Beam-stop

If we know that behind the beam stop the intensity is always 50 counts or less, we can discriminate the valid area of our image by defining a ROI mask (i.e., a shape function) (cf. p. 17) by simply writing... 

Figure 4.1. Typical X-ray setup with 2D detector in normal-transmission geometry. The intensity of the incident X-ray beam is measured in an ionization chamber (a). Thereafter it penetrates the sample which is subjected to some process. At a distance R (cf. Table 2.1 on p. 7) behind the sample the detector is recording the scattering pattern. In its center (b) the detector is protected by a beam stop. It is equipped with a pin-diode which records the intensity of the attenuated beam...

Measure the Beam Profile. Deconvolution is possible if the primary beam profile has been recorded. Recording of the beam profile is readily accomplished during the adjustment of the beamline prior to the experiment as long as the beam stop has not yet been mounted. Damage to the detector is avoided1 either by short exposure or by attenuation of the primary beam itself. 

Mispositioned slits result in discrete slit scattering. Slit scattering is recognized by thin and intense streaks in horizontal and/or vertical direction that extend outward from the beam stop. Take out the sample or rotate it in order to make sure that the streaks are not resulting from an interaction of the beam with the sample. Horizontal streaks can be affected by moving horizontal slit edges. 

The intensity of the X-ray beam is measured by ionization chambers or pin-diodes13. Pin-diodes can only be operated in the beam stop. The variation of the beam intensity during the experiment should be measured both before and after the sample. If the beam intensity monitors are set up properly, the absorption of the primary beam by the sample can be computed for each scattering pattern. The placement of the first ionization chamber in or after the X-ray guide tube to the sample is uncritical. 

The design and placement of the second beam intensity monitor demands more attention. The definition of X-ray absorption does not discriminate between primary beam, USAXS and SAXS. So the second beam intensity monitor should guide primary beam, USAXS and SAXS through its volume, whereas the WAXS should pass outside the monitor. The optimum setup for SAXS and USAXS measurements is a narrow ionization chamber directly behind the sample. For WAXS measurement a pin-diode in the beam stop is a good solution for WAXS. For USAXS and SAXS it may be acceptable, as long as the relevant part of the primary beam is caught, the optical system is in thermal equilibrium and the synchrotron beam does not jump (cf. Sect. 4.2.3.5). 

The primary beam profile is reasonably measured during adjustment of the optics just before the beam stop is inserted. If overexposure of the detector can be avoided by choosing a short exposure interval this method is to be favored. Instead, attenuation of the primary beam by an absorber must be considered. 

Sample Orientation If possible orient the sample in such a way that the beam-stop holder does not cut through an important region (peak). If you expect that the sample exhibits fiber symmetry, check it rotating the sample about the assumed fiber axis and take some patterns. 

This is most easily done at a laboratory source where the current of the X-ray tube is decreased to the lowest possible value. At a synchrotron beamline this is more complicated, because the measurement of the primary beam requires special adjustment. So, technically this should be done before the final optical adjustment of the device, as long as the slits can be narrowed for the purpose of intensity attenuation and as long as the primary beam stop is not yet mounted. It is not advised to use absorbers that are mounted behind the monochromator, because they change the spectral composition of the X-ray beam. 

We need Bp in the perpendicular direction with respect to the direction in which the streak is extending. The beam profile has (hopefully) been measured after the adjustment of the instrument and before installation of the beam stop. Before being used in Eq. (9.27) or Eq. (9.28) Bp is converted to reciprocal space units. 

Ti content in the polymer films was measured with a Princeton Gamma Tech System 4 x-ray Fluorescence Spectrometer. The conditions employed were Cr target, 50 keV source operating at 3 mA, 0.75 mm aperture, 4.8 mm beam stop, helium atmosphere and 100 sec. counting time. A calibration curve was constructed by plotting the fluorescence counts versus the amount of Ti in HB-HPR 206 films determined by Rutherford Backscattering Spectroscopic (RBS) analysis. 

Figure 2.6 Diffraction pattern from a crystal of the MoFe (molybdenum-iron) protein of the enzyme nitrogenase from Clostridium pasteurianum. Notice that the reflections lie in a regular pattern, but their intensities (darkness of spots) are highly variable. [The hole in the middle of the pattern results from a small metal disk (beam stop) used to prevent the direct X-ray beam, most of which passes straight through the crystal, from destroying the center of the film.] Photo courtesy of Professor Jeffery Bolin.

In this photo, the goniostat (a) and area detector (c) are separated by a drum of helium (b) which transmits X rays with less loss than air. The crystal (d) is barely visible in the tiny glass tube. The two arrows (e) mark the collimator (left) and the beam stop (right), which prevents the direct X-ray beam from reaching the detector. Arrows 1, 2, and 3 on the goniostat indicate the X, < >, and u> circles. 

It is clear that rays at the outer rim of the lens contribute most to the depolarization effects. To exploit the depolarization effects for the same type of orientational imaging observed for the SNOM, it has been proposed to use ring-like (annular) illumination [43]. An annular beam can be easily created by removing the central part of the excitation beam using a simple beam stop as symbolized by the black disk in Fig. 7(a). The resulting confocal scan images show very nicely the different effective patterns that can be attributed to different absorption dipole orientations. Some of the patterns are labeled by the respective orientation (see Fig. 7(b)). For details see [42,43],... 

QWP) mounted in a computer-controlled motorized rotator and is focused to the optical cell, which contains the sample, and is absorbed by a beam stop. 

Figure 1 gives a schematic drawing of the basic setup used in the GISAXS experiments. The two-dimensional detector is only recording the intensity reflected above the sample surface. The direct beam is not recorded with the detector to avoid detector saturation as several orders of magnitude in intensity separate the incoming intensity from the reflected one. In addition the specularly reflected peak (condition af = at) is shielded with a beam stop to... 

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