Ray Detection Methods

Position-sensitive X-ray detection systems, which were originally introduced for imaging purposes, particularly in medical applications, have been used in X-ray crystallography for about 20 years. Their virtue is that they count all the electrons, all the time, either along a line or over an area. 

There are three principal types of position-sensitive X-ray detectors of use for crystallography—the vidicon, solid-state devices (largely charge-coupled devices), and proportional counters. Vidicons are area detectors which use image intensifier techniques to produce TV-type images. They are not count rate limited and are therefore well suited for use with high-flux sources 

As with most proportional counters, the linear PSPC consists of a central anode wire between one or two cathodes. The space between the anode and cathode is filled with a conventional counter gas consisting of an ionizable gas, frequently Ar or Xe, and a quencher, e.g., CH4, typically in the ratio 9 1 and often at pressures of a few atmospheres. A large voltage ( - 1.5 kV) is maintained between the anode and cathode. When an X-ray photon enters the counter, there is a finite probability that it will ionize a gas molecule and the emitted electron then accelerates towards the anode under the influence of the field. This electron may then collide with further gas atoms, resulting in the formation of an electron avalanche near the central wire and thus the formation of a charge pulse on the anode. The total charge in this pulse is proportional to the energy of the incident photon. Thus far, this description is that of a conventional proportional counter. The position sensitivity can be achieved in several ways. 

One of the disadvantages of this type of detector is that it requires a very-high-resistance anode. Carbon-coated quartz (25-/im radius and 8-kfl mm resistance) is frequently used, but this suffers X-ray damage. One way of overcoming this is to use lower-resistance wire, e.g., nichrome, and to add extra capacitance to each end of the wire. The maximum count rates of both these detectors are typically several tens of kilohertz, and they may be made to almost any required dimensions, typical values being 5 to 20 cm. The criteria for detector design are given in a review by Radeka, and detector systems of this type are also available commercially for those who do not wish to construct them. 

FIGURE 4. A schematic representation of a resistive anode position-sensitive proportional counter. 

---

The discovery of the rare earth elements provide a long history of almost two hundred years of trial and error in the claims of element discovery starting before the time of Dalton s theory of the atom and determination of atomic weight values, Mendeleev s periodic table, the advent of optical spectroscopy, Bohr s theory of the electronic structure of atoms and Moseley s x-ray detection method for atomic number determination. The fact that the similarity in the chemical properties of the rare earth elements make them especially difficult to chemically isolate led to a situation where many mixtures of elements were being mistaken for elemental species. As a result, atomic weight values were not nearly as useful because the lack of separation meant that additional elements would still be present within an oxide and lead to inaccurate atomic weight values. Very pure rare earth samples did not become a reality until the mid twentieth century. 

The formation of such materials may be monitored by several techniques. One of the most useful methods is and C-nmr spectroscopy where stable complexes in solution may give rise to characteristic shifts of signals relative to the uncomplexed species (43). Solution nmr spectroscopy has also been used to detect the presence of soHd inclusion compound (after dissolution) and to determine composition (host guest ratio) of the material. Infrared spectroscopy (126) and combustion analysis are further methods to study inclusion formation. For general screening purposes of soHd inclusion stmctures, the x-ray powder diffraction method is suitable (123). However, if detailed stmctures are requited, the single crystal x-ray diffraction method (127) has to be used. 

The last three detection schemes apply only under very special circumstances. Transmission EXAFS is strictly a probe of bulk structure, i.e., more than about a thousand monolayers. The electron- and ion-yield detection methods, which are used in reflection rather than transmission schemes, provide surface sensitivity, 1-1,000 A, and are inherendy insensitive to bulk structure. X-ray fluorescence EXAFS has the widest range of sensitivity—from monolayer to bulk levels. The combination of electron or ion yield and transmission EXAFS measurements can provide structural information about the X-ray absorbing element at the surface and in the bulk, respectively, of a sample. 

One should compare capabilities to the electron beam X-ray emission methods of Chapter 3. The major difference is the higher lateral resolution with electron beams and the associated mapping capabilides. Another difference is the shorter probing depth possible with electrons, except when compared to the specialized TXRF method. Comparing electron-beam EDS to X-ray/particle EDS or electron-beam WDS to X-ray/particle WDS, the electron beams have poorer detection limits because of the greater X-ray bacl ound associated with electron... 

