1. Method of determining the distribution of radioactive material within a region comprising: inputting a plurality of radiological measurements and associated geometrical measurements taken at a plurality of positions in said region, wherein the radiological measurements are obtained using an un-collimated total counts radiation sensor, or an un-collimated total absorbed dose radiation sensor, or an un-collimated gamma spectrometer; defining a 3D model of structures within said region; obtaining the position within the model of each radiation measurement by using the geometrical measurements; ascribing the radiological measurements to a distribution of sources restricted to defined locations in the 3D model; parameterizing the source distribution over the defined source locations, where the number of parameters exceeds the number of radiological measurements; relating each parameter to the calculated observable radiation field, calculated using a physical model, at each measurement position; adjusting the parameters to optimize the correspondence between the actual radiological measurements and the calculated observable radiation field, to yield the distribution of radioactive material as defined by the adjusted parameters.