Sealed X-ray tube

The diffractometer has gradually evolved in terms of maximum power of sealed X-ray tubes, rotating anodes, new X-ray optics, better detector efficiency, position-sensitive detection and, lately, real-time multiple-strip (RTMS) fast X-ray detection, which replaces a single detector by an integrated array of parallel detectors to provide an up to 100-fold increase in efficiency compared with traditional detectors without compromise on resolution. Time-resolved powder diffraction is... 

In the case of structural studies with x-rays, sample size is rarely a serious problem since single crystals as small as about 0.001 mm3 (sealed x-ray tube) or 10-9 mm3 (synchrotron radiation) will do. In the case of x-ray diffuse scattering measurements with the assembly described in Section VI.D, the crystal must be 2 mm long at least but can be relatively thin. With neutrons, however, sample size is more critical. Indeed, for structure measurements, 0.1 mm3 is a minimum, with the highest neutron fluxes available nowadays, and more usual sizes are in the range of several cubic millimeters to several tens of cubic millimeters. For polarized neutrons or inelastic scattering studies, much larger samples are necessary. 

The most common laboratory source of X rays is a sealed X-ray tube, that is, a glass (or cerantic) vessel with a metal base, in which under conditions of high vacuum a beam of electrons is created, accelerated by an electric potential of 30 to 60 kV and focused onto a target of one of the abovementioned metals, thereby producing X rays which leave the tube through a beryllium window. When electrons bombard the target, both... 

Figure 2 Cross section of a sealed X-ray tube (schematic)...

The x-ray tube assembly is a simple and maintenance-free device. However, the overall efficiency of an x-ray tube is very low - approximately 1% or less. Most of the energy supplied to the tube is converted into heat, and therefore, the anode must be continuously cooled with chilled water to avoid target meltdown. The input power to the sealed x-ray tube ( 0.5 to 3 kW) is therefore, limited by the tube s ability to dissipate heat, but the resultant energy of the usable x-ray beam is much lower than 1% of the input power because only a small fraction of the generated photons exits through each window. Additional losses occur during the monochromatization and collimation of the beam (see section 2.3). 

Figure 2.4. The schematic explaining the appearance of two different geometries of the x-ray focus in a conventional sealed x-ray tube (left) and the disassembled tube (right). The photo on the right shows the metallic can with four beryllium windows, two of which correspond to line- and two to point-foci. The surface of the anode with the cathode projection is seen inside the can (bottom, right). What appears as a scratch on the surface of the anode is the damage from the high intensity electron beam and a thin layer deposit of the cathode material (W), which occurred during the lifetime of the tube. The cathode assembly is shown on top, right.

The low thermal efficiency of the sealed x-ray tube can be substantially improved by using a rotating anode x-ray source, which is shown in Figure... 

A powder diffractometer in your laboratory is equipped with a sealed x-ray tube, which has Cr anode. You need to design a P-filter to ensure that the intensity of the Kp spectral line is less than 0.5% of the intensity of the Ktti part in the characteristic spectrum. Calculate the needed thickness of a foil made from the most appropriate metal (which one ) and by how much the intensity of Kai and Kp lines will be reduced after filtering. 

A) Sealed x-ray tube source, curved position sensitive detector, the radius of its goniometer is 150 mm and difflaction data are collected in the transmission mode using cylindrical specimens. You employed this equipment to characterize the phase purity of your materials. 

B) Sealed x-ray tube source, Bragg-Brentano goniometer, radius 185 mm, scintillation detector. 

The classification fast , overnight and weekend experiments is usually applied to laboratory powder diffractometers equipped with conventional sealed x-ray tube sources. Obviously, when the brilliance of the available source increases dramatically, the time of the actual experiment decreases. It is worth noting that since specialized beam time (e.g. a synchrotron source) is limited, this normally implies that the majority of samples should undergo a thorough preliminary examination using conventional x-ray sources. 

FIGURE 7.1 Schematic diagram of a conventional, broad focus, sealed X-ray tube. The anode material, usually of a pure element, determines the characteristic X-ray spectrum that is produced. 

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. 

The source is the sealed X-ray tube or a synchrotron (with much higher photon flux). 

This book does not, however, pretend to present the state of the art in crystallographic research. Apart from a few rudimentary ideas, there is no discussion of the fascinating subject of quasi-crystals or aperiodic crystals because these are still quite rare materials. Although synchrotron radiation is the tool of choice for cutting-edge research, the classical sealed X-ray tube is the only source available in most universities and industrial laboratories and will certainly remain so. This book is not an introduction to structure determination, there being a number of modern texts already available in this area. 

Four different types of X-ray sources are employed in X-ray analysis (a) sealed X-ray tubes and (b) radioactive sources are the most commonly employed, while to a lesser extent primary X-rays produced in (c) rotating anode tubes and (d) synchrotron radiation facilities are also utilized for analytical purposes. 

Most commercially available X-ray spectrometers utilize a sealed X-ray tube as an excitation source, and these tubes typically employ a heated tungsten filament to induce the emission of thermionic electrons in a vacuum chamber. After acceleration by means of a high voltage X the electrons are directed towards a layer of high purity metal (e.g., Cr, Rh, W, Mo, Rh, Pd,. ..) that serves as anode. In the metal layer, a bremsstmhlung continuum is produced, onto which the characteristic lines of the anode material are superimposed. The broad band radiation is well suited for the excitation of the characteristic lines of a wide range of atomic numbers. The higher the atomic number of the anode material, the more intense the beam of radiation produced in the tube. Fig. 11.11 shows a schematic cross-section of a sealed X-ray tube. 

It can be seen from Eq. (3.7) that the analyte line intensity excited by the continuum is proportional to the atomic number of the anode. Thus, higher atomic number anodes would be expected to yield higher sensitivities. Unfortunately this is not always the case. In high-power, grounded-anode, sealed x-ray tubes, the higher the atomic number of the anode, the greater is the fraction of incident electrons that are scattered in the direction of the x-ray tube window. This in turn requires a relatively thick window (perhaps 500- to 1000-jim beryllium) to dissipate the... 

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