X-ray transparency

Research. A significant impact on research at high pressure has come about with the use of gem quaHty diamonds as Bridgman-type anvils in a smaU compact high pressure device (40—42). With this type of apparatus, pressures greater than those at the center of the earth (360 GPa = 3.6 Mbars) have been reached, and phase transformations of many materials have been studied. Because of the x-ray transparency of diamond, it is possible to determine the stmcture of the phases under pressure. Because of the strenuous environment, crystals selected for this appHcation have to be of very high quaHty. 

Detectors for quantitative measurement of X-ray absorption spectra must measure the flux (photons s-1) of the X-ray beam. Ionization chambers consisting of X-ray transparent windows on each end of a chamber holding an inert gas work... 

Detectors for quantitative measurement of X-ray absorption spectra must measure the flux (photons s of the X-ray beam. Ionization chambers consisting of X-ray transparent windows on each end of a chamber holding an inert gas work well as transmission detectors for concentrated samples. For transmission detectors, ln(/o//) is proportional to the absorption coefficient of the absorbing atom, p (/o = incident X-ray photon intensity, /= transmitted intensity), according to Beer s Law ... 

A mask consisting of a pattern made with an x-ray absorbing material on a thin x-ray transparent membrane,... 

As discussed previously, the anvil seats on which the diamond anvils are mounted have traditionally been made of X-ray transparent beryllium (Fig. 6a), although other hard, low-Z materials have also been used, such as B4C and BN. The disadvantage of all of these is the large background scatter that arises once the... 

A recent innovation in thin-film detectors has appeared on the commercial market in which a standard Si(Li) detector is protected by an apparently strong, X-ray transparent and radiation insensitive thin-film. The exact nature and composition of this thin-film is yet to be disclosed in the open literature. However, according to manufacturer s reports [13] this detector window allows transmission of boron X-rays (0.185kv for Ka) and can be operated at atmospheric pressure. In a typical spectrum from this type of detector provided by the manufacturer, a strong oxygen Ka line as well as well-defined Si, Al and Na peaks can be observed for albite (NaAlSi Og). Since these detectors have only recently been installed in the field, details of performance have yet to be reported by users. 

A major reason why XAFS spectroscopy has become a critically useful probe of catalyst structure is the fact that it is easily adapted to characterization of samples in reactive atmospheres. The X-ray photons are sufficiently penetrating that absorption by the reaction medium is minimal. Moreover, the use of X-ray- transparent windows on the catalytic reaction cell allows the structure of the catalyst to be probed at reaction temperature and pressure. For example, the catalyst may be in a reaction cell, with feed flowing over it, and normal online analytical tools (gas chromatography, residual gas analysis, Fourier transform (FT) infrared spectroscopy, or others) can be used to monitor the products while at the same time the interaction of the X-rays with the catalyst can be used to determine critical information about the electronic and geometric structure of the catalyst. 

A critical component of the cell is the X-ray-transparent window that allows the X-ray beam to impinge on the sample and the transmitted or fluorescent X-rays to be detected. Typical window materials that have been used are polyimide (Kapton ), beryllium, quartz, diamond, polyester (Mylar ), and titanium. Table 3 shows estimates of the thicknesses of window materials for various X-ray energies from 5 to 25 keV, determined on the basis of the assumption that 25% of the X-rays are absorbed by the window material. 

FIGURE 22 Cell (Kampers et al., 1989) that functions as a continuously stirred tank reactor (a) sample holder (b) heater/cooler cylinder (c) hollow tubes for coolant and electrical wires (d) reservoir for coolant liquid (e) thermocouple (f) flange enclosing in situ chamber (g) O-ring (h) X-ray transparent window (i) gas inlet/outlet. Reprinted with permission from J.A. van Bokhoven, T. Ressler, F.M.F. de Groot, and G. Knopp-Gericke, in In situ Spectroscopy of Catalysts , B.M. Weckhuysen, Ed., published by American Scientific Publishers (2004). Copyright American Scientific Publishers. 

During the experiment, a layer of catalyst powder (or sieved meshed particles) is packed into the center tube holder. The alumina membrane disc holds the powder sample at the downstream side while allowing the reactant to flow through with minimal resistance. At the upstream side, a small amount of quartz wool is inserted to support the powder. This whole sample holder assembly is fastened to the outer connector and this whole tube assembly is enclosed in the quartz tube that is surrounded by a resistive heating unit. At each end of the tube, X-ray transparent windows are mounted. The thermocouple is placed close to the sample holder. The long gas inlet tube is heated by the tubular furnace, ensuring that the gas is preheated before it reaches the catalyst sample. The upper working temperature is 973 K, which is only limited by the materials of construction. 

A) X-ray transparent window (B) window seal (C) sample holder (D) gas connections. Reprinted with permission from (Weiher et al, 2005). Copyright 2005 Wiley-Blackwell. 

The operating principle of a DAC is elegantly simple (Figure 15). A gasket of metal foil (usually steel, W or Re) is placed between the diamond anvils. A hole drilled in the center of the gasket contains the sample immersed in a hydrostatic liquid. The anvils are mounted on beryllium (for X-ray transparency) back-plates, to which force is apphed by an inflatable membrane, a level-arm mechanism or just by tightening screw-bolts. Thus a hydrostatic compression is achieved. Such cells can be operated also at high and low temperature, and are also suitable for spectroscopic studies, since diamond is an ideal transmitter of heat and of all types of radiation. 

This instrument was designed [16] to fill a need for fast, reproducible sedimentation analyses in the sub-micron size range. The heart of the instrument is a hollow, x-ray transparent, disc which, under normal operating conditions, contains 20 ml of suspension at a concentration of around 0.2% by volume. The centrifuge speed is selectable in the range 750 to 6000 rpm. The default condition is for the source and detector to remain stationary for 1 minute at a radial position of 48.00 mm and then to scan towards the surface. Total run time is normally 8 min. 

Hand grenades and antipersonel mines can be easily and cheaply constructed from polyester casting resin, auto body putty, ABS and PVC pipe, plexiglass, vacumn formed styrene sheet etc. These plastic devices won t produce shrapnel fragments or concussions equal to the power of conventional bomba, but they do have two unique and noteworthy features they are both non metallic and x-ray transparent. 

Chromium diffusion and reduction in soil microsites. The X-ray microprobe is useful for in-situ studies of redox reactions within soil columns. T. Tokunaga (LBNL) has pioneered this approach for study of heterogeneous reactions at small scales in soils and has developed mini-soil columns with X-ray transparent windows that incorporate electrodes for redox potential measurement (Fig. 24). 

Figure 6. Design of the Soft X-ray Endstation for Environmental Research (SXEER), optimized to examine the chemistry of light elements under ambient conditions. The samples are placed at 1 atm. pressure in the sample chamber, which is isolated from the high vacuum part of the station with thin X-ray transparent windows (Myneni, unpublished data).

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