Recently developed semiconducting HgI2 polymeric composite x-ray detectors are very promising for digital radiography and non-destructive testing. We present a new model that explains well the variation of the detector's sensitivity with applied bias and crystallite shape and size, based on charge transport between grain boundaries through the polymeric layer gaps. A previously published model, which was developed for a different polymer/semiconductor composite failed to account for many details of our experimental results. The new polymeric binder presented in the present paper showed non-linear dark current versus voltage dependence, and a higher sensitivity. At the low voltage range, it showed a higher sensitivity for a reduced grain size. The new model addresses the grain geometry as an important factor, and accounts well to most sensitivity characteristics of our detectors. The model excludes itself from a debate in the literature concerning specifics of charge transport mechanisms in composites containing electrically conducting particles. However, certain charge transport between neighbor crystallites is necessary in order to explain the experimental results.
Polycrystalline mercuric iodide (HgI2) photoconductor material was directly deposited on flat panel amorphous silicon (a-Si) thin film transistor (TFT) pixel arrays in order to test their application as direct x-ray conversion detectors. The 4' x 4' and 2' x 2' detector plates were fabricated either by Physical Vapor Deposition (PVD) or the Screen-Print (SP) method. Although developed for medical radiological imaging, they can also be used for nondestructive test imaging. The present HgI2 arrays have 100 μm x 100 μm pixels on the 2' x 2' detector and 139μm x 139μm on the 4' x 4' imager. The initial results are very promising and show high x-ray sensitivity and low leakage current. The advantage of these detectors is that they can be directly deposited on the pixellated arrays containing the TFTs and other electronic read out circuits and can be fabricated in large sizes. These polycrystalline PVD-HgI2 thick film detectors have now been fabricated up to 1,800μm thick, which makes them also useful for higher-energy X-ray applications. Imaging results obtained by both PVD- and SP-HgI2 will be shown. The effect of the crystallite size on the imaging properties will be demonstrated and the difference in sensitivity applying positive or negative bias on the top electrode will be discussed. Comparison of x-ray sensitivity to other photoconductor materials such a-Se and PbI2 will also be presented.
For the first time polycrystalline HgI2 photoconductor material directly evaporated on a-Si array for direct conversion of x-rays for imaging purposes, were successfully imaged at Xerox-Palo Alto Research Center. The initial results are very promising and show a high x-ray sensitivity and low leakage current. Since Ti-W alloys are used as pixel electrodes, an intermediate passivation layer must be used to prevent a chemical reaction with the detector plate. The thickness that these Polycrystalline HgI2 thick film detectors have been fabricated until now is up to 1,800 micrometers , which makes them useful also for high energy applications. The characterization of the Polycrystalline HgI2 thick films deposited with or without the passivation layers by measuring their dark currents, sensitivity to 65 and 85 kVp x-rays and residual signals after 1 minute of biasing, will be shown for several detectors. Some preliminary results will be shown for some novel screen-printed HgI2 detectors.
A first image of some tiny screws were obtained for the first time with polycrystalline HgI2 acting as the photoconductor material deposited on a-Si direct conversion X- ray image sensors, produced by Xerox -- Palo Alto Research Center. The initial results are very promising and show a high X-ray sensitivity and low leakage current. The response of these detectors to a radiological X-ray generator of 65 kVp has been studied using the current integration mode. Already its sensitivity expressed in (mu) C/R*cm2, is very high, values of 20 (mu) C/R*cm2 have been measured for films of 100 - 250 microns thickness and bias of 50 - 200 volts respectively, which is superior to the published data for competing materials such as polycrystalline PbI2 and a-Se detectors. The fabrication and characterization measurements of the Polycrystalline HgI2 thick film detectors will be given. The characterization data which will be reported here consists of: (a) sensitivity, (b) dark currents, (c) stability of sensitivity dependence on the number of exposure, (d) X-ray response dependence on dose energy and (e) signal decay dependence on the number of exposures.
The fabrication of polycrystalline HgI2 thick film detectors using the hot wall physical vapor deposition, method is described. The X-ray response of these detectors to a radiological X-ray generator of 60 kVp has been studied using the current integration mode. The response expressed in (mu) A, the dark current expressed in pA/cm2 and sensitivity expressed in (mu) C/R(DOT)cm2 are given for these detectors for several thickness and grain sizes. The optimal sensitivity is compared with published data on the response to X-rays by polycrystalline PbI2 and A-Se detectors.