Level and interface detection

An improved method is described for detecting the interface between two immiscible or partly miscible liquids having different abilities to moderate fast neutrons and/or to absorb slow neutrons. The method is adaptable to continuous monitoring of the position of the interface. The method comprises exposing the liquids to fast neutrons emitted by a source adjacent to the containment vessel and monitoring the slow neutrons emerging from the liquids and may be arranged to operate an alarm system. The neutron source and slow neutron detector may be contained in a portable probe unit that can be moved backward and forward either over the outer surface of the containment vessel or inside an inserted dip-pipe. (U.K.)