Amorphous silicon detector

The setup as seen in Figure 1 mainly consists of a Varian Linatron 3000A linear accelerator (LINAC) as radiation source, a rotational stage for sample manipulation, and a two-dimensional high-energy x-ray detector array consisting of four amorphous silicon area detectors Heimann RIS 256. The source to detector distance is 3.7 m. 

The detector setup consists of four 256 x 256 pixel amorphous silicon technology sensor flat panels with 0.75 x 0.75 mm pixel size, having an active area of 192 x 192 mm [5j. These sensors are radiation sensitive up to 25 MeV and therefor well suited for detecting the LINAC radiation. The four devices are mounted onto a steel Irame each having the distance of one active area size from the other. With two vertical and two horizontal movements of the frame it is possible to scan a total area of about 0.8 x 0.8 m with 1024 x 1024 pixel during four independent measurements. 

Therefore it is reasonable to prepare already the data acquisition for a three dimensional evaluation in cone-beam-technique by means of two-dimensional detectors. The system is already prepared to integrate a second detector- system for this purpose. An array of up to four flat panel detectors is foreseen. The detector- elements are based on amorphous silicon. Because of the high photon energy and the high dose rates special attention was necessary to protect the read-out electronics. Details of the detector arrangement and the software for reconstruction, visualisation and comparison between the CT results and CAD data are part of a separate paper during this conference [2]. 

Fig. 4. Some electronic device applications using amorphous silicon (a) solar cell, (b) thin-fiLm transistor, (c) image sensor, and (d) nuclear particle detector.

Nuclear particle detectors, hydrogenated amorphous silicon in, 22 135 Nuclear power, 6 813... 

Microstructures and systems are typically fabricated from rigid materials, such as crystalline silicon, amorphous silicon, glass, quartz, metals and organic polymers. Elastomeric materials can be used in applications where rigidity is a drawback. We have demonstrated the concepts of elastomeric systems by fabrication of photothermal detectors, optical modulators and light valves. We believe that elastomeric materials will find additional applications in the areas of optical systems, micro analytical systems, biomaterials and biosensors. 

Currently under development are the next generation x-ray detectors such as the pixel-array, amorphous silicon, and solid state detectors. These detectors offer a larger active area, lower background, faster readout, and higher dynamic range than the current CCD or IP systems. 

The prototype system used in the trial was made by General Electric Medical Systems (Milwaukee, WI). The system used a 18 x 23 cm detector incorporating an amorphous silicon thin-film transistor bonded to a cesium iodide crystal scintillator. Other than the detector and associated electronics, the system was identical to one of the GE s clinical units, the DMR. For this reason, the GE DMR was chosen as the film unit for the trial. The digital and film units had almost identical dimensions, allowing the same patient positioning to be used on both. The only noticeable difference was the thickness of the digital detector on the prototype, compared with the film cassette used on the DMR. 

The four machines represented four distinct technologies. The GE machine, the commercial version of the prototype used in the Colorado-Massachusetts trial, used an 18 x 23 cm area ( flatpanel ) detector consisting of a cesium-iodide (Csl) phosphor bonded to an amorphous silicon substrate containing a rectangular photodiode array. The Csl phosphor converts the incident x-rays to light, which are then detected by the photodiode array. A thin-film transistor array deep to the amorphous silicon layer processes and transmits the electronic signal to the external electronics. The pitch (size) of a detector element in this system was 100 pm. 

Hgure 4 Construction of the flat-panel detector. The X-ray sensitive layer consists of cesium iodide (Csl). The amorphous silicon sensor array comprises 3000 x 3000 pixels. (Reproduced with permission from Hamers S and Freyschmidt J (1998) Digital radiography with an electronic flat-panel detector First clinical experience in skeletal diagnostics. Medicamundi 42 2-6 Medicamundi.)... 

The most widely used fiat detector (FD) design is based on a two-level, indirect conversion process of X-rays to light (Granfors et al. 2001 Antonuk et al. 1997 Spahn et al. 2000 Yamazaki et al. 2004 Ducourant et al. 2003). In the first step, the X-ray quantum is absorbed by a fluorescence scintillator screen, e.g., a cesium iodide (Csl) substrate, converting it into visible light. In the second step, this light is received by a photodiode array, e.g., based on amorphous silicon (a-Si), and converted into electrical charge. 

Fig. 3.1. Schematic view of an indirect converting flat detector based on Csl and an amorphous silicon active readout matrix, including driver and readout electronics. An individual pixel is shown in detail. It comprises a large photodiode and a small thin-film transistor (TFT)...

P. J. Chupas, K. W. Chapman, and P. L. Lee, "Applications of an Amorphous Silicon-Based Area Detector for High-Resolution, High-Sensitivity and Fast Time-Resolved Pair Distribution Function... 

Fink C, Hallscheidt PJ, Noeldge G, Kampschulte A, Radeleff B, Hosch WP, Kaufman n GW, Hansmann J (2002) Clinical comparative study with a large-area amorphous silicon flat-panel detector image quality and visibility of anatomic structures on chest radiography. AJR Am J Roentgenol 178 481-486... 

Floyd CEJ, Warp RJ, Dobbins JT, Chotas HG, Baydush AH, Vargas-Voracek R, Ravin CE (2001) Imaging characteristics of an amorphous silicon flat-panel detector for digital chest radiography. Radiology 218 683-688... 

Since 1970 the subject of amorphous semiconductors, in particular silicon, has progressed from obscurity to product commercialization such as flat-panel liquid crystal displays, linear sensor arrays for facsimile machines, inexpensive solar panels, electrophotography, etc. Many other applications are at the developmental stage such as nuclear particle detectors, medical imaging, spatial light modulators for optical computing, and switches in neural networks (1,2). 

Thin film transistor (TFT) arrays are produced inexpensively and in large numbers for use in modern computer monitors and televisions. This readout system can be combined with amorphous hydrated silicon or amorphous selenium which is deposited over the large surface of the TFT array and acts as the X-ray conversion layer. Having established themselves firmly as an X-ray detector for medical imaging already, they have until now failed to make an impact on the field of crystallography, their high noise level being the main drawback. It can be surmised with some confidence that this type of detector will become standard equipment in the near future. 

In the last decade or so, amorphous selenium has been applied as a photoconductor in X-ray image detectors, particularly for biomedical imaging. For this application, flat-panel detectors with large sensing areas (> 30 cm x 30 cm) have been developed. The coating of amorphous Se is typically 500 pm thick, and is deposited over a silicon thin film transistor layer. After applying an electrical potential to the surface, the detector is exposed to an X-ray beam and the electrons released are used to transmit information that ultimately provides an image. 

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