Detection of organic materials by spectrometric radiography method
  In this paper we report a spectrometric approach to dual-energy digital radiography that has been developed and applied to identify specific organic substances and discern small differences in their effective atomic number. An experimental setup has been designed, and a theoretical description proposed based on the experimental results obtained. The proposed method is based on application of special reference samples made of materials with different effective atomic number and thickness, parameters known to affect X-ray attenuation in the low-energy range. The results obtained can be used in the development of a new generation of multi-energy customs or medical X-ray scanners. 
S.V. Naydenov, V.D. Ryzhikov, G.M. Onyshchenko, P. Lecoq, and C. Smith