Rowland spectrometer

The spectrometer is necessarily quite large, and a complicated mechanism has to be precision engineered in order to enable 0 to be altered while keeping both the crystal and the detector on the Rowland circle. In order to cover the whole X-ray spectrum a range of crystals with different lattice spacings is required, which may be interchanged automatically. 

A typical instrument is equipped with four computer controlled crystal spectrometers, as well as an EDS system for preliminary qualitative analysis. A light microscope is provided for examining the specimen and also for ensuring that the specimen height is adjusted until it is on the Rowland circle. The drawing of... 

Fig. 1. Spectrometer configuration at NIST EBIT note that the EBIT source is located well inside the Rowland circle. The spectrometer is in the perpendicular orientation where the axis of the spectrometer is perpendicular to the long axis of the EBIT source. The detector arm moves vertically with changes in diffraction angle.

Estimates of shifts of spectra in curved crystal geometries are often calculated for an ideal detector located on the Rowland circle. However, the detection surface is usually fiat and therefore cannot lie on the Rowland circle. Detectors located on a fixed length detector arm will additionally travel off the Rowland circle as the Bragg angle is scanned unless the crystal curvature is simultaneously scanned (which raises problems of stress hysteresis). Conventional shifts calculated for detection on the Rowland circle do not agree with shifts at a flat extended detector and this systematic error can be 100-200 ppm for any Johann curved crystal spectrometer. We have incorporated fiat surface detectors located off the Rowland circle into the general theory [18,17]. 

FIGURE 63 Schematic of a HERFD crystal array spectrometer, in this case using six spherically bent Ge(620) crystals to collect the Fe Kf region with approximately 1.0 eV resolution. The inset (left) shows the Rowland circles for each analyzer crystal (Heijboer et al., 2004). Reprinted with permission from (Heijboer et al., 2004). Copyright 2004 American Chemical Society. 

J., Jeffery, M., Rowland, B. (2002). Hydrolysis of VX on concrete rate of degradation by direct surface interrogation using an ion trap secondary ion mass spectrometer. Environ. Sci. Technol. 36 4790. ... 

As WDS analysis is sequential, it is practical to use several spectrometers of this type around the electron column but, for obvious reasons of available space (the diameter of the Rowland circle is approximately 30 cm), we are limited to 2 spectrometers mounted obliquely or 5 vertical spectrometers. The second configuration is generally chosen since it can be used to conduct simultaneous analysis of S elements on the same point. 

This effort has concentrated on building and commissioning the Rowland Circle (RS) x-ray spectrometer for recording the elastic portion of the diffuse scattered x-rays from the water solutions. These measurements will complement the XAFS measurements discussed in Sect. A by adding the pair distribution function of atoms about the ion of interest to larger distances. 

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