A photoinduced discharge X-ray detector using photoconductors: influence of the material selection on the electronic properties.
In the Photo-Induced Discharge (PID) method of X-ray detection, image information generated by X-rays is derived using a readout laser. The structure of the PID detector consists of multiple layers with different functionalities. A high voltage is applied between the top and bottom layers, generating an electric field for separating the electron-hole pairs (EHPs) generated by X-rays in a photoconductor layer. A separate dielectric layer is used to trap EHPs. Improvements to detection efficiency are accomplished by selecting new photoconductor materials as a substitute for amorphous selenium (a-Se), commonly used in existing PID methods. The photoconductor material also determines the optimal wavelength of an appropriate readout laser. Lead oxide, lead iodide, mercury iodide, and mercury iodide (with additives) were tested as alternative photoconductor materials with low dark current. The readout laser used here operates at wavelengths of 459 nm and 620 nm. The reaction characteristic on the readout laser was measured in all four types of photoconductors. Detection properties were compared to a flat panel detector. No reaction characteristics could be measured for lead oxide and lead iodide for either wavelength. The mercury iodide photoconductor (including additives) reacted to the 495 nm wavelength, but showed a weak, inconsistent signal for different measurements. The mercury iodide photoconductor showed excellent characteristics for both wavelengths, but was superior for a wavelength of 495 nm. The mercury iodide-based PID sensitivity was measured at 0.4 nc/mR cm2 with a dark current at 0.5 nA/cm2 and signal to noise ratio (SNR) of 340. These results show a lower sensitivity compared to existing flat panel detectors, but show a much lower dark current. Consequently, SNR values approximately 260 times higher were observed in the PID detector as compared with a flat panel detector.