A material discrimination detector comprising: - a single broad energy band X-Ray source adapted to project a range of high energy X-rays of 1 MeV and above, towards an object, - a composite detector arranged to be placed beyond the object, and which on bombardment by transmitted X-rays is configured to produce substantially simultaneously: a) a first component predominantly attributable to the higher energy component of the incident X-rays and b) a second component predominantly attributable to the lower energy component of the incident X-rays, - circuit means adapted to determine the light generated in scintillating crystals of the composite detector by the said two components, said circuit means corresponding to a read-out system which is coupled to the scintillating crystals for reading out the latter, - means for generating from the thus determined light numerical values relating thereto, - means for forming a ratio of one numerical value relative to the other, - and a look-up table of atomic numbers and ratio value, wherein the mean atomic number for material through which the X-rays have passed derived using the derived value of the said ratio, for the material in question, characterized in that the composite detector comprises: - a thin front crystal so that the energy deposited is more or less independent of X-rays energy, and the thin front crystal has to preferentially respond to lower energy X-rays in the range 1 MeV to 1.5 MeV, - a low-Z beam hardener which preferentially removes lower energy X-rays from the X-ray beam, - a series of high-Z converters which alternate with and are thereby sandwiched, by thin crystals , which sample the electrons produced by collisions upstream of the crystals, the thin front crystal configured to be read out from each side as by a photodiode, or fibre, and the outputs from the two opposite sides of the crystal being added. - a single broad energy band X-Ray source adapted to project a range of high energy X-rays of 1 MeV and above, towards an object, - a composite detector arranged to be placed beyond the object, and which on bombardment by transmitted X-rays is configured to produce substantially simultaneously: a) a first component predominantly attributable to the higher energy component of the incident X-rays and b) a second component predominantly attributable to the lower energy component of the incident X-rays, a) a first component predominantly attributable to the higher energy component of the incident X-rays and b) a second component predominantly attributable to the lower energy component of the incident X-rays, - circuit means adapted to determine the light generated in scintillating crystals of the composite detector by the said two components, said circuit means corresponding to a read-out system which is coupled to the scintillating crystals for reading out the latter, - means for generating from the thus determined light numerical values relating thereto, - means for forming a ratio of one numerical value relative to the other, - and a look-up table of atomic numbers and ratio value, - a thin front crystal so that the energy deposited is more or less independent of X-rays energy, and the thin front crystal has to preferentially respond to lower energy X-rays in the range 1 MeV to 1.5 MeV, - a low-Z beam hardener which preferentially removes lower energy X-rays from the X-ray beam, - a series of high-Z converters which alternate with and are thereby sandwiched, by thin crystals , which sample the electrons produced by collisions upstream of the crystals, the thin front crystal configured to be read out from each side as by a photodiode, or fibre, and the outputs from the two opposite sides of the crystal being added. <EOS>