The invention provides a detector assembly for use in a nucleonic level gauge comprising: a) at least one radiation detector; b) electronic apparatus for controlling the at least one detector and processing the electronic signals produced by the at least one detector; and c) a photomultiplier; wherein all of said components (a)-(c) are capable of being enclosed in an elongate, radiation-permeable housing having a cross-sectional area which is substantially uniform along the length of the housing. A detector apparatus may contain a plurality of such detector assemblies contained within a single housing. The detector is preferably an elongate plastic scintillator.