Abstract: A method and system are provided for more accurately and reliably characterising radioactive activity sources within a material through the use of a measurement data set from the detected emissions which is compared with a computed data set produced by a model of the location to which one or more candidate solutions for the position and/or activity for one or more model activity sources are provided. Optimisation of the match between the two data sets provides, for the characterisation.

Abstract: A method and apparatus use a first detector (7) sensitive to the interaction of cosmic rays and/or one or more types of particle generated by cosmic rays and a second detector (5a) sensitive to emissions from radioactive material to investigate a volume of material at the investigation location (1). By detecting signals from the first detector (7) during a first measurement period (W) and detecting signals from the first detector and/or second detector during the second measurement period (X), a ratio for their values can be obtained and used to indicate a possibility, wherein the possibility could be the presence of radioactive material associated with the volume of material; or the absence of radioactive material associated with the volume of material; or uncertainty as to whether there is radioactive material associated with the volume of material; or the presence of high atomic number material associated with the volume of material.