Abstract: A method and materials are described for the monitoring of ionizing radiation in a location or on a person. The method comprises providing a fluorescent organic sensor material in the location where monitoring is required, the fluorescent organic sensor material having a molecular structure comprising at least one pair of fluorophore units, the units being held together by a spacer unit such that they are capable of interacting to produce a distinct fluorescence spectrum different from those of the individual fluorophore units; exposing the sensor material to the radiation; and determining any change in the fluorescence spectrum of the sensor material after exposure.