Use Monte-Carlo simulations for frame unwrapping and WFM #589
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…table instead of choppers
nvaytet
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| import scipp as sc | |||
| from numpy import random | |||
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| from ..chopper import DiskChopper | |||
| from . import chopper_cascade | |||
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I don't know if it is still useful to keep the other FakeBeamline, as well as the non-disk choppers below.
The refactoring of tests means they are no longer used in the tests, but they may still be useful in the future?
src/scippneutron/tof/unwrap.py
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| from typing import NewType | ||
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| import numpy as np | ||
| import scipp as sc | ||
| from scipp.core.bins import Lookup | ||
| import tof |
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I have only briefly skimmed through this file so far. Consider how the use of tof in now deeply embedded in the entire file. At the same time, you already know that you want to use (also) McStas in place of tof in the future. Would it be possible to follow common design practices and define a minimal and well-defined interface/wrapper, such that the architecture is hopefully modular enough so you can swap out tof for McStas without rewriting all the logic again?
Probably half of that is already achieved by moving all tof-using functions to a different file (module), and by being clear about the public interface, ensuring the same interface would work with McStas.
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It's not that embedded actually, it's almost entirely in a single provider (run_tof_model).
All one needs to do to replace the table with a mcstas one is change the provider for SimulationResults.
But yes, we should move the tof stuff into a separate module, and make room for a mcstas provider.
I guess we would then have a set of unwrap_basic_providers (tof-based), and a set of unwrap_advanced_providers (mcstas-based)?
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Looks very interesting! I'll take a closer look today. |
src/scippneutron/tof/unwrap.py
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| Facility where the experiment is performed (used to determine the source pulse | ||
| parameters). | ||
| """ | ||
| return PulsePeriod(1.0 / tof.facilities[facility].frequency) |
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Should we also remove the use of tof here? It doesn't influence the lookup table.
But we could also keep our own library of frequencies in scippneutron...
Multithreading would be nice. What parameters are you talking about? As far as I see it, there are no parameters? |
I meant the entries in the lookup table. But maybe it's already quite small? |
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Conclusion from discussion: the |
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The PR in |
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Pushed to the wrong repo. Update in |
In this PR, we move away from our analytical computations of frame boundaries and fitting the polygons with single lines for frame unwrapping and WFM.
This method yielded errors typically towards the edge of the frames where the polygon fit would be less accurate (the lines would in some cases be outside of the polygons). Using a 3rd order fit helped but still wasn't optimal.
In this work, we instead use the
tofpackage to illuminate the chopper cascade with a pulse of neutrons.We then propagate the neutrons that exit the cascade to the distances of the pixels (this is just a single broadcast operation) and build a lookup table that gives us wavelength (or time-of-flight) as a function of distance and time-of-arrival.
The results are better than with the previous method (tested on a DREAM simulation), and the code simpler.
The computation is somewhat more expensive than before, but for now it is not very noticeable.
More importantly, this opens the door to other simulation methods. Instead of
tof, one could in the future use McStas to make a much more realistic simulation of neutrons travelling through the cascade, including effects like non-instantaneous opening of choppers or neutron reflections along the beam guide.See the
frame-unwrappingandwfm-time-of-flightnotebooks for a description of how the algorithm works.Comparison with previous version:
Plots of dspacing vs 2theta:
true: using the true wavelengths of the neutrons from the simulation datafrom_tof: the current version implemented in this PRfrom_main: the version on main using linear fit to polygonsfrom_cubic: an (unpublished) improvement on main using a cubic fit to polygons (see https://github.com/scipp/scippneutron/tree/cubic-polygons)1D dspacing plot: