Point Collimation

In addition to point-focus apparatus there are scattering devices with an extremely elongated cross-section of the primary beam. Historically this geometry has been developed as a compromise between ideal collimation and insufficient scattering power. Their practical importance is decreasing as more powerful point-collimated sources become available. Kratky camera (Alexander [7], p. 107-110) and Rigaku-Denki camera (BaltA Vonk [22], p. 83) are the most frequent representatives of slit-focus devices. 

Since powerful X-ray sources and sophisticated beam shaping have generally become available, point-collimated setups for the study of X-ray scattering have lost their former handicap of low intensity. Today they benefit from their simple and versatile geometry. This section is devoted to an overview of modern apparatus -beginning with the source of X-radiation and ending with the detector and the data acquisition system. 

X-Ray Scattering. A small angle X-ray scattering camera with a point collimation has been used for the determination of Bragg spa-cings of many liquid crystal samples. Temperature wa monitored between 20° and 30°C, and the samples were contained into a capillary tube. The measurements were made by the photographic method on the first two reflections. 

Considering that the factor Lq for point collimation and for laminar particles (one-dimensional model) is q, substituting Equation 19.10 in Equation 19.9 and rearranging gives... 

In the following, a description of an improved Kratky-camera [73] will be discussed together with an extended discussion of the treatment of data. This device is capable of measuring latex particles up to a diameter of 200 nm and reaches the q-range provided by SAXS-cameras which work in point collimation and use synchrotron radiation (cf. below [73]). 

Precise collimation systems are needed to make measurements close to the primary beam. In addition, scattering at small angles is usually very weak. Both pinhole and line collimation has been used block collimation systems require mathematically transforming the data from line collimation to point collimation. During the last 15 years, the development of synchrotron sources and better area detectors has led to a proliferation of scattering experiments that occur in real time. 

To change the resolution in q, or to extend the q-range, the wavelength of the incident beam or the sample-detector distance can be adjusted. The colUmation of the incident beam can have either point or slit geometry. In the case of sUt geometry, the measured scattered intensity represents the convolution of the scattering pattern of the sample for point collimation with the slit profile [23]. 

X-ray low- and wide-angle diffraction studies of the liquid crystalline phases were performed in a manner previously reported [lOj. The low-angle camera was a point-collimated... 

The wide angle X-ray scattering (WAXS) regime was collected nsing an evacnated Statton camera (Warhus, DE) with point-collimated X-ray beam via two pinholes (0.05 mm), separated by 15 cm at Cu Ka wavelength 1.5418 A. In order to examine scattering from dry membranes, SPTES-50, SPTES-60, SPTES-70, and... 

So far in our consideration of small-angle diffraction behaviour we have not considered the effective of diffuse interfacial layers. Porod showed that the tail i.e. the asymptotic behaviour at high angles) of a scattering curve for an ideal two-phase system with sharp boundaries between the phases should have an intensity proportional to s " for a system studied with point collimation and proportional to s when studied with infinite slit collimation. 

For experiments carried out with point collimation the observed scattering intensity in the tail for systems in which the interface is a linear slope and for which positive deviations have been subtracted is given by ... 

---