Section 7.2.4.  Detector system integration

Hardware interfacing. CCDs are operated, during both data acquisition and readout, by a dedicated hardware controller attached to a host computer, generally a PC. The requirements for the controller are complex and quite stringent in order to obtain low-noise operation. As with any high-speed electronics, noise increases with speed. Typically, pixel read rates of 100–500 kHz are used, although higher read rates can be used (and still preserve low-noise performance) for CCDs with multistage on-chip amplifiers. Some CCDs have multiple output amplifiers that increase pixel throughput by using parallel digitization channels. The entire CCD can also be read at reduced resolution by analogue summation of adjacent rows and/or columns on the chip (binning). This has the added benefit of increasing the signal-to-noise ratio for signals with low spatial frequency since there are fewer digitizations. Binning is highly recommended whenever the reduced resolution can be tolerated.

The time resolution of the detector can be further increased if the CCD array is used as frame storage. In this case, a portion of the imaging area is masked to X-rays, making it available for storage. The exposed area can be shifted rapidly into the masked area and a second exposure begun. Storage for five to ten subframes can easily be configured before readout is necessary. The time resolution is ultimately limited by the phosphor decay time and the time needed to shift the image. Although most CCDs are capable of being operated in very flexible ways, flexible CCD controllers are expensive. The consequence is that few commercial CCD X-ray detectors permit use of all the available options.

The detector itself is contained in a cryostat with the low-noise parts of the controller nearby, either in a separate box connected by a short cable or mounted inside the cryostat itself. A longer cable carries the time-multiplexed digitized data to the computer. High-speed serial data technologies are under investigation to simplify this connection and will become imperative for the much larger format detectors that are being developed.

Several installations have constructed a safety shield in front of the detector that opens only when data are being collected. This device helps to protect the delicate front surface of the detector and is highly recommended.

Data acquisition software. There is a wide spectrum of computer configurations surrounding CCD detectors. The major tasks to be performed are operating the detector, controlling the beamline, storing raw data, correcting images and analysing diffraction patterns. In home laboratories, where exposure times are relatively long, a single PC typically handles these tasks. In another arrangement, the detector controller is really an embedded system, mostly unseen by the operator, making the detector a remote image server. The raw data or corrected images come to the user's workstation where subsequent analysis is performed. This circumvents the problem that the detector computer may be running a different operating system from the workstation. At storage-ring sources, where the data volume is very large, the detector is almost always configured as a remote image server; the user's workstation does not even need to be nearby. Clusters of remote computers that can perform tasks in parallel become attractive for streamlined data collection, correction and analysis from large data sets. Remote analysis over the internet is being explored by several storage-ring facilities.

Control software should be easy to use, but flexible and extensible. It should be easy to set up experiments and sequence the individual steps in an experiment: exposure, readout, correction, storage and crystal movement, and wavelength change for MAD experiments. Extensible software would permit a user-written macro to be run at each step in place of the detector primitive that is provided. For instance, if it were desired to collect two images at different exposure times for each position of the crystal, extensible software would make it easier to set up the experiment. Finally, the software should permit access to all of the readout modes of the detector. For instance, a detector may be capable of rapidly scanning a small region of interest for alignment purposes, or it may be capable of streak-mode operation for certain types of time-resolved experiments. Available CCD detector software for macromolecular applications has room for much improvement. Hopefully, software will continue to undergo rapid development. Standardization is especially needed.