POWGEN | BL-11A | SNS#
Step-by-step Instruction#
Advanced Setup#
This tab of the GUI is for hosting some advanced configurations for the data reduction, as can be glimpsed from the following snapshot,
… Put items worth of special mentioning here …
Absorption Correction#
This section is for setting up the absorption correction, given some basic inputs about the sample under the beam. The required inputs are Sample Formula, Measured Mass Density, Container Type, and Type of Correction for general samples, together with the Vanadium Rod Radius for vanadium correction. The Sample Formula input should be following a certain format, typically like this,Yb2-Ti2-O7. The Measured Mass Density refers to literally the measured mass density given in g/cc. The Number Density refers to the theoretical number density of the sample, in the unit of \(Å^{-3}\) – it can be obtained from, e.g., a CIF file for the crystallographic structure of the sample. With both the Measured Mass Density and Number Density present, the Packing Fraction will be calculated and the corresponding box will be populated with the calculated value automatically - thus the input box for Packing Fraction is not editable. The Container Type dropdown menu is for specifying the geometry of both the container and the sample, which is needed for the absorption calculation. Instead of manual input for the geometry information (container inner, outer radius, and container height), a dropdown list is provided for all the typically used containers on POWGEN, where general users could select according to which of those containers were being used for their experiments. The Type of Correction dropdown is a list of alternative methods for the absorption correction. Here, details about those different methods will not be covered, and interested readers could refer to the documentation page located at https://docs.mantidproject.org/v6.5.0/concepts/AbsorptionAndMultipleScattering.html. The Vanadium Rod Radius entry is literally for providing the radius of the vanadium rod being used for normalization purpose. The Sample Height (given in cm) entry is optional – if it is not given, the program will try to read the value in from the sample log (embedded in the saved raw data of the measurement in NeXus format).
Here, it is also worth mentioning the Caching Options in the middle of the GUI interface presented above – it is for caching the absorption calculations. For a specific sample associated with specific geometry and among other configurations (which one would set up following the instruction above), the absorption calculation only needs to be performed once, and any further data reduction associated with the identical setup of the sample should be able to use the already performed absorption calculation. A typical use case is the absorption correction for a series of temperature measurement for the same sample, for which we should not repeat the absorption calculation which is computationally heavy. There are three input boxes for cache directory input. The first entry is for users to save and load their own version of the cache files. The second one should be an IPTS specific directory and it will be only used for loading (i.e., the absorption calculation will not be saved to that directory). Similarly we have the third entry, which should be a central place hosting the common absorption calculations. e.g., for vanadium, and again, this directory will only be used to load existing cache files. So, in practice, if we want to populate the specified second or third directory, we need to first run the normal reduction by specifying a save directory to host the calculated absorption in the first entry. Then, we need to copy the saved absorption cache file to either the second or the third directory. As a general user, one can copy the saved absorption cache file to their IPTS folder (under shared) and specify the corresponding directory path as the second entry. As an instrument team member, one should grab the saved absorption cache file (e.g., for vanadium) and put it somewhere under the instrument shared folder so that in the future general users can specify that central path as the third entry.
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