Dear Frantisek,
the best way to measure a clean diffraction pattern for clay mineral
analysis is of course to avoid beam overflow, either by choosing a
bigger sample length or a smaller fixed (or automatic) divergence slit.
However, if you are using the ADS (what should make sure that you
primary beam bundle hits the sample only) and you have still a "positive
effect of the knife edge" on background slope at angles below 7 deg,
this is simply a message that your primary beam bundle isn't "clean"! If
this "knife edge" really keeps away any kind of radiation from the
detector, you can simply calculate (or draw) the connection from the
detector channel at this angle over the knife edge position (2.5 mm
above the sample) towards the "source" of your unwanted background. The
line ends about 6-8 mm below the beam center, in front of the tube
window. This radiation is neither from the sample nor any "air scatter",
but simply from the material region around the lower edge of the tube
housing or the shutter opening. This area is hit by the intense primary
bundle (before the divergence slit) and becomes a source of
fluorescence/scatter. From here this radiation can pass the divergence
slit inclined and above the sample, and so it reaches the detector. In
fact, if there would be a second slit position close to the tube shutter
(as it was constructed in older instruments), this unwanted radiation
could be blocked effectively before reaching the ADS. We tried this by
inserting some tungsten or steel blades glued in the filter frame of the
Panalytical Empyrean instrument, and it works very well, without cutting
the beam bundle at higher angles as the fixed knife edge does. And this
solution is much cheaper than a "motorized knife edge" ;-)
Best regards
Reinhard
Am 22/10/2016 um 06:46 schrieb iangie:
Dear Frantisek,
>I have tried the 0.26 and 0.14° FDS, however a large "beam overflow"
has occurred. Moreover, the intensities of diffractions at higher
angles are very low in comparison to the data collected with ADS slits.
[TW] At θ=1° , 0.26 and 0.14° FDS coresponding to ~39 and ~74 mm
footprint on sample, respectively, which is much longer than your
sample length 15mm. This beam overspill will induce air
scattering which increases background at that angle.
Using ADS, the sample volume illuminated is increasing with Sinθ
dependance, therefore you observe "higher" peak intensity at high angle.
At 2θ=2°, the knife edge hieght should be lower; At 2θ=50°, the knife
edge height should be higher. Therefore, I recommanded you use
Bruker's Motorised Knife Edge, which retracts itself in real time
accoding to beam divergence and # of detector openning channels
at each θ.
Your LynxEye XE PSD should be able to opearte in Variable Detector
Openning mode, which opens fewer channels at low angle and all
channels at higher angle. You may want to try the scan type: "Coupled
2Theta/Theta (VDO)".
However, the ultimate solution would be combining ADS, MKE, VDO
together, which gives lowest possible background at low angle, ideal
for recording clay basal reflections.
--
Yours Sincerely,
Dr. Tony Wang
At 2016-10-21 21:59:23, "František Laufek"
<frantisek.lau...@geology.cz> wrote:
Dear all,
I would like to ask you about your experience in collecting the
XRD data from low angles (for me from 2° of 2Theta) to 50° 2Theta
using the Bruker D8 Advance diffractometer
(Bragg-Brentano geometry) with the Lynx Eye XE position sensitive
detector.
The studied samples are clay minerals and the main purpose of the
task is qualitative and later (semi)quantitative phase analysis.
I have fixed beam knife (=Anti-Scatter Screen) and ADS/FDS slits,
280 mm is the goniometer radius. The length of my samples is
around 15 mm.
After a few experiments and calculation, the optimal data
collection strategy seems to be:
- 10 mm automatic divergence slits (ADS), beam knife 2.5 mm above
the sample (beam knife at this position does not interfere with
the primary beam (to 50°) and still reduces the background at low
angles).
I have tried the 0.26 and 0.14° FDS, however a large "beam
overflow" has occurred. Moreover, the intensities of diffractions
at higher angles are very low in comparison to the data collected
with ADS slits.
Any suggestions are welcome.
Frantisek Laufek
Czech Geological Survey
Prague
Czech Republic
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--
TU Bergakademie Freiberg
Dr. R. Kleeberg
Mineralogisches Labor
Brennhausgasse 14
D-09596 Freiberg
Tel. ++49 (0) 3731-39-3244
Fax. ++49 (0) 3731-39-3129
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