During World War II and thereafter, the methods of x-ray detection were improved until it is now a matter of simple routine to measure relative x-ray intensity easily and precisely. This improvement, which was accelerated by the rapid progress in nuclear physics, has promoted a rapidly growing appreciation of the great advantages that can attend the application of x-ray absorption and emission to chemical analysis. In their rush to make these applications, analytical chemists have occasionally made discoveries predictable from earlier work, usually by physicists, in the field of x-rays. 

But Roentgen s achievements transcended mere discovery. He studied the properties of the new rays so well that he laid the foundations not only for important methods of x-ray detection (fluorescence of a phosphor, darkening of a photographic plate, ionization of a gas) and for radiography, but for the application of x-ray absorption to analytical cKemistry as well. 

The effects in question are often translated into electric currents, pulsed or continuous. For the convenient reading or recording of these currents, complex electronic circuitry (2.3) may be needed. Modern methods of measuring x-ray intensity are therefore primarily a concern of the experimental physicist. Nevertheless, the analytical chemist must know something about them because x-ray detectors are now among the tools of his trade. This chapter, which cannot hope to do justice to modern x-ray detection, will attempt to provide him with an acceptable minimum of knowledge. 

Tossing a mental coin, the decision was to analyze the case of noise proportional to the square root of the signal. This, as you will recall, is Poisson-distributed noise, characteristic of the noise encountered when the limiting noise source is the shot noise that occurs when individual photons are detected and represent the ultimate sensitivity of the measurement. This is a situation that is fairly commonly encountered, since it occurs, as mentioned previously, in UV-Vis instrumentation as well as in X-ray and gamma-ray measurements. This noise source may also enter into readings made in mass spectrometers, if the detection method includes counting individual ions. We have, in... 

Local composition is very useful supplementary information that can be obtained in many of the transmission electron microscopes (TEM). The two main methods to measure local composition are electron energy loss spectrometry (EELS), which is a topic of a separate paper in this volume (Mayer 2004) and x-ray emission spectrometry, which is named EDS or EDX after the energy dispersive spectrometer, because this type of x-ray detection became ubiquitous in the TEM. Present paper introduces this latter method, which measures the X-rays produced by the fast electrons of the TEM, bombarding the sample, to determine the local composition. As an independent topic, information content and usage of the popular X-ray powder dififaction database is also introduced here. Combination of information from these two sources results in an efficient phase identification. Identification of known phases is contrasted to solving unknown stmctures, the latter being the topic of the largest fiaction of this school. 

An X-ray fluorescence method has been developed for the determination of technetium in solution . At concentrations of less than 1.0 mg Tc per ml there are not interelement effects. Therefore, it is possible to ascertain technetiiun in its compounds without previous decomposition, provided that the compounds are soluble in water or dioxane. The detection limit is about 4 Tc. 

A X-ray crystallographic method for detecting the transient accumulation of intermediates in enzyme catalysis, protein folding, ligand-binding interactions, and other processes involving macromolecules. The approach is premised on the well documented retention of substantial reactivity of biological macromolecules, even in the crystalline state. 

The original and still useful method of X-ray detection is the exposure of photographic film. In the early days of X-ray diffraction, films were also used for the quantitative recording of X-ray intensities. Densitometer measurements of optical density, D, were made of the film. The optical density is defined by... 

The statistical methods are valuable because they detect symmetry elements which are not revealed by a consideration of absent reflections, or by Laue symmetry5. In principle, it is possible to distinguish between all the crystal classes (point-groups) by statistical methods in fact, as Rogers (1950) has shown, it is possible by X-ray diffraction methods alone (using absent reflections as well as statistical methods) to distinguish between nearly all the space-groups (see p. 269). 

The accuracy of the x-ray fluorescence method was evaluated by calculating, from the 50 whole coals analyzed, the mean variation of each element from its mean concentration, determined by the other independent methods previously mentioned and listed in Table III. Detection... 

---