Rotating anode source

A variant of the typical X-ray tube described above is the rotating anode, which is capable of generating much higher intensities. Although rotating anode sources can and have been used in EXAFS experiments, the intensities are of such magnitude that data acquisition for extended periods of time is required and their application is furthermore limited to bulk samples. 

Typical exposure time is 4 - 8 hours using a rotating anode source, Gobel mirror, and a bent ID detector for simultaneous recording of the complete curve. 

The XAS spectrometer is similar to a UV-visible system in that it consists of a source, a monochromator, and a detector. The most favorable XAS source, synchrotron radiation, is tunable to different wavelengths of desirable high intensity. A laboratory instrument for analysis of solids and concentrated solutions may use a rotating anode source (further described in Section 3.3). The monochromator for X-ray radiation usually consists of silicon single crystals. The crystals can be rotated so that the wavelength ( i) of the X-rays produced depends of the angle of incidence (0) with a Bragg lattice plane of... 

An EXAFS experimental set-up has three primary components (i) a source of X-rays, (ii) a monochromator (and collimator) and (iii) a detector. Synchrotron radiation is being widely used for EXAFS, but where this facility is not available, a rotating anode source would be suitable. Progress in EXAFS instrumentation has been comprehensively reviewed in the AIP proceedings (1980). 

In the laboratory of one of the authors (VKP) the direct drive rotating anode source manufactured by Rigaku/MSC has been in continuous operation (the anode is spinning and the x-rays are on 24 hours per day, 7 days per week) for 18 months at the time of writing this book and it is still operating without a breakdown. 

C) Rotating anode source, Bragg-Brentano goniometer, radius 285 mm, scintillation detector. 

Because radiation should be as monochromatic as possible in order to maximize the diffraction effect from atoms in a crystal, the continuous spectrum is either suppressed by selective filters, also illustrated in Figure 7.2, or a discrete wavelength of X ray is isolated with a monochromater of some sort. For most biological structure analyses in the laboratory where a sealed X-ray tube or a rotating anode source (see below) is employed, the anode is of pure copper or occasionally molybdenum. These two elements have strong, characteristic Ka peaks in their spectra at 0.154 nm (1.54 A) and 0.071 nm (0.71 A), respectively. These... 

Currently there are three commonly used X-ray sources the first two, sealed X-ray tubes and rotating anode sources, are found in most protein crystallography laboratories. The third source of X rays is synchrotrons, which are available only at specialized facilities, generally national laboratories. X rays produced by synchrotrons, which have a number of unique and highly desirable features, are generated by a completely different principle than that described above for conventional sealed tubes and rotating anode sources. 

X-ray diffraction patterns were recorded using a Searle toroidal focusing camera with nickel filtered CuKa radiation generated by a Rigaku-Denki rotating anode source. 

A comparison of the intensity of a rotating anode source with the DESY synchrotron (section 5.6.2) by Harmsen et al (1976) was favourable enough to encourage data collection apparatus to be established on DORIS by Harmsen and Rosenbaum (described in Rosenbaum and Holmes (1980) and Rosenbaum (1980)). This workstation (X-11) accommodated small angle diffraction as well as protein crystallography. 

Instrumentation. A setup includes a source for X-rays (usually a fixed or a rotating anode source more recently, synchrotron radiation has become an attractive choice as a source of radiation), a monochromator or at least a filter to select the desired wavelength, the spectroelectrochemical cell and the detector (for an overview on detectors see [3, 9]). 

Structural information on electrodes can also be obtained as a function of potential by X-ray using monochromatic synchrotron radiation and similar transmission cells to those used for XAS (Figure 7B). Transmission cells (Laue mode) are best suited to adsorbates, while reflection (Bragg) cells are used for thicker films. Laboratory rotating anode sources can also be used, and here the instrument usually demands a reflection cell geometry. Stable systems such as solid-state cells and batteries give better... 

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