Dream workflow: units are weird after Lorentz correction

[nvaytet]

Computing CorrectedDetector[SampleRun] in the dream geant4 workflow from the docs yields data with a strange unit:

dim='event',
content=DataArray(
          dims=(event: 1533595),
          data=float64[1.00000000000000074e-40*m^4*counts],
          coords={'event_time_zero':datetime64[ns], 'tof':float64[ns], 'wavelength':float64[Å],
                  'dspacing':float64[Å]})

According to the docstring, the corrections is computed as $d^4 \sin\theta$ (https://github.com/scipp/essdiffraction/blob/main/src/ess/powder/correction.py#L358).
This is indeed what is written in the gsas book:

This is weird because the other formulas listed in that document seem to be dimensionless?
But d^4 is obviously not dimensionless...

[jl-wynen]

@celinedurniak Are you aware of a possible definition of $d$ that is dimensionless? I tried to look through the source code of GSAS but it's quite dense and I couldn't find a definition for d.

[celinedurniak]

There is also this publi https://journals.iucr.org/paper?S2053273322010427

[jl-wynen]

That paper shows the factor we use but does not seem to derive or explain it.

Regardless, the unit will cancel after vanadium normalisation.

But in general, I am confused about this correction. If we have identical binning in the sample and vanadium data, doesn't the correction cancel exactly? So do we only use it because we have different bins? How does this apply to using events in the sample and a histogram in vanadium?

[jokasimr]

I don't think the lorentz correction factor should be applied to the vanadium measurement,
@celinedurniak can you verify that?