On Thu, 3 Feb 2000, Mitchell, Brian J. wrote:
> mu*R < 1 when dealing with CW neutron diffraction experiments. I am not,
> however, familiar with the determination of sample absorption using a TOF
> instrument and at what "hard values" I should consider that the absorption
> is causing a real problem with the corresponding Rietveld refinement.
>
> Any help or input in this matter would be greatly appreciated.
I'm not sure how much this helps but my understanding is that the twotheta
dependance of the absorbtion is unimportant, unless you are using multiple
banks and constraining the scale factors across them (most people don't).
The wavelength dependence is important, even if mu*R seems quite small,
and for most elements the absorbtion cross section varies as 1/v, the
neutron velocity. Values are usually tabulated at v=2200m/s. This *is*
important and it tends to correlate with the Debye-Waller factor. Having
said that, it usually only means the absolute values are off, but relative
values for different atoms in the structure are still meaningful.
You can run into problems which depend on how well the data was normalised
for the incident intensity. See C.G.Windsor's book, 'pulsed neutron
scattering', which goes through one methodology. Assuming it's all been
done correctly, you can either correct the raw data, or put mu*R into a
refinement program. Some say you should never refine the absorbtion, since
it is not an unknown.
As for when does absorbtion cause a problem? I've seen an example of an
annular sample containing Gd (HUGE absorber) giving a dataset which was
refinable for the crystal structure, with no absorbtion correction. The
thermal factors were duff, but in theory (!) they should come out right
when absorbtion is taken into account. So absorbtion is only a problem
when the peaks disappear.
Hope this helps,
Jon Wright
PS: Can anyone tell me if sigma_abs goes as 1/v for Gd?
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