Progress in the science and technology of direct conversion X-ray image detectors: The development of a double layer a-Se based detector

One very important requirement for an X-ray photoconcluctor to be usable in a practical detector is that the dark current at the operating field should be as low as possible. We examine, in terms of an order of magnitude calculations, the maximum allowed dark current density, which is similar to 10 pA mm(-2). Dark currents in presently competing X-ray photoconductors are compared and discussed. We pay particular attention to a-Se based photoconcluctors inasmuch as the progress in the science and technology of a-Se based detectors has led to their commercialization. We report and discuss dark current measurements on single and double layer a-Se based photoconcluctor structures, and show how these measurements paved the way to the development of a new a-Se based X-ray detector fabricated on a CCD that is suitable for slot scanned X-ray imaging. The a-Se CCD detector has provided excellent images, and has a remarkable high resolution, better than 16 lines mm(-1).