It has been reported and discussed that electrical current can be produced when an insulating material interacts with ionizing radiation. We have found that high-resolution images can be obtained from insulating materials if this current is guided by an electric field to the pixels of a TFT array. The charge production efficiency of insulators is much smaller than that of photoconductor materials such as selenium, silicon, or other conventional semiconductors. Nevertheless, when the intensity of the ionizing radiation is sufficiently high, a charge sensitive TFT imaging array with only dielectric material can produce high MTF images with contrast resolution proportional to the intensity of the radiation. The function of the dielectric in this new detector may be similar to that of an ionization chamber. Without the semiconductor charge generating material, the dielectric imaging detector does not exhibit charge generation fatigue or charge generation saturation. Prototype detectors have been tested using diagnostic x-ray beams with energy ranging from 25 kVp to 150 kVp, and therapeutic 2.5MV, 6MV, 10MV, and 15MV photon beams (with and without an electron built-up layer), electron beams, broad area proton beams, and proton pencil beams in the energy range of 150 MeV. High spatial resolution images up to the Nyquist frequency have been demonstrated. The physics, structure, and the imaging properties as well as the potential application of this detector will be presented and discussed.
Using electric field to partition the selenium layer into a low field charge drift region and a high field avalanche gain region was first proposed in 2005<sup>(1)</sup>. Engineering and fabricating such a grid structure on a TFT array have been a challenge. High dielectric strength material (up to several hundred volts/um) is required. Furthermore, it is very difficult to achieve or control a stable and uniform avalanche gain for imaging without too much excess noise from the elevated grid structure about the pixel plane. Image charge gain is non-uniform depending on the distance from the center of the avalanche well. A novel coplanar detector structure is now being tested. All image charges collected on a dielectric pixel surface will transfer to the central pixel readout electrode along a converging field. Uniform gain via a stable avalanche process can be achieved. This new structure does not require a conventional TFT platform and higher temperature fabrication process can be used. Imaging charges generated from x‐ray are first directed to a dielectric charge collection interface surface. During the sequential rolling image readout, imaging charges in each line are re-directed to an orthogonal lines of central readout electrode by a convergent field with high electric field strength at the rim of each pixel central electrode. All accumulated image charges need to pass through the end point of this converging field and therefore undergo a uniform impact ionization charge gain. This gain mechanism is similar to a proportional counter in radiation detection.