An apparatus for detecting radioisotopes in fluids (e.g., in gas/liquid chromatography) comprises a bundle of scintillating optical fibres 12 extending along the interior of a conduit 10 through which a fluid may flow. The bundle of scintillating optical fibres has ends 14, 16 passing through the side of the conduit 10 to connect with light detecting means 18, 20. Other embodiments and methods of use are disclosed.