CorrectTOFAxis v1¶

CorrectTOFAxis dialog.

Table of Contents

Summary
Properties
Description
- Using a reference workspace
- Calculating new TOF axis
Usage
Source

Summary ¶

Corrects the time-of-flight axis with regards to the incident energy and the L1+L2 distance or a reference workspace.

Properties ¶

Name	Direction	Type	Default	Description
InputWorkspace	Input	MatrixWorkspace	Mandatory	An input workspace.
OutputWorkspace	Output	MatrixWorkspace	Mandatory	An output workspace.
ReferenceWorkspace	Input	MatrixWorkspace		A reference workspace from which to copy the TOF axis as well as the ‘Ei’ and ‘wavelength’ sample logs.
EPPTable	Input	TableWorkspace		An input EPP table.
IndexType	Input	string	Detector ID	The type of indices used in ReferenceSpectra or ElasticBinIndex (default: ‘Detector ID’). Allowed values: [‘Detector ID’, ‘Spectrum Number’, ‘Workspace Index’]
ReferenceSpectra	Input	int list		A list of reference spectra.
ElasticBinIndex	Input	number	Optional	Bin index of the nominal elastic TOF channel.
EFixed	Input	number	Optional	Incident energy if the ‘EI’ sample log is not present/incorrect.

This algorithm is meant to adjust the time-of-flight axis so that the elastic peak is at zero energy transfer after unit conversion from ‘TOF’ to ‘DeltaE’. The algorithm has two modes: either it uses a given ReferenceWorkspace, or it calculates a constant time-of-flight shift using the L1 + L2 distances of the InputWorkspace and incident energy.

Using a reference workspace ¶

If ReferenceWorkspace is set, this algorithm copies the X axis as well as the ‘Ei’ and ‘wavelength’ sample logs, if present, to OutputWorkspace. The rest of the input properties are discarded.

Calculating new TOF axis ¶

If no ReferenceWorkspace is given, the algorithm takes the L1 distance $l_1$ from the instrument attached to InputWorkspace and calculates the average L2 distance $l_2$ using the histograms specified by ReferenceSpectra. The algorithm also needs to know the TOF $t_{elastic}$ corresponding to the zero-energy transfer. This is either taken from the first spectrum in InputWorkspace as the bin centre of the bin specified by ElasticBinIndex, or calculated from the elastic peak positions given in EPPTable. EPPTable should be in the format returned by the FindEPP v1 algorithm. In this case the algorithm averages the PeakCentre column for histograms listed in ReferenceSpectra. Finally, the algorithm needs the incident energy $E_i$ which can be either specified by EFixed or is taken from the sample logs of InputWorkspace. In case EFixed is specified, the ‘Ei’ and ‘wavelength’ sample logs of OutputWorkspace are updated accordingly.

The TOF shift $\Delta t$ is calculated by

$\Delta t = t_{elastic} - (l_1 + l_2) / \sqrt{2 E_i / m_n}$ ,

where $m_n$ is the neutron mass. The shift $\Delta t$ is then added to all X values of OutputWorkspace.

The algorithm assumes micro seconds as units of time and meV as units of energy.

Whether the ReferenceSpectra input property refers to workspace indices, spectrum numbers or detector IDs is specified by IndexType.

Usage ¶

Example - CorrectTOFAxis by specifying the elatic bin

import numpy
from scipy import constants

L1 = 2.0 # in metres.
L2 = 2.0
Ei = 55.0 # in meV
v = numpy.sqrt(2 * Ei * 1e-3 * constants.e / constants.m_n)
elasticTOF = (L1 + L2) / v * 1e6 # in micro seconds.

