SANS (ZOOM & LARMOR): rocking scan

[Tom-Willemsen]

As an instrument scientist on ZOOM, I would like to be able to perform a rocking scan.

This scan involves:

• Changing a motor (angle) of the sample in the beam
• Measuring from two distinct regions-of-interest on the detector (call these measurements A & B). For clarity these regions-of-interest will be provided as numpy arrays of spectrum ids.
  • Scientifically, A and B correspond to a region on the left-hand side and the right-hand side of the detector. The aim of this scan is to center a crystal in the rotation axis, aiming to get the same intensity on both the left and right hand side of the detector.
• No time-of-flight or wavelength chopping is required for this scan. The ROIs are purely spectrum numbers.
• As the angle is varied, both A and B will trace out non-overlapping, but similarly-shaped, gaussians. In other words the two gaussians are expected to have similar fit parameters (height, width, background), except that the center (x0) parameter will be different.
• We want to find the location where the "A" peak and "B" peak are equal to one another, or in other words, a point equidistant from the centre of peak A and the centre of peak B.
  • This is achievable with two independent Gaussian fits, for A and B, and then taking the average position of the x0 parameter of the two fits in an instrument-level plan wrapper (check using historical data linked below - but this has been confirmed with RD as a sensible approach)
  • Reducer(s) need to be written/extended to expose multiple regions of interest simultaneously, derived from the same DAE data. We should use the spectra-map under the hood as the number of pixels in both regions of interest may be large.

Acceptance criteria

• Implement
• Test on ZOOM or another SANS beamline

[Tom-Willemsen]

Diego will send us some representative rocking-scan data.

Edit: data from a previous rocking scan is in ZOOM run numbers 47511-47542, if we need real data to test with.

[Tom-Willemsen]

RD suggests approach of fitting two independent peaks and taking the average of the two fitted centres is likely suitable.

So the work in this ticket is:

• Implement a DAE reducer which exposes multiple independent regions-of-interest from a single reducer. Implement it using spectra-mapping, as the number of pixels in each region could potentially be quite large.
• Once we have the reducer above, our standard fitting callback will simply be added twice for the two independent pieces of y data (corresponding to different regions on the detector), and the instrument-level plan wrapper will take the average. This instrument-level wrapper should be written as part of this ticket.

This will also be useful for LARMOR & possibly other SANS instruments.