charge-coupled device, CCD
charge-coupled device
"CCD" is a two-dimensional digital semiconductor photoelectric conversion device. For electron beam detection, electrons are converted into light by a fluorescent material or a YAG crystal. When the CCD is irradiated with light, electron charges are accumulated in the depletion region (potential wells). Then, these charges are transferred to successive wells. Finally, these charges are taken out as electric signals. Since the CCD contains a dark current, the device is cooled to suppress this current (cooled to -30 ℃ by Peltier cooling). A normally-used CCD has a size of 2K × 2K (a square of approximately 3cm) with a spatial resolution (pixel size) of 14 μm (7 μm for visible light available). Compared to the imaging plate, CCD has a smaller dynamic range of 4 digits or a gray scale of 16 bits than an imaging plate. However, the most advantage of CCD is that it can be used online (only offline use in the imaging plate). CCD is used for acquisition of a high-resolution image and for detection of X-rays in WDS. Recently, a CCD with a larger area (4K × 4K) have been getting popular.
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