A smoke detector comprises a soft x-ray source (10) and an air space (16) communicating with ambient air (18) and positioned so that soft x-rays produced by the soft x-ray source (10) pass into the air space (16) to ionise air in the air space (16). A detector (14) is positioned for detecting ionised air in the air space (16). Smoke particles entering into the air space (16) mop up ionised air This leads to a reduction in the number of ions detected which triggers an alarm. The soft x-ray source (10) preferably comprises a substrate (22) on which are provided a plurality of nanometer scale elongate structures (28). The elongate structures (28) are provided in an evacuated chamber (42) formed between the substrate (22), a spacer (30), and a laminated film (36). The laminated film (36) comprises an aluminium foil layer (38) and a supporting nylon layer (40). A voltage converting circuit (44) is powered by a 9 volt DC battery and applies a 1.5 kV DC voltage between the substrate (22) and the aluminium foil layer (38). In use, the electrical voltage induces field emission of electrons from the elongate structures (28). The electrons are accelerated across the chamber (42) to the aluminium foil layer (38), where they collide with the foil layer (38) and produce the soft x-rays.