SLIDE 35 130
- J. Hoszowsku et al. I Nucl. Instr. and Meth. in Phys. Res. A 376 (1996) 129-138
Fig.
drawing
von Hamos spectrometer geometrical principle
The crystal is curved around the x-axis. The figure is not to scale. (Note. that the z-coordinate is the vertical axis in Fig. 2).
- surface. For a fixed position of the components,
an incident X-ray location on the detector corresponds geometrically to a particular Bragg angle and hence to a particular X-ray energy through the Bragg relation: E [keV] = 12.398/(2d[b;] sin 0))
(1)
where E and 13 are the energy and Bragg angle, respective-
- ly. Such focusing, called vertical focusing,
permits at one position
components, data collection
limited primarily by the crystal length (x-extension) and detector length (x-extension). In
- rder to study a greater energy interval the central Bragg
angle is adjusted by translation
spondingly
- f the detector along their axes. The source-to-
crystal and crystal-to-detector distances are varied but kept
- equal. The position
- f the crystal center from the slit is
given by: x,=RcotB, (2) where R is the crystal radius of curvature and 0 is the central Bragg angle. The detector distance x,, is twice that
2.2. Spectrometer chamber The target, crystal and detector are all contained in a stainless steel vacuum chamber (180 X 62 X 24.5 cm3) with 15 mm thick side walls, 8 mm thick cover and 10 mm thick bottom (see Fig. 2). The chamber is pumped to about 1O~‘Torr by a turbo-molecular pump (exhausting power 880 l/s) and a two stage rotary pump (10 l/s), and is mounted on a mobile stand. The top and bottom of the chamber are reinforced by 20 mm thick ribs of stainless
- steel. The chamber is equipped with four beam ports and
can be rotated about the target. Thus the angle between the beam direction and the crystal axis of curvature can be varied from 0” to 90” in 30” steps. Three circular ports permit access for the target, crystal and detector replace-
the X-ray tube is mounted in place
- f the circular port above the target system. This port can
be rotated around its center, so that the angle between the ionizing radiation and the crystal axis of curvature can be set to any value between 0” and 90”. The carriages
view of the facility: (1) crystal, (2) CCD detector, (3) target barrel, (4) X-ray tube, (5) beam ports and (6)
vacuum pump. supporting the detector and crystal move on special tracks (Schneeberger linear rail system), that allow very precise and almost frictionless movements. Each carriage is set in motion through a hardened stainless steel screw driven by a stepping motor (400 steps per turn) and a bronze nut fixed to the bottom of the carriage. The crystal and detector screws are 16 mm in diameter and are 69 and 99 cm long, respectively. Both have a 2 mm thread which gives a 5 p,m displacement
- f the carriage per step of the driving motor.
To achieve high precision in the positioning
and the detector, the screw-nut systems were manufactured without any play and are lubricated with a special high- vacuum lubricant (TorrLubeTM, Sputtered Films, Santa Barbara, CA 93103). The two carriages are stopped automatically at both ends of the rails by two inductive proximity detectors. The latter serve also as absolute position references. The distances between the slit and sensors for the detector axis are 28 and 112 cm. For the chosen crystal radius of curvature
range of the spectrometer is thus 24.4” to 6 I. 1”. The crystal and detector assembly is mounted
25 mm thick Al platform fixed to the bottom of the chamber. in
- rder to avoid any strain of the tracks which could be
caused by possible deformations
2.3. Construction detuils 2.3.1. Target-slit system The effective X-ray source viewed by the crystal is defined by a vertical (z-direction) rectangular slit, consist- ing of two juxtaposed tantalum pieces 0.3 mm thick and 10 mm high. The slit system has the following advantages: a high definition
- f the beam profile on the target is not
required, the variations of the beam position leading to a shift of the spectral position and as a consequence to a distortion of the X-ray line profile can be avoided, and the line shape is not affected by possible thermal target deformations. The width of the slit is adjustable. Typical
- Fig. 1. Schematic illustration of
K-LL RAE X-ray fluorescence.
- Fig. 4. (A) Si and (b) SiO2 X-ray fluorescence spectra measured by quick scan (25 minutes) for
the range of interest (XRF), compared with XANES spectra measured by Ohta et al. [16] at the Photon Factory. Kawai ¡1998. ¡
The ¡chemical ¡state ¡of ¡atoms ¡can ¡be ¡tracked ¡in ¡,me ¡by ¡emission ¡spectra ¡
EXAFS ¡would ¡required ¡tuning ¡EXFEL ¡to ¡each ¡energy ¡in ¡turn. ¡ XES ¡von ¡Hamos ¡spectrometer ¡records ¡every ¡wavelenth ¡at ¡once ¡ and ¡has ¡same ¡informaCon ¡as ¡XANES ¡and ¡EXAFS. ¡(call ¡it ¡EXEFS) ¡
EXAFS ¡requires ¡lower ¡dose ¡than ¡XRD, ¡ ¡damage ¡can ¡be ¡monitored ¡and ¡controlled, ¡ ¡allowing ¡for ¡data ¡collecCon ¡from ¡intact ¡ Mn4Ca ¡clusters. ¡EXAFS ¡also ¡provides ¡higher ¡Mn ¡distance ¡resoluCon ¡than ¡3.2-‑3.5 ¡Å ¡MX. ¡ ¡ ¡ ¡Kern ¡2013, ¡ ¡Yano ¡'05, ¡'08, ¡ ¡ ¡Glatzel ¡ ¡….Yulia ¡Pushkar ¡
How ¡the ¡RAE ¡makes ¡XES ¡ depend ¡on ¡DOS ¡(empty). ¡
