A method of determining the dose rate of a radiation source includes locating three or more detectors in the vicinity of the source. Each detector provides an output indicative of the amount of radiation received from the source and determining the location of the source from at least three of the detector outputs, wherein as many of the detector outputs is required to provide an acceptably accurate result are used in determining the location. Thus, the dose of radiation from the source can be determined from the determined location of the source and either a known activity of the source or a measure of the activity of the source determined by means of the detectors.