The researchers started with an existing neutron radiation detector, which they adapted to form a system made up of a gadolinium plate capable of capturing neutrons that have been slowed by a plastic shell around the plate. The plastic, which is sensitive to gamma radiation, slows the neutrons down. The neutrons are then captured by the gadolinium, emitting gamma rays in the process. The challenge was to distinguish between the gamma rays coming from outside sources (whether environmental or artificial, including potential threats) and those generated by the neutrons captured by the gadolinium plate.

Researchers at List, a CEA Tech institute, tackled the problem with signal analysis methods capable of identifying the unique characteristics of the gamma rays emitted by the gadolinium plate. Here's how: First, gamma rays' energy varies depending on their origin. Second, the fact that the gadolinium simultaneously emits rays in several directions when it reacts (coincidence). And, last, the time that elapses between when the nuclei are pushed back in the plastic by the neutrons, and when they are captured by the gadolinium.

The different possible scenarios were simulated to identify the best detection windows, for example. An initial prototype of the physical detector was built, and its capacity to discriminate between the gamma rays emitted by the neutrons captured by the gadolinium was confirmed in lab tests.