[Nexus] Triple Axis Spectrometer definition

[Nick Maliszewskyj]

Greetings,

The NCNR is in the process of building a state of the art triple axis
spectrometer on our BT7 beam tube which should go into operation by
early 2004. As a part of this undertaking, we are redesigning our
software for triple axis spectrometry and are making the step of
breaking away from our traditional formatted ASCII data file format.

I submit for your review a series of meta-DTD files to fill in the
blanks in our instrument descriptions. The BT7 triple axis will have
the capability of doing neutron spin polarization analysis, so it is
important to us that not only will the file structure accommodate
polarization information, but that it be possible to store all
polarization states in the same dataset.

Scans performed by the spectrometer may trace out trajectories in
reciprocal lattice coordinates ([hkl]), momentum transfer Q, energy
transfer dE, or simply in motor angles. Regardless of what path is
followed during a scan, we prefer to record all experimental
observables with each point (e.g., all motor angles, sample
temperature, sample magnetic field, incident beam monitor counts, beam
polarizer states, as well as the physically meaningful quantities
[hkl], Q, dE, etc.).

The most flexible and appealing way to write the data would be to form
"tuples" of related quantities which are recorded sequentially. The
catch is that any reading application would have to "mine" these
tuples to reconstruct the data set. As a compromise, I propose
maintaining one NXdata group for each set of polarization cross
sections. Within each NXdata group the requirements for automatic
determination of plottable data would be satisfied. Matching indices
of each element of the NXdata group would form the association of the
members of a tuple (i.e., Q[3] is the momentum transfer for the fourth
point, whose events are stored in counts[3]). In the odd case in which
the primary detector is not a point detector (say it's a linear or
area detector), the most slowly varying index of a multidimensional
data item is the "point number" index.

The definition files for NXpolarizer and NXflipper attached are meant
to start some discussion of the needs of polarized beam experiments.
I've not tried to be overly comprehensive in specifying these
components, but rather have merely filled in the items I feel that we
will actually use internally.

Thanks for your attention and I welcome any comments you have.

Nick

-- 
ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
o Dr. Nicholas C. Maliszewskyj
o Center for Neutron Research
o National Institute of Standards & Technology
o 100 Bureau Drive, Stop 8562
o Gaithersburg MD 20899-8562
o nickm at nist.gov
ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
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