[GSAS-II] Absorption correction of TOF neutron powder diffraction

[Toby, Brian H.]

Hi Diogo,

   The value for the absorption coefficient is mu.R/lambda for CW data and mu.R (e.g. lambda = 1) for TOF instruments. Note that this is adjusted for the packing fraction, so it is a good idea to weigh absorbing samples and estimate volume to get an estimate the density of the sample as packed (for both with x-rays and neutrons). For TOF, the correction will be adjusted for wavelength across the pattern, but this causes problems for instruments that mix data over a wide angular (wavelength) range. One really can’t properly do a post-merge absorption correction on Powgen data.

   In general, absorption corrections correlate very highly with Uiso values, so they are very hard to refine. It is usually best to make an estimate and not vary them.

   I’m rusty on multiple scattering as I have not thought about it since my PDF days, but it is a slowly varying effect and I think it is somewhat similar in form to absorption, so I think the comment in the old GSAS manual is intended to mean that the effects can’t be separated.

Brian

From: GSAS-II <gsas-ii-bounces at aps.anl.gov> on behalf of Diogo Fernandes De Almeida via GSAS-II <gsas-ii at aps.anl.gov>
Date: Friday, July 25, 2025 at 10:00 AM
To: Brown, Matthew via GSAS-II <gsas-ii at aps.anl.gov>
Subject: [GSAS-II] Absorption correction of TOF neutron powder diffraction
Dear all, Apologies in advance if this is something I could easily find in the documentation, but I've spent some time looking and couldn't find anything. If there is a suitable reference I could be pointed to I would be very appreciated! I'm
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Dear all,

Apologies in advance if this is something I could easily find in the documentation, but I've spent some time looking and couldn't find anything. If there is a suitable reference I could be pointed to I would be very appreciated!

I'm having some trouble finding how absorption correction works in time-of-flight neutron diffraction data. I have a dataset which has already been absorption corrected in the data-reduction process from an experiment at a neutron source. However, I notice a significant improvement in goodness-of-fit by visual inspection of the pattern by refining absorption in Sample Parameters, particularly of the higher d-spacing / lower Q peaks. Extinction is also refined but alone can't account for the difference I see, and neither do isotropic atomic displacement parameters.

My problem is that the GSAS-II GUI says the absorption value being refined is mu*r/lambda, which as far as I understand only works for monochromated radiation. I found some documentation in the GSAS technical manual related to this:
It is assumed for TOF data that the linear absorption of all components in the sample varies with 1/velocity, and it is indistinguishable from multiple scattering effects within the sample.
Does this mean I should not refine absorption at all for TOF data, and all effects should be account for during the data-reduction process? Or does it mean by refining this parameter I'm actually accounting for multiple scattering instead of absorption?

Thank you for reading this far and for any help you can provide!
Best wishes,
Diogo
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