Nuclear Customs: How to Inspect a Warhead Without Opening It express
A customs officer can't take a suitcase apart to see what's inside. They shine X-rays through it and, from the shadows on the screen, tell: here's metal, here's organic matter. In much the same way, an inspector for future nuclear disarmament treaties should be able to check a warhead — without opening it or seeing its secret internals.
Neutrons are better suited for this than X-rays. When you direct a neutron beam at an object, some neutrons get absorbed by uranium or plutonium nuclei. But absorption isn't uniform: at certain neutron energies, the nuclei 'gobble them up' especially eagerly. It's like light passing through a stained-glass window: the glass lets some colors through and blocks others. By which colors are missing, you can tell what metals were added to the glass.
The scientists built a device about the size of a small table. Inside, neutrons travel just two meters, and their time of flight is measured — from which their energy is calculated. If the sample contains, say, uranium-235, a characteristic dip appears on the graph at an energy around 1–100 electron-volts. Such a dip is like a fingerprint: each isotope has its own, and it tells you exactly what's in front of the inspector.
They tested it on three materials: highly enriched uranium, depleted uranium, and reactor-grade plutonium. In two hours, they gathered enough data to see all the needed dips. Then a program called REFIT compared them with calculated ones and produced the composition: the fraction of uranium-235 was determined to within 5%, and the isotopic composition of plutonium to within 6%. For a first proof of concept, that's a good result.
Such a device could become a tool of trust: one side shows that the warhead really contains uranium, not a dummy, without revealing the exact design. The neutron 'stained glass' reveals only composition, not blueprints. It's like a signature on a document: it confirms authenticity but doesn't disclose the contents.
Original abstract
as written by the authors arXiv:2603.08912
Neutron Resonance Transmission Analysis (NRTA) was explored as an arms control verification approach to support potential future nuclear weapon limiting treaties. A compact and portable neutron Time of Flight (ToF) system was developed to enable proof-of-concept NRTA measurements of special nuclear material (SNM). Using a short 2-meter flight path, the NRT system is sensitive to cross-section resonances of isotopes such as 235U, 238U, 239Pu and 240Pu between 1-100 eV incident neutron energies due to their physical nuclear structure. The detected neutron ToF spectrum exhibits transmission dips at resonance energies that are characteristic of SNM isotopic composition in the inspected item. The proof-of-concept measurements of Highly Enriched Uranium (HEU), Depleted Uranium (DU), and Reactor Grade Plutonium (RGPu) confirmed the characteristic resonance features within two hours of data collection time. Analysis via the REFIT resonance fitting tool accurately predicted the 235U enrichment and Pu isotopic composition within 5% and 6% of the known values, respectively.
The time for a neutron to fly through the two-meter tube is about 200 microseconds — a hundred times faster than the blink of an eye.
In the TV series 'The Expanse', Mars customs officers scan ships for nuclear materials — looks like the future is already here.