Marathon Fusion Says It Turned Mercury Into Stable Gold With Neutrons

Marathon Fusion says it used fusion neutrons to produce stable gold from mercury in September, a result whose scale is difficult to assess because the company has not disclosed its yield.

In an Oct. 21 post, the San Francisco startup claimed its production rate was 10 times CERN’s 2025 benchmark. It has not specified whether that comparison measures output per second or total output. The company shared a gamma-ray spectrum with signals recorded eight to 44 hours after irradiation, but no yield, purity figure or experimental details.

Lead-nucleus collisions at ALICE produced a peak rate of about 89,000 gold nuclei per second. From 2015 to 2018, the Large Hadron Collider’s four main experiments produced about 86 billion gold nuclei, weighing a combined 29 picograms. The nuclei struck the collider’s pipe walls and broke apart in less than a second.

Those figures come from a different process than Marathon’s neutron-based method, and the company has not defined the measurement behind its 10-times claim. The comparison therefore does not establish how much gold Marathon produced.

Mercury-198 absorbs a neutron and emits two neutrons, becoming mercury-197. That isotope then decays into stable gold-197. Marathon’s proposal places mercury-198 in a blanket around a fusion reactor core; neutrons from the reaction could also be used to breed tritium, a fusion fuel. The company’s gold-production plan also describes the process as a potential addition to fusion power generation.

The resulting gold could contain radioactive isotopes and might need 14 to 18 years of storage before it could safely circulate. Marathon’s planned San Francisco reactor is focused on medical isotopes. The company lists a 1-kilowatt fusion system designed to produce neutrons rather than electricity, and says it has used fusion neutrons to irradiate zinc and make radioactive copper isotopes for cancer diagnosis and treatment.

Marathon estimates that a plant producing 1 gigawatt of fusion power could yield thousands of kilograms of gold annually. No commercial fusion power plant is operating, so the estimate has not been tested at commercial scale.