For multi-energy digital radiography, a sandwich structure is proposed, comprising "scintillator-photodiode" (S-PD) type detectors. This will allow obtaining information on an object simultaneously for two or three energies in the 20-400 keV X-ray range.
Criteria have been formulated and a procedure proposed that determine requirements to detector characteristics for distinction of specified substances. Under this procedure, the following characteristics have been determined for two- and three-energy detecting systems.
For a two-energy detector system used for organic substances (Zeff=6-8) and iron/steel (Zeff=26), the X-ray tube anode voltage VA=140 kV, sensitivity range of the low-energy detector (LED) and the high-energy detector (HED) 40-60 keV and 100-140 keV, respectively. The average detector sensitivity of LED should be closer to 40 keV, and that of HED - to 140 keV. There is a limitation that is imposed upon the detector sensitivity by sensitivity of the radiographic system. For such variant, LED are made on the basis of a photodiode and a ZnSe(Te) scintillator, ensuring good sensitivity in this range and transparence in the HED range. HED can be made on the S-PD base using CsI(Tl), CdWO4 or Cd2O2S. In a three-energy system, where elements with Zeff=6,7 and 8 should also be determined detector sensitivity ranges should be as follows: LED - 10-35 keV, medium-range energy detector (MED) - 40-80 keV, HED - 100-140 keV. Such subdivision into ranges is ensured by the use of a semiconductor sensor (e.g., silicon) as LED, S-PD with ZnSe(Te) as MED, and CsI(Tl), CdWO4 or Cd2O2S - as HED.