In real life, Jure Zupan of the University of Cincinnati, along with collaborators from Fermi National Accelerator Laboratory, MIT, and Technion–Israel Institute of Technology, has now explained how such production could take place-not as a joke, but as an actual proposal worthy of study.
Axions are very light, weakly interacting particles that could explain the 85% of the Universe’s matter believed to be dark. While invisible to light, their gravitational fingerprints clearly show in the rotation curves of galaxies and the motion of stars. Historically, searches have been directed to the Sun or nuclear fission reactors, whereas fusion devices have been largely overlooked. That this is so is surprising: a 2,000 MW-class deuterium-tritium plant could sustain a neutron flux near the inner wall of 10¹⁵ neutrons/cm²/s-about 100 times more in a similar class fission reactor.
In the proposed apparatus, high-energy 14.1 MeV neutrons from the fusion core will impinge on lithium-based breeding blankets and steel structures, causing nuclear reactions. In a few of these reactions, rather than a gamma ray, the excited nucleus emits an axion-like particle, or ALP. Another channel involves neutron scattering accompanied by bremsstrahlung “braking radiation” in which energy radiated by the decelerating neutron could emerge as an ALP. These mechanisms differ from the Primakoff effect that dominates in the solar axion production, sidestepping the unfavorable detection odds that doomed Sheldon and Leonard’s fictional attempt.
Lithium’s role here is not just structural. In advanced reactor designs, such as those studied for ITER and DEMO, lithium serves as a tritium breeding medium and, in liquid form, as a plasma-facing component (PFC) with unique thermal and radiation-handling properties. Simulations of liquid lithium divertors show they can form a dense, low-radiation secondary plasma that shields components from intense heat loads, reducing photon radiation power to as little as 0.36% of incident energy far below tungsten’s 51.34%. This resilience makes lithium-lined walls attractive not only for sustaining fusion but also for surviving the neutron bombardment crucial to axion generation.
It is ingenious to detect these particles. The team envision a heavy-water Cherenkov detector, similar to the Sudbury Neutrino Observatory, located about 10 meters from the reactor. ALPs that managed to escape the vessel could break up deuterons in the detector into protons and neutrons with energies well above the 2.2 MeV threshold. In a year’s operation, the anticipated signal might be distinguishable from the 4800 yearly background events from solar neutrinos by comparing running times with the reactor-on and reactor-off.
This is complementary to well-established axion searches, such as the microwave-cavity-based ADMX haloscope, that aim to detect galactic dark matter axions converting into photons within strong magnetic fields. While haloscopes cover the cold dark matter halo, the fusion-based axion searches would explore axion coupling within a high-flux nuclear environment, thus extending the coverage for the particle’s possible parameter space.
The bremsstrahlung channel is particularly interesting from the nuclear physics point of view: under fusion conditions, neutron slowing down is accompanied by complex scattering dynamics, and the spectrum of emitted radiation is well-characterized for photons. Using measured gamma emission rates from lithium and iron, Zupan’s group can estimate ALP production rates with fewer theoretical uncertainties than for scalar particles, whose emission lacks a direct electromagnetic analogue. If implemented, this dual-purpose use of fusion reactors-energy generation and fundamental particle searches-could turn facilities such as ITER or DEMO into unprecedented laboratories for beyond-Standard-Model physics. As Zupan said, “Neutrons interact with material in the walls. The resulting nuclear reactions can then create new particles.” In other words, the very engineering challenges of fusion-managing extreme neutron fluxes and plasma-wall interactions-could become the tools for probing one of cosmology’s deepest mysteries. The kicker, for anyone who loved science, is that a decades-long pop-culture punchline about axion math gone wrong could be the prelude to a far more astonishing ending-real fusion reactors become axion factories, and the dark sector driving the shaping of the cosmos may light up. It might be Sheldon’s sad face that has reason to smile.
