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 (40, 54,56, 58) which alternate with and are thereby sandwiched, by thin crystals (42, 48, 50, 52), 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 (40, 54,56, 58) which alternate with and are thereby sandwiched, by thin crystals (42, 48, 50, 52), 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.