Key Takeaways
- Marathon Fusion says its fusion neutrons can make medical isotopes, including terbium-149 for targeted cancer therapy.
- The company said in an X post on 21 October 2026 that its neutrons turned mercury into stable gold, beating CERN's record by a factor of 10.
- A December 2025 preprint says gold co-production could cut the plasma gain needed for economic viability from roughly 10 to 100 to roughly 3 to 5, a cut of about 3.3 times at the low end (10 divided by 3) and 20 times at the high end (100 divided by 5).
- BusinessToday reports the company's gold output figures are modelled and its experimental claims still need independent validation.
Marathon Fusion, a San Francisco startup, said in an X post on 21 October 2026 that its neutrons turned mercury into stable gold, beating CERN's record by a factor of 10. It also says its fusion neutrons can make medical isotopes, including terbium-149, a radioactive isotope studied for targeted cancer therapy.
Some targeted radiotherapies attach radioactive isotopes to molecules that bind to specific cancer cells, BusinessToday explained. Marathon Fusion said on 24 September 2026 that researchers have studied terbium-149 for decades as a candidate for targeted cancer therapy but have never been able to produce it in quantities sufficient for clinical use.
What Marathon Fusion Said on X
The post, dated 21 October 2026, made three claims. The first sentence reads: "Last month, we demonstrated high-rate alchemy: turning mercury into real, stable gold, beating CERN’s record by a factor of 10." The second reads: "Last week, we enriched isotopes in the world’s most powerful plasma centrifuge, beating the Soviet record that stood for 40 years." The third reads: "Now, we’re building a fusion reactor in San Francisco to achieve record power density."
BusinessToday, which reproduced the post, said the energy-spectrum plots label copper-64, copper-67, mercury-197 and gold-197 signals but do not establish total gold output, purity, efficiency or commercial scalability. It described the company's radiotherapy chart as showing cumulative targeted-radiotherapy trials rising to approximately 390 by 2025. The article also noted that CERN's ALICE experiment estimated only about 29 picograms of gold over four years. Related WYDE coverage: CERN Shuts Down the Large Hadron Collider to Prepare Its High-Luminosity Upgrade.
Why Medical Isotopes Carry the Patient Case
Jason Parisi, Marathon Fusion's principal research scientist, said in the September release: "Electricity is the application everyone associates with fusion, but those neutrons can also be used to mass-manufacture medical isotopes, creating a new supply chain for this critical industry." Adam Rutkowski, chief technology officer and co-founder, put the patient case this way: "Our work shows that there's a practical solution leveraging existing infrastructure to not only get Tb-149-based drugs through clinical trials, but to reach patients all over the world."
The same release said the route still needs testing. Parisi said: "With the model in place, it's now critical to move forward with experimental confirmation," and BusinessToday added that commercial supply would depend on production yields, isotope purity, processing facilities, regulatory approval and reliable delivery.
The Preprint Math: Gold Output and Plasma Gain
The gold rate traces to Marathon's 2025 paper, an arXiv preprint whose abstract says computer simulations of neutron behaviour (neutronics) show a tokamak (a doughnut-shaped fusion reactor design) can produce gold at about 2 tonnes per gigawatt of thermal power per year. BusinessToday called the company's gold projections modelled, not demonstrated commercial output.
A December 2025 preprint says co-producing electricity and gold can reduce the plasma gain needed for economic viability from roughly 10 to 100 down to roughly 3 to 5. Plasma gain is the ratio of fusion power produced to the heating power put into the plasma. Read as thresholds, that is a cut of about 3.3 times at the low end (10 divided by 3) and 20 times at the high end (100 divided by 5). BusinessToday's account of the October claims does not mention plasma gain.
Funding, Stage and Founders
Marathon Fusion's July 2024 release says it began operations in 2023. TechCrunch reported that the company had raised a $5.9 million seed round led by 1517 Fund and Anglo American, and its release put total funding at $6.9M with an ARPA-E CREATE award. Heatmap described the company in August 2026 as a seed-stage startup eyeing a Series A. The September 2026 release names Kyle Schiller as chief executive and co-founder.
People Also Ask
Did Marathon Fusion turn mercury into gold?
Marathon Fusion said in an X post on 21 October 2026 that its neutrons turned mercury into stable gold, beating CERN's record by a factor of 10. BusinessToday reported that the claim still requires independent scientific validation.
How could fusion neutrons make medical isotopes?
Marathon Fusion's 24 September 2026 release says fusion reactors produce an extraordinary number of high-energy neutrons that can mass-manufacture medical isotopes. The company's Jason Parisi said the route needs experimental confirmation.
Is the two-tonne gold figure measured output?
No. BusinessToday says the two-tonne figure per gigawatt of thermal power per year comes from Marathon's 2025 paper, which rests on neutronics simulations, and calls it modelled, not demonstrated output.
Sources: BusinessToday, Marathon Fusion on X, PR Newswire, arXiv, TechCrunch, PR Newswire, Heatmap.