Heavy water (the kind where ordinary hydrogen is replaced by its beefier cousin, deuterium) was subjected to an electric current. Palladium settled on the cathode, and in this modest electrochemical bath, detectors repeatedly caught something unexpected: traces of slow neutrons.
There were two kinds of detectors. The first kind is coated with a layer of boron: when a slow neutron hits the coating, charged fragments are produced, leaving visible tracks in the plastic. The second kind is identical but without boron; they don't see slow neutrons and only show background from other particles. The difference between the two detectors' readings is the sought-after signal.
In runs with heavy water and a magnetic field of 0.25 tesla, this difference was clear. It corresponded to a flux of about 6.7 neutrons per square centimeter per second, as if a few invisible particles were landing on your palm every second. When the field was removed, the difference dropped by a factor of six. And in an identical run with ordinary water, there was no signal at all.
The authors are in no hurry to explain. They simply report: there is a reproducible signal that behaves like slow neutrons. And they offer a protocol — how to run such experiments to distinguish a real effect from chance. For the physics of metal-deuterium systems, this is not an answer but a carefully marked trail toward future questions.
Boron-10 is one of the best absorbers of slow neutrons: that's why it's used both in detectors and in control rods of nuclear reactors.