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" 
 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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