A method for investigating a volume of material at an investigation location for radioactive material includes: detecting signals from a first detector caused by the interaction of one or more types of emission from the radioactive material with the first detector; detecting signals from a second detector caused by the interaction of cosmic rays and/or one or more types of particle generated by cosmic rays with the second detector; processing the signals from the first detector to provide information on the amount of radioactive material associated with the volume of material; processing the signals from the second detector to provide information on the amount of cosmic rays and/or one or more types of particle generated by cosmic rays incident on the investigation location; and correcting the information on the amount of radioactive material associated with the volume of material using the amount of cosmic rays and/or one or more types of particle generated by cosmic rays incident on the investigation location, thereby providing corrected information on the amount of radioactive material associated with the volume of material.