DIRECT CONVERSIONS
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Figure 2: Flat
Panel Detector with direct conversion amorphous selenium.
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Figure 3:
Direct-conversion thin-film
transistor detectors use a uniform layer of evaporated amorphous selenium to
convert x rays into electron-hole pairs. Electric fields that are established
within the selenium by means of bias voltages channel the charge to the nearest
collector, which preserves spatial resolution. Very high fill factors are
achievable with appropriate electrode design.
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- · It constructed by adding an x-ray photoconductor as the top layer of the electronic thin-film transistor sandwich.
- · Amorphous selenium is used as the photoconductor material because of its excellent x-ray detection properties and extremely high intrinsic spatial resolution (>20 line pairs per milimeter [lp/mm] at 100 keV).
- · Before exposure, an electric field is applied across the amorphous selenium layer through a bias electrode on the top surface of the selenium.
- · As x-rays are absorbed in the detector, the ionization created by x-ray photons results in releasing electrons and holes within the selenium atom and owing to the electric field within the selenium, electric charges are drawn directly to the charge-collecting electrodes.
- · The electrons are transferred and stored in the TFT detectors.
- · The thin-film transistor (TFT) is a photosensitive array comprised of small pixels.
- · Each pixels contains a photodiode that absorbs electrons and generates electrical charge.
- · Pixels are effectively separated by means of field shaping within the selenium layer, and the entire selenium surface is available for x-ray charge conversions. The TFTs are positioned in a matrix which allows the charge patterns to be read pixel by pixel.
- · This process is extremely fast where more than 1 million pixels can be read and converted into a digital image in <1 second.
