Gas Proportional Detectors

As X-ray counters, gas proportional detectors provide unrivaled dynamic range and sensitivity for photons in the range which is important for macromolecular crystallography (for a review, see Kahn and Fourme, 1997). The classical gas proportional detector is a multiwire proportional chamber (MWPC), widely used as an in-house detector with conventional X-ray sources. Two MWPC diffrac-tometer systems are commercially available. Gas proportional detectors use as a first step the absorption of an X-ray photon in a gas mixture high in xenon or argon. This photoabsorption produces one electron-ion pair, the total energy of which is simply the energy of the initial X-ray photon. The ion returns to its

Fig. 2.15 Expanded view of a multiwire proportional chamber (MWPC) showing the anode plane sandwiched between the two cathode planes. A is the position of the avalanche. The centers of the induced charge distributions are used to determine the coordinates, x and y, of the avalanche. (Reproduced by permission of Academic Press, Inc., from Kahn and Fourme, 1997.)

Fig. 2.15 Expanded view of a multiwire proportional chamber (MWPC) showing the anode plane sandwiched between the two cathode planes. A is the position of the avalanche. The centers of the induced charge distributions are used to determine the coordinates, x and y, of the avalanche. (Reproduced by permission of Academic Press, Inc., from Kahn and Fourme, 1997.)

neutral state either by the emission of Auger electrons, or by fluorescence. Since the kinetic energy of these first electrons is far greater than the energy of the first ionization level of the xenon or argon atoms, fast collisions with atoms (or molecules) in the gas very quickly produce a cascade of new electron-ion pairs in a small region extending over a few hundred micrometers around the conversion point. The total number of primary electrons produced during this process is proportional to the energy of the absorbed X-ray photon, and is thus a few hundred for ~ 10 keV photons. These primary electrons then drift to the nearest anode wire where an ionization avalanche of 10000-1000000 as many ion pairs results. The motion of the charged particles in this avalanche (chiefly the motion of the heavy positive ions away from the anode wire) causes a negative-going pulse on the anode wire and positive-going pulses on a few of the nearest wires in the back (cathode) wire plane (see Fig. 2.15).

The disadvantages of the MWPC detector are the limited counting rate due to the build-up of charges in the chamber, together with limitations in the readout electronics and the lower sensitivity at shorter wavelengths. This makes the application of MWPCs with synchrotron radiation poorly effective.

Charge-Coupled Device-Based Detectors

A remarkable development for the use with synchrotron radiation is the design and construction of charge-coupled device (CCD) detectors (for a review, see Westbrook and Naday, 1997). CCDs were developed originally as memory devices, but the observation of localized light-induced charge accumulation in CCDs quickly led to their development as imaging sensors. These CCD detectors are integrating detectors like the conventional X-ray-sensitive film, IPs and analog electronic detectors using either silicon-intensified target (SIT) or CCD sensors. Integrating detectors have virtually no upper rate limits because they measure the total energy deposited during the integration period (although individual pixels may become saturated if the signal exceeds its storage capacity).

The first commercially available analog electronic detector was the fast area television (FAST) detector produced by Enraf-Nonius (Delft, The Netherlands). This detector contained a SIT vidicon camera as an electronically readable sensor. The SIT vidicon exhibits higher noise than CCDs, which have therefore replaced SIT sensors during the past few years. Because of their high intrinsic noise, detectors with SIT vidicon sensors need an analog image-amplification stage, and this limits the overall performance of such detectors. Several CCD detector systems have also been developed that incorporate image intensification. The most important development in detector design for macromolecular crystallography has been the incorporation of scientific-grade CCD sensors into instruments with no image intensifier. These detector designs are based on direct contact between the CCD and a fiber-optic taper. There are several commercial systems available based on this construction (Hamlin Detector; Mar Research, Norderstedt, Germany).

Phosphor Fiber-optic Thermoelectric Heat Water

Phosphor Fiber-optic Thermoelectric Heat Water

Fiber-optic

Fig. 2.16 Schematic representation of a CCD/taper detector. (Reproduced by permission of Academic Press, Inc., from Westbrook and Naday, 1997.)

Fiber-optic taper

Fig. 2.16 Schematic representation of a CCD/taper detector. (Reproduced by permission of Academic Press, Inc., from Westbrook and Naday, 1997.)

A schematic representation of such a detector is shown in Figure 2.16. An X-ray phosphor (commonly Gd2O2S:Tb) is attached to a fiber-optic faceplate, which is tightly connected to a fiber-optic taper. The X-ray-sensitive phosphor surfaces at the front convert the incident X-rays into a burst of visible-light photons. Although it is possible to permit the X-rays to strike the CCD directly, this method has several drawbacks, such as radiation damage to the CCD, signal saturation, and poor efficiency. The use of a larger phosphor as an active detector area and the demagnifying fiber-optic taper is also necessary because the size of the scientific-grade CCD sensors is not as large as needed for the demands of the X-ray diffraction experiment. The fiber-optic taper is then bonded to the CCD, which in turn is connected to the electronic readout system. The CCD must be cooled to temperatures ranging from -40 °C to -90 °C, depending on the various systems. The great advantage of CCD detectors is their short readout time, which lies in the range from 1 to a few seconds.

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