While trying to compare measurements with an old and a new Cu-tube without monochromator or Ni-filter, I have found a strange behaviour in TOPAS. The Kb peak positions within the same measurement were often calculated at angles higher or lower than expected (Ka1 and Ka2 peaks are correct). The calculated positions seem to randomly fluctuate around the right value.
The activated "LAM-cursor" always shows the right positions, when moved to the corresponding Ka1 peak.
In this picture below I used a simple 3 line emission profile (Cu-Ka1, Cu-Ka2 and Cu-Kb):
lam
ymin_on_ymax 1e-005
la 0.6605 lo 1.540598 lh 0.5 'Cu-Ka1
la 0.3395 lo 1.544426 lh 0.5 'Cu-Ka2
la 0.1 lo 1.39225 lh 0.5 'Cu-Kb
Doing the same refinement and using only the Kb emission line the calculated positions are right (lower part of the picture).
I tried with and without fundamental parameters with same results.
Can someone explain this behaviour?
The picture is showing a measurement of a corundum plate (SRM1976).
[calculated profile = orange; LAM-cursor moved at position of 1 1 6 - Ka1 = vertical red lines; arrows visualising the Kb peak shift]
