A method and apparatus for obtaining radiation transmission data including providing a radiation source, e.g., x-ray or gamma-ray source, and a radiation detector system, e.g., x-ray or gamma-ray detection system, spaced therefrom to define a scanning zone therebetween, the detector system capable of detecting and collecting spectroscopically resolvable information about incident radiation. Collecting a dataset of information about radiation incident including transmissivity of an object in the scanning zone at at least one scanning position from radiation transmitted through the object and received at the detector system. Resolving each dataset spectroscopically across a plurality of frequency bands within the spectrum of the source; at least one of the frequency bands corresponding to a characteristically scattered wavelength of a target species to be identified. The absence of or substantial reduction in a transmitted signal intensity at the frequency band is interpreted as the presence of the said target species.