[GSAS-II] 2D image Calibration of CeO2 (Ceria) Using a GE2 detector - Possible Detector Specific Issue

[Serio, Joseph A.]

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:wright at esrf.fr]
Sent: Thursday, July 6, 2017 2:55 PM
To: Serio, Joseph A. <serioja at ornl.gov>
Cc: gsas-ii at mailman.aps.anl.gov <gsas-ii at 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." <serioja at ornl.gov<mailto:serioja at 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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