KEYWORDS: Sensors, Selenium, Photon counting, Electric field sensors, Spatial resolution, Time metrology, Surgery, Spectroscopy, Spectroscopes, Single photon
In this study, we fabricated a pixelated unipolar charge sensing detector based on amorphous selenium with a 20-μm pixel pitch using standard lithography process. A pulse-height spectroscopy (PHS) setup with a very low noise front-end electronics was designed, and experiments were performed to investigate the achievable energy resolution with the unipolar detector, as well as with a conventional detector for comparison purposes. PHS measurement results are presented that demonstrate, for the first time, a measured energy resolution of 8.3 keV at 59.5 keV is for the unipolar charge sensing device in contrast to 14.5 keV at 59.5 keV for conventional a-Se devices, indicating its promise for the contrast-enhanced photon counting imaging with an unsurpassed spatial resolution.
KEYWORDS: Sensors, Electrodes, Signal detection, Photon counting, Selenium, Photoresistors, Electric field sensors, Near field, X-ray imaging, Photodetectors
Practical photon counting detectors that have been adopted for commercial use are typically based on crystalline or polycrystalline materials. However, these types of materials are challenging to scale to large-area medical imaging applications because of yield and cost issues associated with the crystal growth and bonding technology required to interface the sensor with the readout IC. An alternate approach is to use a large-area-compatible, mature, direct conversion X-ray-detection sensor such as amorphous selenium (a-Se). The technical challenges for photon counting with a-Se lie in overcoming (1) the slow carrier-transport material property of a-Se, which leads to count-rate limitations due to pile-up, and (2) the lower X-ray-to-charge conversion gain, which degrades SNR and can be resolved by improved design of pixel readout circuits. In this paper, we address the a-Se material limitation by leveraging a unipolar charge sensing detector design. We demonstrate that the proposed unipolar charge sensing detector provides an effective method to detect charge of the polarity type having a higher mobility-lifetime product, obviating the need for detection of the opposite polarity slow transport charge. Transient signal measurements indicate that a quasi depth independent signal rise-time is achieved with the unipolar charge sensing detector. Moreover, two orders of magnitude improvement is observed compared to the conventional a-Se detector rise-time (0.15 μs vs. 25 μs).
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(1). 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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.