X-ray region

When the target of an x-ray tube is struck by electrons, these are retarded by the atoms of the target. The energy the electrons lose is radiated in a spectrum that ranges from the x-ray region into the infrared ye say this spectrum has been produced by electron excitation. 

It is customary to express the empirical data concerning term values in the X-ray region by introducing an effective nuclear charge Zeff e in the place of the true nuclear charge Ze in an equation theoretically applicable only to a hydrogen-like atom. Often a screening constant S is used, defined by the equation... 

Like positron emission, electron capture is never observed directly. However, after electron capture, the product atom is missing one of its 1 J electrons, as shown schematically in Figure 22-6b. When an electron from an outer orbital occupies this vacancy in the 1 orbital, a photon is emitted whose energy falls in the X-ray region of the... 

There are a number of experimental factors to be considered in a surface EXAFS experiment. First of all, one needs access to a synchrotron source (for the reasons previously mentioned) with significant flux in the hard X-ray region. In the United States, three such facilities exist and these are ... 

Monochromators for dispersing X-radiation utilize single crystals which behave like a diffraction grating. The spacing of the crystal lattice determines the angles at which radiation is reflected and generally two or more different crystals are required to cover the X-ray region of the spectrum. 

An important observation was that the emission lines are not confined to the narrow visible region of the electromagnetic spectrum. Instrumental detection showed that discrete lines are also present in the infrared and ultraviolet wavelengths, and eventually it showed this in the X-ray region also. It became clear that the wavelength of the line simply corresponded to the energy... 

For many of the analytical techniques discussed below, it is necessary to have a source of X-rays. There are three ways in which X-rays can be produced in an X-ray tube, by using a radioactive source, or by the use of synchrotron radiation (see Section 12.6). Radioactive sources consist of a radioactive element or compound which spontaneously produces X-rays of fixed energy, depending on the decay process characteristic of the radioactive material (see Section 10.3). Nuclear processes such as electron capture can result in X-ray (or y ray) emission. Thus many radioactive isotopes produce electromagnetic radiation in the X-ray region of the spectrum, for example 3He, 241Am, and 57Co. These sources tend to produce pure X-ray spectra (without the continuous radiation), but are of low intensity. They can be used as a source in portable X-ray devices, but can be hazardous to handle because they cannot be switched off. In contrast, synchrotron radiation provides an... 

In the hot X-ray region, the American observatory Chandra-AXAF will be a serions rival for XMM. It has better spatial resolntion dne to its exceptionally smooth mirrors, bnt lower sensitivity becanse of a lower collecting area. Japanese astronomy is also very present at these wavelengths. 

The availability of synchrotron sources with an almost continuous spectrum in the X-ray region allows the use of effects of metal atoms such as iron, zinc and copper which are often present in proteins as well as the atoms commonly used in derivative preparation, and a choice of the wavelengths at which the absorbtion edge is sampled. 

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