[Nexus] UB matrix in NXsample

[Osborn, Raymond]

Dear Armando,
Busing and Levy is quite specific in calling U the orientation matrix, so the description in the NeXus base class, i.e., "Orientation matrix of single crystal sample using Busing-Levy convention: W. R. Busing and H. A. Levy (1967). Acta Cryst. 22, 457-464”, implies that we are talking about U. Whether everyone else uses it that way, I can’t say.

In our own software, we assume that orientation_matrix refers to U and then dynamically generate B from the unit cell parameters. That way, the orientation matrix should, at least in principle, be fixed during an experiment while heating or cooling the sample. Of course, tweaks might be necessary if the sample shifts. I suppose we could add a UB matrix to the list of stored parameters in a NeXus file even though it is technically redundant.

We have another issue to do with where we store the orientation matrix (sample or detector). In principle, the NXsample group is the correct location, but there are circumstances where we would prefer it to be in the NXdetector group, but that should be the subject of another thread. 

With best regards,
Ray

> On Apr 10, 2017, at 10:44 AM, V. Armando Solé <sole at esrf.fr> wrote:
> 
> Dear colleagues,
> 
> I am trying to map the orientation matrix (UB in the SPEC jargon) to the
> appropriate place in the NXsample group
> (http://download.nexusformat.org/doc/html/classes/base_classes/NXsample.html#nxsample)
> 
> My understanding is that it should be under orientation_matrix however I
> face some difficulties:
> 
> - The format is [n_comp, 3, 3]. I would be expecting a 3x3 matrix. So, I
> guess if there is only one composition or type of crystal, the format
> has to be [1,3,3]
> 
> - Do you call orientation_matrix to the UB product in Busing and Levy or
> just the matrix U? Very often one just has the product UB and it would
> be cumbersome to have different meanings as function of having the
> (optional) cell parameters or not in order to build the B matrix. If you
> actually mean the U matrix, I would strongly advice to add the
> ub[n_comp, 3, 3] representing UB as a sample dataset field.
> 
> As a side question, what is the NXsample/sample_orientation that has
> three floats? The only thing I find in Busing and Levy corresponding to
> three floats is hkl. If it is that, hkl would have been more explicit.
> If it is not that, what is it?
> 
> Thanks a lot for your help.
> 
> Best regards,
> 
> Armando
> 
> 
> 
> 
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-- 
Ray Osborn, Senior Scientist
Materials Science Division
Argonne National Laboratory
Argonne, IL 60439, USA
Phone: +1 (630) 252-9011
Email: ROsborn at anl.gov