Scattering parasitic

As a result slow variation of the adjustment is observed the intensity of the primary beam will abnormally increase or decrease, the parasitic scattering background will grow, slit scattering will change (cf. Sect. 4.2.3.7). It should be clear that changes of the primary beam intensity which are paralleled by respective changes of the synchrotron current are normal. 

If the synchrotron is operated in discontinuous mode, the storage ring will be refilled two or three times every day. The interval between two consecutive refills is called a synchrotron radiation run. The parasitic scattering (machine background) should at least be recorded once within each synchrotron radiation run. You might consider... 

Gas-stream cryostats. These were the first kind of setup available. Their advantage is that they are cheap and that no shields or windows are required (no absorption, no parasitic scattering). The device is fixed while the crystal rotates in the cold gas stream. With nitrogen the limit temperature is about 100 K helium is rarely used since it cannot be recuperated easily in such an arrangement. The reliability is not particularly good. Icing... 

X-Ray detectors may be classified as point, linear or area, depending on whether they record the diffraction pattern in zero, one or two spatial dimen -sions. Point detectors must be scanned to measure the diffraction pattern, whereas linear or area detectors can be fixed. Point detectors are easily compatible with post-sample optical elements. Linear and area detectors allow the data to be acquired much faster, but as more open systems they are prone to detecting parasitic scatter from the air or sample environment. Both linear and area detectors are types of position sensitive detector (PSD). 

Figure 10.5 shows the GOD scans for two Oj < of varying sample thickness. A number of peaks are visible for films of different thickness. All peaks appear independent of the film thickness but they vary in intensity. As in GID, intense (100) and (200) peaks are visible (modified by parasitic scattering of beamline components), which is a hint to the random orientation of nanocrystallites (powder rings). At the same time, further peaks are visible at = 1.28, 1.44, 1.57 and 1.70 A where the second one is most intense. These peaks are located in the region of the amorphous halo centred at Q A A b Again we have observed the polymorphism in the out-of-plane scans only at (100) and (100) indexes for the thickest sample. 

Figure 10.5 Out-of-plane GOD scans for samples of different thicknesses. The incidence angles are 0.10° and 0.11°, respectively. In low-g range the diffraction curves suffer from parasitic scattering of beamline components.

Because of the beam stop and parasitic scatter the lowest observable S values are around 0.01 nm"1, In the following the intensity integrated between about 0.01 and... 

The maximum entropy method can handle Laplace transforms such as those found in pulse fluorometry, without restricting the validity of the solution or suffering from any instability. It allows the recovery of the distribution of exponentials describing the decay of the fluorescence (i.e., inverting the Laplace transform), which is, in turn, convolved by the shape of the excitation flash. Also, it can determine the background level and amount of parasitically scattered radiation. 

Subtraction of solvent-induced background. Although the block-coUimation system used in the camera discussed above suppresses most of the parasitic scattering at low angles, several other effects may lead to a considerable backgroimd which must be subtracted carefully from the measured intensities. To assess this problem in further detail. Fig. 10 gives a comparison of the different contributions to the measured scattering intensity of a polystyrene latex of 150 nm diameter [73]. 

Electron microscopy uses a beam of primary electrons that is focused onto the surface of the sample (Figure 21.2) a voltage is applied to accelerate the electrons to energies up to 30 keV (SEM) or 100-300 keV (JEM). While in the SEM mode electrons and X-rays emitted into the hemisphere of the primary beam are analyzed (reflection setup), TEM probes the transmission of the sample with respect to high energy primary electrons. All high resolution electron microscopes are operated under a chamber vacuum better than 10 mbar to reduce parasitic scattering. 

Slits. Slits are used to shape the beam and to reduce parasitic scatter aroimd the primary beam. The latter is of overriding importance in small-angle scattering beamlines since it is the amoimt of parasitic scatter that is the most important factor in determining the possible low angle resolution and therefore the maximum size of the structures that can be observed (see Fig. 3). 

Fig. 3. Schematic description of an x-ray scattering beamline. The optical system is represented by a lens. The parasitic scatter cone is represented by the gray area. The scattering information which falls in this region is useless since the parasitic scatter is orders of magnitude more intense than the scattering pattern itself The importance of the positioning of the slits in determining the extent of the parasitic scatter cone, which is the limiting factor in achieving low angle resolution, can be observed. Most beamlines are in fact equipped with three or more slit systems between the last optical element and the sample position.

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