Jon,
Thank you for the response. Unfortunately, I think I only have a single ceria
file, and this along with the accompanying test runs were performed by someone
else about 5 years ago. Perhaps someone whom has worked on a similar system has
done that “sanity check”?
Best,
Joe Serio
From: Jon Wright [mailto:wri...@esrf.fr]
Sent: Thursday, July 6, 2017 2:55 PM
To: Serio, Joseph A. <seri...@ornl.gov>
Cc: gsas-ii@mailman.aps.anl.gov <gsas...@aps.anl.gov>
Subject: Re: [GSAS-II] 2D image Calibration of CeO2 (Ceria) Using a GE2
detector - Possible Detector Specific Issue
Hello,
Out of curiosity, did you check with a few different detector distances or
positions? If a flat panel is not flat there might be less of a problem as the
detector goes further back. A real radial distortion shouldn't depend on
distance and is clearer when you put the beam center in the detector corner.
Good luck,
Jon
On Jul 6, 2017 20:36, "Serio, Joseph A."
<seri...@ornl.gov<mailto:seri...@ornl.gov>> wrote:
Hi everyone,
I have a concern regarding a 2D image calibration obtained from a GE2 amorphous
Silicon detector for a CeO2 (ceria) standard. In performing the calibration in
GSAS-II, the obtained values for tilt angle and rotation agree well with those
independently determined using a different method. However, when integrating
across the entire azimuthal region for all visible full diffraction rings,
there is a systematic departure of the peak positions that increases as 2theta
increases (decreasing in d spacing). In fact, the GSAS-II
calibration/integration only gives the “accurate” match to the CeO2 peaks if
the penetration variable is used, which yields a negative value. The
documentation on GSAS-II says about this variable - “Coefficient for
penetration correction to distance; accounts for diffraction ring offset at
higher angles. Optionally determined by calibration”. This is generally
considered to be a positive two-theta shift, so a negative shift from the
expected positions gives a negative depth of penetration, obviously incorrect.
Something interesting I found, however, is that in a 2008 paper written about
these detectors, entitled “Synchrotron Applications of an amorphous silicon
flat-panel detector”
(https://www.researchgate.net/publication/23195015_Synchrotron_applications_of_an_amorphous_silicon_flat-panel_detector),
it is mentioned that when a CeO2 standard was applied and the instrumental
parameters were fully calibrated, a trend still existed where the calculated
positions were incorrect with an increasing discrepancy as the diffraction line
position was increased. This discrepancy is attributed to “radial distortion in
the underlying image”, and an equation is proposed for rescaling the radial
axis accordingly.
My thinking is that the increasing error I see (up to del(2theta) = -.0278 at
2theta=9.7242) could be actually due to the “radial distortion” of the
detector itself, which is discussed in the aforementioned paper but not
immediately dealt with in GSAS-II. The “penetration depth” variable in GSAS-II
just applies a linear del(2theta) shift, which attempts to compensate for this
possible correction by giving an illogical negative value. Would it be possible
to include this functionality described in this paper when calibrating data
from 2D GE a-Si detectors if this is indeed what I am seeing? I have compiled
various screenshots of what I have seen if necessary. Thanks!
Joseph Serio
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