A process for detecting an interface by neutron diffusion

The invention concerns a process and an apparatus for detecting an interface of substances having different hydrogen contents, present in a pipe or receptacle in metal (steel). It can concern the interface of a diphasic flow (hydrocarbons or water in the presence of gases, air or other gases) or of the level of an organic solid powdery substance containing hydrogen. A neutron source containing 0.1 to 1 μg of 252Cf and a neutron detector are placed on or near the external face of the steel wall. The neutron detector is not located at more than 50 cm from the neutron source; it is a counter tube filled with He-3 more sensitive to neutrons diffused by collision with the hydrogen than to the fast neutrons of the source. In particular, the invention makes it possible to detect the liquids (water, hydrocarbons) present in the gas conveying facilities, the presence of foam or the level of a fluidised bed of polymer substances. It can be used for warning or control purposes