Theoretical And Experimental Studies For X-ray Imaging Detectors Based On Mercuric Iodide

Hard X-ray phase-contrast radiography is a promising novel technique for the observation of internal structures of low absorbing materials. By this technique, the extraction of structural details does not rely anymore on the analysis of the X-ray intensity fluctuations due to the sample's absorption but on the reconstruction of the phase changes induced by the refraction of different domains inside the object and that causes a better imaging contrast than conventional absorption imaging. But applications of this technique rely on digital X-ray imaging detectors with large area, high sensitivity and high spatial resolution. In ordr to satisfy above all the imaging requirements, this paper will deal with the development of a high performance X-ray imaging detector.A direct conversion X-ray imaging detector based on mercuric iodide (HgI₂) and photoinduced discharge (PID) readout by green wavelength semiconductor lasers was put forward. Two-dimension images were achieved by ultra-thin linear laser beam scanning and the parallel signal readout based on multiple narrow strip electrodes, so it is possible getting an X-ray imaging detector with large area, high sensitivity and high spatial resolution.The overall plan of this detector system was given and also its operating principle, features and difficulties were analyzed. Further, the designs of films and electrodes and processing flow of this detector fabrication were given, and also the preliminary schemes of readout circuits, opticmechanical scanning and system synchro control were given.Several primary performance parameters of this detector, sensitivity, spatial resolution, pixel discharge time and Detective Quantum Efficiency DQE, were analyzed in detail from theory and simulated numerically. The computing model and method of every performance parameter were given, Monte Carlo algorithm and iterative algorithm was applied respectively for computing the spatial resolution and pixel discharge time, relative affecting factors were investigated and their relation curves were given, and also some optimized values for the design of this detector were given.A unique physical vapor deposition (PVD) device for HgI₂ deposition was developed and it can endure the corrosion of HgI₂, keep high vacuum and long life, and also prevent the surroundings pollution since HgI₂ is highly toxic. Depending on this device and low purity (99.5%) low-cost HgI₂ source material, polycrystalline HgI₂ films have been grown with dimensionsΦ130 mm in diameter onto ITO-coated glass substrate. The growth techniques used can be easily extended to produce much larger film areas and the thickness of the grown layers, size of the grains and crystallinity can be regulated in a controlled way by adjusting the growth parameters.The basic physics characteristic, dark current and response characteristic to X-ray were tested and the results show that the films have preferred crystalline orientation (00l), dark current density less than 10 pA/mm², volume resistivity in the order of 1013 ??cm and X-ray response sensitivity of about 16μC/cm²Â·R. The results can comparative with the foreign level and the fabrication of high performance HgI₂ polycrystalline films grown from the low purity source material is very significative to decrease the development cost of detectors.