3.11

DirectILLSelfShielding v1

../_images/DirectILLSelfShielding-v1_dlg.png

DirectILLSelfShielding dialog.

Summary

Calculates self-shielding correction for the direct geometry TOF spectrometers at ILL.

Properties

Name Direction Type Default Description
InputWorkspace Input MatrixWorkspace   Input workspace.
OutputWorkspace Output MatrixWorkspace Mandatory The output corrections workspace.
Cleanup Input string Cleanup ON What to do with intermediate workspaces. Allowed values: [‘Cleanup ON’, ‘Cleanup OFF’]
SubalgorithmLogging Input string Logging OFF Enable or disable subalgorithms to print in the logs. Allowed values: [‘Logging OFF’, ‘Logging ON’]
SimulationInstrument Input string Sparse Instrument Select if the simulation should be performed on full or approximated instrument. Allowed values: [‘Sparse Instrument’, ‘Full Instrument’]
SparseInstrumentRows Input number 5 Number of detector rows in sparse simulation instrument.
SparseInstrumentColumns Input number 20 Number of detector columns in sparse simulation instrument.
NumberOfSimulatedWavelengths Input number Optional Number of wavelength points where the simulation is performed (default: all).

Description

This algorithm calculates self shielding correction factors for the input workspace. It is part of ILL’s direct geometry reduction suite. InputWorkspace should have a sample defined using SetSample. Beam profile can be optionally set using SetBeam. The algorithm uses MonteCarloAbsorption as its backend.

To speed up the simulation, the sparse instrument option of MonteCarloAbsorption is used by default. The number of detectors to simulate can be given by SparseInstrumentRows and SparseInstrumentColumns.

By default the correction factors are calculated for each bin. By specifying NumberOfSimulatedWavelengths, one can restrict the number of points at which the calculation is done thus reducing the execution time. In this case CSplines are used to interpolate the correction factor over all bins.

The correction factor contained within the OutputWorkspace can be further fed to DirectILLApplySelfShielding.

Usage

Example - Absorption corrections of fake IN4 workspace

import numpy
import scipy.stats

# Create a fake IN4 workspace.
# We need an instrument and a template first.
empty_IN4 = LoadEmptyInstrument(InstrumentName='IN4')
nHist = empty_IN4.getNumberHistograms()
# Make TOF bin edges.
xs = numpy.arange(530.0, 2420.0, 4.0)
# Make some Gaussian spectra.
ys = 1000.0 * scipy.stats.norm.pdf(xs[:-1], loc=970, scale=60)
# Repeat data for each histogram.
xs = numpy.tile(xs, nHist)
ys = numpy.tile(ys, nHist)
ws = CreateWorkspace(
    DataX=xs,
    DataY=ys,
    NSpec=nHist,
    UnitX='TOF',
    ParentWorkspace=empty_IN4
)
# Manually correct monitor spectrum number as LoadEmptyInstrument does
# not know about such details.
SetInstrumentParameter(
    Workspace=ws,
    ParameterName='default-incident-monitor-spectrum',
    ParameterType='Number',
    Value=str(1)
)
# Add incident energy information to sample logs.
AddSampleLog(
    Workspace=ws,
    LogName='Ei',
    LogText=str(57),
    LogType='Number',
    LogUnit='meV',
    NumberType='Double'
)
# Elastic channel information is missing in the sample logs.
# It can be given as single valued workspace, as well.
elasticChannelWS = CreateSingleValuedWorkspace(107)

DirectILLCollectData(
    InputWorkspace=ws,
    OutputWorkspace='preprocessed',
    ElasticChannelWorkspace=elasticChannelWS,
    IncidentEnergyCalibration='Energy Calibration OFF', # Normally we would do this for IN4.
)

sampleGeometry = {
    'Shape': 'Cylinder',
    'Height': 8.0,
    'Radius': 1.5,
    'Center': [0.0, 0.0, 0.0]
}
sampleMaterial = {
    'ChemicalFormula': 'V',
    'SampleNumberDensity': 0.05
}
SetSample(
    InputWorkspace='preprocessed',
    Geometry=sampleGeometry,
    Material=sampleMaterial
)

DirectILLSelfShielding(
    InputWorkspace='preprocessed',
    OutputWorkspace='absorption_corrections',
    SimulationInstrument='Full Instrument', # IN4 is small enough.
    NumberOfSimulatedWavelengths=10
)
# The correction factors should be applied using DirectILLApplySelfShielding.
corrections = mtd['absorption_corrections']
f_short = corrections.readY(0)[0]
f_long = corrections.readY(0)[-1]
print('Absoprtion corrections factors for detector 1')
print('Short final wavelengths: {:.4f}'.format(f_short))
print('Long final wavelengths:  {:.4f}'.format(f_long))

Output:

Absoprtion corrections factors for detector 1
Short final wavelengths: 0.4047
Long final wavelengths:  0.2267

Categories: Algorithms | Workflow\Inelastic