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A new high speed/high resolution X-ray detector called the XEBIT (X-ray sensitive Electron Beam Image Tube) is currently under development at the University of Connecticut Health Center. This large area (9' diameter) direct conversion detector is based on an X-ray photoconductor called thallium bromide. The device utilizes cathode ray tube technology to provide a 30 frame per second raster scanned electron beam to both charge and readout the photoconductor. Thallium bromide is a high Z material with a linear attenuation coefficient of 28.11 cm-1 at 60 kev. This high stopping power results in a quantum efficiency of 57% at 60 kev for 300 micron thick layers. Thallium bromide is a very good X-ray photoconductor that requires 6.5 ev to create an electron-hole pair. For 60 kev photons, this results in a gain 9230 per absorbed photon. With a hole-mobility lifetime product of 1.5 X 10-6 cm2/volt, good charge collection can be achieved at reasonable field strengths. Thallium bromide has a very high band gap of 2.7 ev and a dielectric constant of 33. Its resistivity, which is 5 X 109 ohm-cm at room temperature, is dominated by ionic conductivity. Fortunately, ionic conductivity has a strong temperature dependence that can be significantly reduced with moderate cooling to -25 degrees centigrade. The XEBIT uses thallium bromide as an X- ray photoconductor in a vidicon type image tube. Its principals of operation are very similar to the standard light sensitive vidicon that were utilized extensively in the commercial television industry. A scanning electron beam charges the TlBr surface, with respect to the positively biased front surface, providing the necessary electric field across the photoconductor for charge transport. X-rays then penetrate the window and are absorbed by the thallium bromide. The absorbed photons generate large numbers of electron-hole pairs due to the high conversion gain. Electrons drift under the electric field to the positive bias electrode and the holes drift to the vacuum surface and annihilate stored charge. This results in an image dependent charge pattern on the vacuum surface of the photoconductor. A subsequent scan of the photoconductor generates the capacitively coupled signal by replacing the annihilated electrons. The XEBIT utilizes well-developed display tube technology to provide a very cost effective alternative to image intensifier and screen/film based systems. The XEBIT is currently under development as a replacement for X-ray Image Intensifiers in medical imaging applications. The first devices are 9 inch prototypes designed to be no larger than standard intensifiers. It will replace the image intensifier/optics/video camera with one direct conversion device. The XEBIT suffers from no veiling glare and has far superior contrast resolution with over 50 percent modulation at 5 line pairs per millimeter. The XEBIT is capable of full field imaging as well as under scanning to view smaller regions with higher detail.
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