In o rd er to perform an an g u lar dispersive exp erim en t in a diam o n d anvil cell it is im portant to have a high p hoton flux as this type o f ex p erim en t requires a m o n o ch ro m atic X -ray beam . W e p erform ed an g u lar dispersive X -ray diffraction (E D X R D ) ex p erim en ts on secto r 13 G eo S o ilE n v iro C onsortium for A dvanced R adiation S ources C o llab o rativ e A ccess Team (G S E C A R S -C A T ) at A dvanced P hoton Source (A P S ) at the A rg o n n e N ational L aboratory (A N L ) south o f C hicago (Illinois, U S A ) and at the S ynchrotron R adiation Source (S R S ) in D aresbury (U K ) beam line 9.1.
A s show n above, the X -rays p ro d u ced by the synchrotron are not m ono ch ro m atic (fig u re 2-6 and figure 2-7). A m ono ch ro m ato r, eith er chan n el-cu t m onocrystal (ty p ically silicon (220) or (11 1)) or a dou b le crystal (typically m ade o f 2 single cry stals i.e. silicon or diam o n d ) (figure 2-9), renders the w hite beam m o n ochrom atic. The “ w h ite ” X -ray beam en ters the m o n o ch ro m ato r at a set angle. T he beam com es out o f the m o n o ch ro m ato r after tw o reflections. T he m ono ch ro m atic output beam is parallel to the input w h ite beam but has a d ifferen t height. The angle betw een the crystal and the incom ing w hite X -ray beam d eterm in es the energy o f the m onochrom atic beam exiting the m onochrom ator.
M o n o c h r o m a tic D o u b le cr> stal b e a m ' ' ' ^ ^ m o n o c h r o m a to r M o n o ch ro m a tic X -ra \ B eam W hite X -ray a ) B ea m '"’^ - ^ ^ * ® ^ ^ W h i t e b ea m
Figure 2-9 Illustration o f (a) channel-cut crystal m onochrom ator an d (b) double crystal m onochrom ator.
W e co llect the diffraction pattern w ith an X -ray sensitive area detector. In a first ap p ro x im atio n , the X -ray w av elen g th , the distance betw een the sam ple and the im aging d etecto r, and the spatial resolution o f the d etecto r determ ine the resolution o f the d iffractio n pattern. W e used tw o types o f detectors: C C D detectors and im age plates. T he im age plate has a better resolution and covers a larger area than the C C D . T able 2-1 helps to m ake a choice betw een C C D d etecto r and im age p late. For ex am p le, it m ay be better to use a C C D detector for studies o f p hase tran sitio n s at
high tem perature w hen one collects a lot o f spectra. H ow ever, fo r a fine structure analysis one m ay prefer the use o f an image plate, even though the im age plate requires a longer reading time.
The m ain advantages o f using a CCD detector are the speed, the good spatial and geom etrical correction. H ow ever, one should not use a C CD detector for long collection tim e (m ore than a few m inutes) as it has a small dynam ic range (num ber o f intensity levels) and is likely to saturate or induce som e electronic noise. The im age plate is excellent for long exposure tim es w ell over an hour. Thus, it is m ore appropriate to use the im age plate for w eakly scattering sam ples. The spatial distortion correction o f im age plates w ith online readers is excellent. Im age plates read using an independent reader can give rise to non reproducibility in the reading o f the im age plate. H ow ever, in som e cases, the reader is particularly accurate and gives rise to an exceptionally good correction (like in the case o f the reader used at D aresbury by the Edinburgh group).
CCD detector Im age plate
D etector size 100 X 100 mm 200 X 300 mm
Pixel size 88 pm 100 pm
Typical distance from the sample 200 m m 300 m m
R eading time Few seconds Several m inutes
Spatial and geom etrical corrections G ood See above
R equires opening o f the hutch for each NO YES
new diffraction spectrum
Table 2-1 C om parison between instru m en tal p aram eters o f C CD detector (B ru ker IK ) a n d
im age p la te u sed a t GSECARS-CA T (APS).
T he m onochrom atic beam is fairly large (in the order o f 1 x 1 m m ) and it is im portant to keep as m any photons as possible in order to obtain good quality spectra. So it is often im perative to focus the beam using for exam ple K irkpatrick-B aez m irrors [101] (K-B m irrors). K -B m irrors allow the focusing dow n to beam sizes o f less than 30 pm . In order to stop any beam shape im perfections, w e also use cleanup slits w hich in som e cases also reduce the size o f the beam .
In the experim ents perform ed on the superconducting w iggler beam line 9.1 at the SRS, w e did not use a focused beam. A pinhole defined the beam size. Therefore, the incident X -ray beam w as significantly w eaker than on the bending m agnet beam line 13 B M -D at A PS/G SE C A R S w hich provides a focused beam.