SANS: Need an algorithm that can account for the flood source distance in calibrations

[Steve Williams]

Provide a way for the SANS reduction process to rescale the flood calibration to an arbitrary sample-detector distance. The orignal flood file must contain the information about from where the source was placed. This algorithm must be used by the SANS group flood source correction

[Nick Draper]

Bulk move of tickets at the end of iteration 28

[Nick Draper]

"New" tickets moved at the code freeze of iteration 29

[Steve Williams]

from Richard Heenan: The current LOQ flood source file is the raw per pixel counts, rescaled to average to 1.0 over some sensible radius range near the middle of the detector.

At some convenient point in the data reduction the counts in each detector pixel are divided by the rescaled flood source. Note that this ALSO does the normalisation to solid angle, assuming the flood source to detector and sample to detector distances are the same.

[Actually on LOQ, they are not quite the same, we put the source, which is “larger” than the typical sample, closer to the detector, next to the vacuum tank window, then assume it is the same effective distance. Would do the same on SANS2d. The beam stop is left in on LOQ, but would have to be removed for SANS2d]

For LOQ data, you then either have to ignore the solid angle correction, or as COLETTE does, multiply by it when you divide by the flood source, so it cancels later when you divide by solid angle.

For SANS2d we do not yet have any flood source data. We would have to collect it at a short sample-detector distance, then rescale by the expected ratio of solid angles to the distances and sideways offsets used for a given run. [Steve K, can you start the planning to run flood source once the front end work has ended – poss over Christmas hols if “security” is OK ]

Hence we need to actually divide the initial flood source data by solid angle for both LOQ and SANS2d.

Thus we need a Mantid python routine to sum all the flood source counts in each pixel, then divide by the solid angles for the set-up used to record the flood source, and renormalize to average 1.0 We will likely be able to write this ourselves. Would need to restore from archive & reprocess old LOQ flood source data files in this way, but there are not many of them, to make new flood source files for Mantid data reduction.

Then the present Mantid data reduction “flood source correction” will work - divide by “normalised flood source” early in process, then correct for solid angle of actual set up later.

[Nick Draper]

Bulk move of tickets to iteration 31 at the iteration 30 code freeze

[Anders Markvardsen]

From discussion with Richard it was agreed that this one not required.

The flood files measured will be corrected by any variation due to solid angle. This means when adding such a file to the GUI (or command line) everything should just work, and the same on LOQ and SANS2D (Q1D and Qxy will add correct solid angle correction separately).

[Michael Reuter]

OK, sounds like the knowledgeable party has declined this work.

[Stuart Campbell]

This ticket has been transferred to github issue ​3640