Conventional x-ray sources

Laue Method for Macromolecule X-Ray Diffraction. As indicated above it is possible to determine the stmctures of macromolecules from x-ray diffraction however, it normally takes a relatively long period of data collection time (even at synchrotrons) to collect all of the data. A new technique, the Laue method, can be used to collect all of the data in a fraction of a second. Instead of using monochromated x-rays, a wide spectmm of incident x-rays is used. In this case, all of the reflections that ate diffracted on to an area detector are recorded at just one setting of the detector and the crystal. By collecting many complete data sets over a short period of time, the Laue method can be used to foUow the reaction of an enzyme with its substrate. This technique caimot be used with conventional x-ray sources. 

Despite considerable efforts very few membrane proteins have yielded crystals that diffract x-rays to high resolution. In fact, only about a dozen such proteins are currently known, among which are porins (which are outer membrane proteins from bacteria), the enzymes cytochrome c oxidase and prostaglandin synthase, and the light-harvesting complexes and photosynthetic reaction centers involved in photosynthesis. In contrast, many other membrane proteins have yielded small crystals that diffract poorly, or not at all, using conventional x-ray sources. However, using the most advanced synchrotron sources (see Chapter 18) it is now possible to determine x-ray structures from protein crystals as small as 20 pm wide which will permit more membrane protein structures to be elucidated. 

An alternate method for obtaining angular information is to make use of the plane polarized nature of synchrotron radiation. It has long been known that XAS should exhibit a polarization dependence for anisotropic samples (18) however it is only recently that attempts have been made to exploit this effect. Early attempts to observe anisotropic XAS suffered from the low intensity and incomplete polarization of conventional x-ray sources. This work has been reviewed by Azaroff (19). 

Figure 49 is a simplified diagram of an x-ray proximity printer showing the source, x-ray confinement cavity, mask, and wafer. As discussed previously, it is necessary to design a source with as small a radiating area as possible in order to minimize penumbral shadowing and with a maximum intensity of x-rays to minimize exposure time. Watts and Maldonado have extensively reviewed conventional x-ray sources, and the reader is referred to that work for additional details. 

Since their discovery in 1895, X-rays have been one of the primary probes with which chemists and physicists have investigated the stractuie of matter. Although most well known for their use in crystallography, the earliest studies with X-rays emphasized spectroscopic measurements, hi the early years of this century X-ray spectroscopy, in particular tiie work of Moseley, played a key role in the discovery and characterization of new elements. Following Aese early successes however, the potential chemical applications of X-ray spectroscopy were largely unappreciated until the mid-seventies. "Die reasons for this can be traced both to the weak interactions of X-rays with matter and to the low intensities available horn conventional X-ray sources. These factors combine to limit conventional X-ray spectroscopy to relatively concentrated samples. 

The above technique, which is very easy to implement (all that is needed besides the usual matrix isolation equipment is a conventional X-ray source for crystallography), was used first to record UV-vis spectra and later IR spectra of numerous radical cations isolated in Ar matrices. The method seems to be quite generally apphcable to any substrate that can be volatilized without decomposition. 

The power output of photolithographic projection printers in the deep UV range (X < 300 nm) is relatively low thereby requiring very sensitive resists in order to achieve economical throughput. The same is true of x-ray printers utilizing conventional x-ray sources. 

Figure 2 X-ray diffraction patterns of samples obtained with various PTES/TEOS ratios, following preparation A. Data recorded with a conventional X-ray source.

Conventional X-ray sources produce beams that contain rays of different wavelength. Equation (3) is satisfied by any wavelength for which a set of lattice planes exists in the crystals at an angle 0 to the incident beam. As a large number of wavelengths can meet this criterion, the diffraction pattern is potentially quite complex (Bueche, 1986). 

The elucidation of catalytic mechanisms of enzymes by diffraction methods is very difficult, even when the three-dimensional structure has been determined to a reasonably high resolution. With conventional X-ray sources, diffraction data collection for macromolecules usually takes days, while chemical reactions that are catalyzed by enzymes may occur in fractions of a second. Nonetheless, it has been possible to study catalysis by... 

