kunyang
Hi all~~
I'm using TOPAS to analyse TOF neutron data in order to do residual strain/stresses analysis. The macros like TOF_PV, TOF_CS_G/L have been used. The question is: sometimes after refinement the values in macros "TOF_PV" and "TOF_CS_G" "TOF_CS_L" hit limit badly. For example,TOF_PV(@, 198.44748`_0.62158, @, 0.00046648421`_0.0184159709_LIMIT_MIN_0, t1), TOF_CS_G(@, 5443.08793`_184361.90229_LIMIT_MIN_3, t1), TOF_CS_L(@, 1387.69942`_352.98410, t1) But the profile always fitted well. I'm doubt that if this is allowed and then I can use refinement results. Hope someone would help me confirm it and maybe some way to solve it.
Another question is: The default initial setting in these macros is
TOF_PV(@, 100, @, .5, t1)
TOF_CS_G(@, 300, t1)
TOF_CS_L(@, 300, t1)
if I make some obvious change for their initial value(when I met limit hitting, I would like to adjust their initial value to see any possibility for improvement), like
TOF_PV(@, 200, @, .5, t1)
TOF_CS_G(@, 1000, t1)
TOF_CS_L(@, 50, t1)
Then refine it. I found the profile fitting is also good, but the obtained lattice parameters have changed obvious, which makes obvious difference in strain calculate results. I do feel confused with such results, how to judge which refinement result is authentic? Hope some one would help me.
mfisch
If there isn't a big difference between TOF and "conventional" diffraction techniques, you cannot refine the profile shape function (e.g. PV) together with CS_L and CS_G as they are highly correlated.
You need to obtain values for PV from a standard with high crystallinity and minor strain (measured with same instrumental settings as your sample) and then only refine CS_L and CS_G. You will find more about this in the macro file and the manual.
kunyang
Many thanks @mfisch!
We used CeO2 for standard. Now I corrected refinement procedure as your guide. Thank you very much. (I feel more confident with my results now thanks to more correction). However, the phenomena of "TOF_CS_G" "TOF_CS_L" hit limit still exists. For example, in CeO2 standard refinement, it showed:
TOF_CS_G(@, 3338.88450`_8157.38491_LIMIT_MIN_3, t1)
TOF_CS_L(@, 9383.42976`_1908.54101_LIMIT_MIN_3, t1)
Is it a problem I need to solve or maybe just ignore it? Any more suggestion? I appreciate a lot for all your help.
Regards,
Kunyang
mfisch
I am not sure what triggers the LIMIT_MIN warning. Check the significance of your values, and notice that CS_G is certainly not significant as its error is larger than the value itself. Check what happens to CS_L and its error if you comment out CS_G.
kunyang
@mfisch
Hi~~
Sorry to bother you again. And many thanks in advance. :)
Another question: As your guide, after refined CeO2 powder(standard), I fixed its refined TOF_PV result to do refinement of our real samples(for example, al2o3 bulk sample), just refine TOF_CS_L for real samples（TOF_CS_G is comment out because of large error). Because of TOF_PV fixed, the peak fitting for samples looks kind of worse than previous no fixed fitting. Is that normal?