Polychromatic X-ray beam

The alternating component of the applied voltage causes a continuous variation in the efficiency of the x-ray excitation (Step II) during each cycle of operation. A polychromatic x-ray beam has a different total absorption coefficient and a different ratio of photoelectric to scattering absorption coefficient for each wavelength (Step V). It is very difficult to take account of these factors exactly. Fortunately, reasonable estimates will suffice in the making of the calculations for Table 4-4 and Figure 4-15. The efficiency has been taken as that for the root-mean-... 

Edges, absorption, see Absorption edges EDT (EDDT) as analyzing crystal, 116-118, 220, 257, 260, 318-327 Effective wavelength of a polychromatic x-ray beam, determination, 8, 72, 76-78... 

Figure 7.7 A predicted Laue pattern of a pea lectin crystal set with a zone axis parallel to the incident, polychromatic X-ray beam. There is a pronounced blank region in the centre of the film. The spot marked N is one example of a nodal spot.

Immediately after discovering X-rays, Rontgen laid the foundation for X-ray absorptiometry with polychromatic beams this can be compared to colorimetry or photometry with white light. As all atoms absorb X-rays, the method is not specific for any element or group of elements. Polychromatic X-ray beams are... 

In common X-ray sources, a metal target is bombarded with high energy electrons produced by a tungsten filament or an electron gun. The emitted polychromatic X-ray beam is composed of a series of lines and a continuous background called Bremsstrahlung. 

The volume of defects is calculated using intensity evaluation. Considering the polychromatic radiation of microfocus X-ray tubes the X-ray beam is represented by an energy dependent intensity distribution Io(E). The intensity Ip behind a sample of thickness s is given by integrating the absorption law over all energies ... 

The two effects just described help to determine the x-ray distribution at the target. Before an x-ray beam strikes a sample to be analyzed, the beam is usually modified further. For example, there may be absorption (and filtering) by the window of the x-ray tube, by an air path between tube and sample, by the walls of a cell containing the sample, and finally by the sample itself. Analogous considerations govern the absorption (and, with polychromatic beams, the filtering) of the beam entering the detector from the sample. 

Equation 1-7 may be considered always valid for absorptiometry with monochromatic x-rays, and usually valid for absorptiometry with polychromatic x-rays also. Difficulties sometimes arise with polychromatic beams. 

The idealized calculations of the efficiency of the parts of an x-ray spectrograph (4.3, 4.4) can be modified to apply to a laboratory instrument if account is taken of the pulsating character of the applied voltage, the polychromatic nature of the x-ray beam, and the absorption in the beam path. 

Principal advantages of 7-rays over polychromatic x-rays- (1) Greater source stability (radioactive isotopes as compared with x-ray tubes) (2) simpler equipment (3) greater compactness of source (4) beams more nearly monochromatic (5) wider energy range available, 2(IQ4) to 2(107) ev (6) lower cost ... 

Generally speaking. X-ray beams produced with a tube are polychromatic, in other words, they contain characteristic emission peaks in addition to a continuous spectrum over a wide range of wavelengths. In order to characterize the material to be studied, it is important to have a monochromatic beam which makes it possible to associate a single diffraction peak with each family of crystal planes. Therefore, it is necessary to select one peak among all of those emitted by the tube. Naturally, the most intense one is chosen. 

The polychromatic beam produced by the X-ray source irradiates a plane crystal cut parallel to a family of crystal planes. This single crystal is oriented around a rotation axis parallel to the reference family of planes. For the chosen wavelength X, when the incidence angle of the X-ray beam is eqtral to the Bragg angle 0j, a diffracted beam with wavelength X is observed. 

At the various synchrotrons all these geometries have been exploited for macromolecular crystal data collection as they have also on conventional X-ray sources. Once the polychromatic synchrotron X-ray beam has been rendered monochromatic the single crystal data can be measured and processed as for a conventional X-ray source. These standard procedures will be discussed briefly before moving on to cover the synchrotron specific aspects. These latter include SR instrument... 

Beam hardening artifacts (for polychromatic X-rays or neutrons) prevent exact quantification of the attenuation coefficients. 

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