Question on the static/dynamic air scatter shield vs. Intensity

Peter_Schultz

Hello Everyone,

when using the "Air scatter shield" during a regular Th/2Th powder diffraction measurement, the said shield is placed directly over the sample in a static distance (e.g. 1 mm above) or adjusted during the measurement via the dynamic airscatter mode, i observed the strong effect on the measured intesities.
This effect is especially strong on the very high diffraction angles, which makes a lot of sense as the shield will block more and more reflected intensity when the source and detector move more and more upwards each other.
To check for that, i measured a regular sample with the airscatter
a) up (very far distance to the sample, no interaction assumend, strong air scatter signal thus in the low angle region.
b) dynamic (... starting rather low, moving upwards during the measurement)
c) static 0.8 mm
I put up some screen shots with each 3 measurements overlapped. In the high/low angle zooms i substracted the backgrounds to get rid of the huge air scatter underground on the low angles and make the relative drop differences of the intensity visible.
So far i have not found a macro/function to correct the intensities according to the chosen static distance value or the dynamic mode in a rietveld refinement.

I guess i just missed the right line in the manual, can anyone experienced give me a hint how to correct that intensity drop as a function of theta when using static values or dynamic mode?

best regards,
Peter Schultz

rowlesmr

You've nerd sniped me. wait out.

rowlesmr

A = R sin(delta/2) / sin(theta + delta/2)
B = R sin(delta/2) / sin(theta - delta/2)
a=b = R sin(epsilon) / sin(theta - epsilon)
x² = R² + h² - 2 R h sin(theta)
epsilon ~= sin(epsilon) = h cos(theta) / x

Unaltered footprint: F = A + B
Altered footprint: f = min(A,a) + min(B,b)

scale peaks: s = f / F

rowlesmr

I think this should be a reasonable approximation. The knife is infinitely thin, the view of the detector is described solely by the incident divergance slits and the knife edge.

For a fixed knife, this should be fairly simple to implement. For a moving knife, you'd need a function to describe the height of the knife as a function of theta. and if you're using a moving knife, it should always be set such that it doesn't interfere?

The peaks are scaled by the ratio of the actual beam footprint to the actual beam footprint visible to the detector as set by the knife.

The change in background due to the presence/absence of the knife is a separate thing.

Peter_Schultz

Thanks a lot for the time and effort! I am not too much into formula implementation in the .inp, ill check if i can alter the imported Fo² for each Theta value with your derived s(Thetha) = f(Theta) / F
huh i think Footprint is dangerous to abbreviate here -
before calculating Fo-Fc in the Rietveld. Editing .xy whould be tiresome.
I remember that i saw weird misfits likely due to this effect for rietvelds with simple structure input (i assumed the Fcalc correct) from cif, they appeared as peak misfits which were a "linear" (at least it appeared approx. linear, i call it linear thus) function in the "uncorrected" difference plot. The "good" center region was about 25-30°, depending on the strongest peaks positions of the structure and the scale factor i guess. It was overfitting below that center Th value and underfitting below. At first i thought it was about abs. correction but that never solved the misfit properly, the temperature factors can partially cover that if refined but thats obviously not physical then.

As i use Rietveld only for determining phase composition in mixtures, it is more of a nerd thing as you mentioned 🙂, but for people who try to solve things from powder data that linear error function in the Fobs must be annoying, and for refining coordinates and U/B values its deadly i guess. Thinking longer on it, even for phase composition rietvelds with 2 "unlucky" crystal phases say CdTe and some C-H-N-O it might cause trouble, due to very difference atomic form factor.. i am not entirely sure if such error linear function with an "accidentaly refined" Theta axis intersection affects the ratios.

Your point on the "moving knife function" is interesting, i might ask someone from Bruker in Hannover this year (we use a D8 Discover) how the auto knife height vs Th is programmed. I originally thought the auto knife "height function" also serves to keep the illuminated area and thus s = f/F constant, although with very low angles the knife will brush the sample then if its not perfectly flat?

DEctors

Dear Peter
The idea of the automatic mode for the airscatter is not to touch the beam, only cutting airscatter at low angle.
When starting very low the knive is very close to the sample thous can touch the sample if the sample is too high, therefor I commonly add an additional offset of around 0.2 mm.
I guess the knive is missaligned (too low) in this case since it should not alter the intensity at high angles.
I commonly do this alignment using the corrundum plate. When the manual distance is set to 0 the knive should ideally rest a couple of microns above the sample. I would suggest to carefully go stepwise down until the knive almost touches the corundum plate. I typically put a small piece of paper on top of the corrundum plate, ideally it should be movable still at the ideal position. Note the distance and add set a small height offset of 0.1 or 0.2 mm to prevent touching powder samples if prepared too high. Next go to a high angle peak of corrundum and perform a scan with the motorized knive in at the maximum position around _24mm. Then a scan with the automatic mode. There should be no difference in intensity.
If there is, increase the parameter "Slit Center Offset" until there is no difference in intensity should be at a maximum of 1mm, commonly around 0.5mm or 0.7 mm.

While aligning you can set both values "Height Offset" and "Slit Center Offset" in the tools plugin.
You find the settings under Instrument -> Device -> Goniometer -> Center -> StageAttachments -> Airscatter
Once your happy with the low angle dataquality without interfering with high angle intensity you can update the configuration with the new values for Height Offset and Slit Center Offset.

Let me know if you need some assitance under dominique.ectors(at)bruker.com
Kind regards
Dominique