In general, there is no principal difference in the diffraction phenomena using the synchrotron and conventional x-ray sources, except for the presence of several highly intense peaks with fixed wavelengths in the conventionally obtained x-ray spectrum and their absence, i.e. the continuous distribution of photon energies when using synchrotron sources. Here and throughout the book, the x-rays from conventional sources are of concern unless noted otherwise. 

Figure 2.7. Schematic diagram of a synchrotron illustrating x-ray radiation output from bending magnets. Electrons must be periodically injected into the ring to replenish losses that occur during normal operation. Unlike in conventional x-ray sources, where both the long-and short-term stability of the incident photon beam are controlled by the stability of the power supply, the x-ray photon flux in a synchrotron changes with time it decreases gradually due to electron losses, and then periodically and sharply increases when electrons are injected into the ring.

Various P-filters are most often used to monochromatize the diffracted beam (e.g. see Figure 3.6, left), but sometimes they are used to eliminate Kp radiation from the incident beam in conventional x-ray sources. The advantages of P-filters are in their simplicity and low cost. The disadvantages include (1) incomplete monochromatization because a small fraction of Kp spectral line intensity always remains in the x-ray beam (2) the intensity of the Ka spectral line is reduced by a factor of two or more, and (3) the effectiveness of a p-filter is low for white x-rays above Ka and it rapidly decreases below Kp. Therefore, p-filters are nearly helpless in eliminating the background, especially when the latter is enhanced by x-ray fluorescence (the filter itself fluoresces due to the true Kp absorption). 

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. 

The presence of dual wavelengths in conventional x-ray sources, or in other words the presence of the Ka2 component in both the incident and diffracted beams, complicates powder diffraction patterns by adding a second set of reflections from every reciprocal lattice point. They are located at slightly different Bragg angles when compared with those of the main (Kai) component. This decreases resolution and increases overlapping of Bragg peaks, both of which have adverse effect on an unbiased peak search. 

Figure 4.25. Powder diffraction pattern collected using a conventional x-ray source.

In this section, we are concerned with a powder diffraction experiment, which consists of a single pattern (profile). The Rietveld technique may also be used to conduct refinement of the crystal structure employing multiple patterns collected from the same material. For example, powder diffraction data collected using conventional x-ray sources with different wavelengths, conventional and synchrotron x-rays, conventional or synchrotron x-rays and neutron source may be used simultaneously in a combined Rietveld refinement. The fundamentals of the combined Rietveld refinement are briefly considered in section 7.3.8. 

However, resists with a sensitivity higher than 1 jiC/cm seem to be rather difficult to handle for practical direct wafer writing technology and an optimum resist sensitivity for a well-designed exposure plant may be 1 - 2 jiC/cm. Extremely fast resists are required for the x-ray lithography, where apart from synchrotron radiation the limited out-put of conventional x-ray sources causes a real limitation in the through-put (22). FBM-G will be the most important resist in x-ray lithography. 

A conventional x-ray source is large and divergent, therefore not suitable for this task. It could in principle be converted into a small-size source by using a screen with a pinhole. The negative side... 

Within the last few years high resolution data collected either from synchrotron or neutron sources has allowed new structures to be solved from powder samples. Powder data from these new sources are superior to that collected from a conventional x-ray source both in resolution and peak to background ratio these improvements in data quality have the capability of making the determination of new molecular sieve topologies easier but not necessarily routine. Synchrotron radiation has possible future capabilities of being used to collect data from crystals that are only a few microns in size. At that time most newly synthesized molecular sieve materials will be solved by single crystal techniques instead of powder techniques. At the present time an acceptable structure refinement can be obtained from a crystal with a... 

Harmson A, Leberman R, Schulz GE (1976) Comparison of protein crystal diffraction patterns and absolute intensities from synchrotron and conventional X-ray sources. J Molec Biol 104 311... 

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