Create a time-of-flight lookup table for BIFROST#

This notebook shows how to create a time-of-flight lookup table for frame unwrapping at BIFROST.

[1]:

from ess.reduce import nexus
import sciline
import scipp as sc
import scippnexus as snx
from ess.reduce.nexus.types import RawChoppers, DiskChoppers
from ess.reduce.time_of_flight.lut import (
    LtotalRange,
    NumberOfSimulatedNeutrons,
    SourcePosition,
    TofLookupTableWorkflow,
)
from scippneutron.chopper import DiskChopper

from ess.bifrost import BifrostSimulationWorkflow
from ess.bifrost.data import simulated_elastic_incoherent_with_phonon
from ess.spectroscopy.types import *

Load and process beamline parameters#

First, load all required beamline parameters from an input NeXus file. We only need to know the geometry up to the sample which is the same for all banks, so choosing a single detector is enough.

[2]:

input_filename = simulated_elastic_incoherent_with_phonon()
with snx.File(input_filename) as f:
    detector_names = list(f['entry/instrument'][snx.NXdetector])

Downloading file 'BIFROST_20240914T053723.h5' from 'https://public.esss.dk/groups/scipp/ess/bifrost/2/BIFROST_20240914T053723.h5' to '/home/runner/.cache/ess/bifrost/2'.

[3]:

bifrost_workflow = BifrostSimulationWorkflow(detector_names)
bifrost_workflow[Filename[SampleRun]] = input_filename

M = nexus.types.CalibratedMonitor[SampleRun, FrameMonitor3]
C = RawChoppers[SampleRun]
choppers, monitor = bifrost_workflow.compute((C, M)).values()
beamline = sciline.compute_mapped(bifrost_workflow, BeamlineWithSpectrometerCoords[SampleRun])[0]

Compute the required distance range to include the monitor and sample:

[4]:

l_monitor = sc.norm(monitor.coords['source_position'] - monitor.coords['position'])
l_min = l_monitor
l_max = beamline.coords['L1']

The choppers in the simulated file need to be processed before they can be used for computing a lookup table. The following works for the specific simulation but is not usable in general.

[5]:

def extract_chopper_plateau(chopper):
    processed = chopper.copy()
    # These are constant in the simulated data.
    processed['rotation_speed'] = processed['rotation_speed'].data.mean()
    processed['phase'] = processed['phase'].data.mean()
    # Guessing here as this is not stored in the file.
    processed['beam_position'] = sc.scalar(0.0, unit='deg')
    return DiskChopper.from_nexus(processed)


disk_choppers = choppers.apply(extract_chopper_plateau)

Compute the lookup table#

Construct a lookup table workflow:

[6]:

workflow = TofLookupTableWorkflow()
workflow[DiskChoppers] = disk_choppers
workflow[LtotalRange] = (l_min, l_max)
workflow[SourcePosition] = beamline.coords['source_position']

# Increase this number for more reliable results:
workflow[NumberOfSimulatedNeutrons] = 200_000

[7]:

workflow.visualize(TimeOfFlightLookupTable, graph_attr={"rankdir": "LR"})

[7]:

../../_images/user-guide_bifrost_bifrost-make-tof-lookup-table_12_0.svg

Compute a lookup table:

[8]:

table = workflow.compute(TimeOfFlightLookupTable)
table

[8]:

[9]:

table.plot()

[9]:

../../_images/user-guide_bifrost_bifrost-make-tof-lookup-table_15_0.svg

Save to file#

[10]:

table.save_hdf5('BIFROST-simulation-tof-lookup-table.h5')

This Page

Create a time-of-flight lookup table for BIFROST#

Load and process beamline parameters#

Compute the lookup table#

Save to file#