Basic Design

Drawing from the experience of several years of use with similar designs at the ALS, the design is based on the Hettrick-Underwood design. Two optical elements are used in the X-ray emission spectrograph with a spherical mirror for focusing the X-rays onto an in-vacuum CCD detector and a variable line spacing (VLS) plane grating for energy monochromatization. The grating is operated in the constant incident angle mode; thus, variation in the detection photon energy is accomplished by moving the detector along the focal plane.

MXS-qRIX has two main components; the optics chamber and a cantilevered swing arm detector assembly.

The Optics Chamber:

The mirror and grating are mounted inside their own carriages and are driven by motorized feed-through and stage-through wire linkage. The relative position of the mirror and grating poles can be adjusted by turning the fine-pitch screws on the back side of the substrate. The ease of ex-situ adjustment allows the setting of mirror and grating angles and their positions, using the high precision coordinate measuring machine (CMM) to stay within the installation tolerance. The mirror and grating carriages are mounted on a common U-shape aluminum backbone via flexures. Their angles can be changed dynamically by motorized feed-through and stage-through. These motorized degrees of freedom will be extremely useful in the initial alignment phase and during the normal operation. The entire optics assembly is attached to a stainless steel top flange and is sealed inside the UHV aluminum chamber. A set of apertures in front of the mirror can be adjusted to block out the scattered x-rays from entering the spectrometer.

The Detector Assembly:

Held inside a light-weight aluminum swing arm, a fly tube holds the 2D imaging detector mounted on the tube that is connected to the optics chamber by a large bellows. The bellows allows the detector angle and position to be changed while maintaining ultra-high vacuum (UHV). The swing-arm fly tube assembly is supported by the brackets mounted on top of two translation stages, which in turn are driven by a motorized jack to change the detector angle relative to the horizon and move the detector along the optical path. This degree of freedom changes the distance between grating and detector, which is needed to correct for defocusing when MXS is recording photon energies far away from the nominal photon energy.

To detect x-rays with different photon energy during the measurements, one only needs to change the detector angle and the distance between grating and detector to correct for defocusing.

Comparing Scientia model XES-350 to the Atlas MXS-qRIX-1000 Soft X-Ray Grating Based Spectrometer, ours has a resolving power of 1,000~5,000 (0.05eV ~ 0.2eV). The MXS can use all types of gratings, whether constant line spacing or varied line spacing, with different surface figures; such as planar, spherical or even elliptical. Different types of gratings can have their respective strengths and limitations. Being able to choose gratings freely with MXS will offer the optical designer the freedom to select ideal optical parameters.

On the other hand, XES 350 can only use fixed radius, constant line spacing, spherical gratings. Although such gratings were easier and less expensive to produce decades ago, they cost almost the same as other types of gratings these days, thanks to the advanced grating fabrication technology. MXS has a far more advanced design that takes full advantage of the latest grating fabrication technology.

XES 350 has a fixed opto-mechanical design with very little room for modification.  The constant line spacing gratings have a fixed incidence angle and radius. The optical parameters, like source-to-grating and grating-to-detector distances, cannot easily be changed. Even with a smaller source beam (nominally 10 microns in the grating dispersive direction) the best resolving power (photon energy divided by the energy resolution) is only around 2,000.

MXS uses a high quantum efficiency CCD detector, while XES 350 can only use a micro-channel plate with a fluorescent coating at the grazing incidence angle. There is almost an order of magnitude difference in the detection efficiency just from the detector. That is one reason why the MXS is more efficient than the XES 350.

Reference: Lawrence Berkeley National Laboratory, R&D 100 entry information