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

[Serio, Joseph A.]

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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