Abstract: A photomultiplier (2) is disclosed having an element (16) for modifying the gain thereof either during manufacture (after the normal activation process) or in use, by changing the secondary electron emission characteristics of dynodes of the photomultiplier (2) and/or by modifying the electromagnetic field within the photomultiplier (2). The element (16) is made of a different material than the emissive surface of dynodes (8) of the photomultiplier (2). Methods of manufacturing a photomultiplier (2) and of tuning the gain of a photomultiplier (2) are also disclosed.