| 摘要 |
A method and apparatus are provided which 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.
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