Abstract: An apparatus for detecting the interface between a hydrogen-containing substance and a second substance of different hydrogen content in a vessel comprising a first elongate source body containing a source of fast neutrons and having an L/D of at least about 4 and providing a substantially uniform flux density of fast neutrons radiating outwardly along the length of the elongate body, a second elongate detector body, the detector body including a slow neutron detecting substance providing substantially uniform detection of slow neutrons along the length of the detector body to provide an output proportional to the number of slow neutrons detected, the source body and the detector body being disposed in substantially parallel relationship to one another, in sufficient proximity to one another and to the vessel and in overlapping relationship to the interface whereby the output from the detector body varies substantially linearly with the change in position of the interface in the vessel.