X-ray detectors for structural biology from Multiwire and Image Plate to CCD de
Main steps:
- Protein purification- Crystallisation
Special:
- Programs for crystallography- X-ray detectors
- ADSC
Basic tutorials:
- Chemistry- Protein
- Peptide
- Amino Acids
Xtal community:
- CCP4BBX-ray detectors
X-ray experiments in structural biology involve measurement of thousand or even million intensities for separated reflections. That is why the single canal X-ray detectors (diffractometers) are used in structural biology not very often. Much better results are arising from usage of area detectors. Area detectors are just any type of X-ray detectors that can collect diffraction information on array at once. There are for common types of area detectors are used:• Film-based detectors
• Image plate
• Charge Coupled Device or CCD detectors
• Multiwire detectors
More detailed description of these detector types with most popular makes and models are given below.
Film-based detectors
Image plate
Charge Coupled Device or CCD detectors
CCD detectors are now used in a variety of ways for x-ray imaging. They are available with up to few thousand pixels, and this size is permanently grown up. From the other hand the pixel size is permanently reduced and is about 10-50µm with readout times of less than 1 s. In most scientific applications, CCD detectors are cooled to below -30ºC to reduce background noise. In most systems, a thin phosphor screen converts the incident X-rays into optical photons, which the CCD detects. A commonly used phosphor is Gd2O2S(Tb), which has a high efficiency and a light decay time of a few hundred microseconds. When used as a detector for macromolecular crystallography, a large phosphor screen (up to thousand mm2) is usually coupled to the CCD with a tapered optical fiber. Under the light beam, the CCD chip is charged and this electric charge is read out and digitized in order to transfer to the computer.Furthermore, modern detectors are based on modular design, where CCD modules can be tightly stacked into 2 x 2 or larger arrays to increase the active area without adding additional readout time (all the modules are read out in parallel).
Very good illustration of this principle is given in the following picture (taken from ASDC)
Principles of modular design for 3 x 3 CCD detector system it is possible to see on the schematic diagram of ADSC Quantum 315r cut-away view. This view is clearly shows tightly stacked phosphor modules with reducing fibre optics linked to the CCD chip.
