Pixels: charge-coupled devices

An x-ray area detector can be used to collect the intensities of many reflections at a time. The crystal must be oriented in many different settings with respect to the incident beam but the detector needs to be positioned at only a few positions to collect all of the data. A charge coupled device (CCD) is used as the area detector on the Siemens SMART single crystal diffractometer system. The SMART detector consists of a flat 6-cm circular phosphorescent screen that converts x-ray photons to visible light photons. The screen is coupled to a tapered fiber optics bundle which is then coupled to a one inch by one inch square CCD chip. The CCD chip has 1024 x 1024 pixels each of which stores an electrical charge proportional to the number of... 

Figure 20-15 Schematic representation of a charge coupled device, (a) Cross-sectional view, indicating charge generation and storage in each pixel. ( >) Top view, showing two-dimensional nature of an array. An actual array is about the size of a postage stamp.

Electrons from adjacent pixels can be combined to create a single, larger picture element. This process, called binning, increases the sensitivity of the charge coupled device at the expense of resolution. 

For X-ray detection, Charge-Coupled Devices (CCDs) are used. Being pixel detectors, they have a built-in two-dimensional position resolution and an energy resolution even better than conventional semiconductor detectors. They allow an efficient background reduction by analyzing the hit pattern and simultaneously... 

Figure 1. The pixel layout for the RCA charge-coupled device (CCD) used in these studies provides a 512 320 array or 163,840 sensor elements (pixels).

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