# Make a workspace with wrong TOF axis.
TOFMin = 0.0
TOFMax = 100.0
binWidth = 0.5
# Lets say the elastic bin is in the centre of the spectrum.
elasticBinIndex = int(((TOFMax - TOFMin) / binWidth) / 2.0)
# Build a Gaussian elastic peak in the workspace.
peakCentre = TOFMin + elasticBinIndex * binWidth
spectrumDescription = '''name=Gaussian, PeakCentre={0},
Height=100, Sigma={1}'''.format(peakCentre, 0.03 * peakCentre)
ws = CreateSampleWorkspace(WorkspaceType='Histogram',
    NumBanks=1,
    BankPixelWidth=1,
    Function='User Defined',
    UserDefinedFunction=spectrumDescription,
    BankDistanceFromSample=L2,
    SourceDistanceFromSample=L1,
    XMin=TOFMin,
    XMax=TOFMax,
    BinWidth=binWidth)

# Do the correction.
correctedWs = CorrectTOFAxis(ws,
    IndexType='Workspace Index',
    ReferenceSpectra='0',
    ElasticBinIndex=elasticBinIndex,
    EFixed=Ei)

# Convert TOF to energy transfer.
convertedWs = ConvertUnits(correctedWs,
    Target='DeltaE',
    EMode='Direct')

# Check results
# Zero energy transfer should be around elasticBinIndex.
for index in range(elasticBinIndex-1, elasticBinIndex+2):
    binCentre = (convertedWs.readX(0)[index+1] + convertedWs.readX(0)[index]) / 2
    print('DeltaE at bin centre {0}: {1:0.4f}'.format(index,binCentre))

Output:

DeltaE at bin centre 99: -0.0893
DeltaE at bin centre 100: -0.0000
DeltaE at bin centre 101: 0.0891

Example - CorrectTOFAxis using EPP table

import numpy
from scipy import constants

L1 = 2.0 # in metres
L2 = 2.0
Ei = 55.0 # in meV
velocity = numpy.sqrt(2 * Ei * 1e-3 * constants.e / constants.m_n)
elasticTOF = (L1 + L2) / velocity * 1e6 # in micro seconds.

# Make a workspace with wrong TOF axis.
TOFMin = 0.0
TOFMax = 100.0
# Build a Gaussian elastic peak in the workspace.
peakCentre = TOFMin + 2.0 * (TOFMax - TOFMin) / 3.0
spectrumDescription = '''name=Gaussian, PeakCentre={0},
Height=100, Sigma={1}'''.format(peakCentre, 0.03 * peakCentre)
ws = CreateSampleWorkspace(WorkspaceType='Histogram',
       NumBanks=1,
       BankPixelWidth=1,
       Function='User Defined',
       UserDefinedFunction=spectrumDescription,
       BankDistanceFromSample=L2,
       SourceDistanceFromSample=L1,
       XMin=TOFMin,
       XMax=TOFMax,
       BinWidth=0.5)

# Prepare for the correction.
EPPTable = FindEPP(ws)

# Do the correction.
correctedWs = CorrectTOFAxis(ws,
    EPPTable=EPPTable,
    IndexType='Workspace Index',
    ReferenceSpectra='0',
    EFixed=Ei)

# Check results.
print('Original TOF for the elastic peak: {0:0.1f}'.format(
    ws.readX(0)[numpy.argmax(ws.readY(0))]))
print('Corrected TOF for the elastic peak: {0:0.1f}'.format(
    correctedWs.readX(0)[numpy.argmax(correctedWs.readY(0))]))
print('Actual elastic TOF: {0:0.1f}'.format(elasticTOF))

Output:

Original TOF for the elastic peak: 66.5
Corrected TOF for the elastic peak: 1232.7
Actual elastic TOF: 1233.1

Example - CorrectTOFAxis using a reference workspace

import numpy
from scipy import constants

L1 = 2.0
L2 = 2.0
Ei = 55.0 # in meV
v = numpy.sqrt(2 * Ei * 1e-3 * constants.e / constants.m_n)
elasticTOF = (L1 + L2) / v * 1e6 # in micro seconds.

# Make two workspaces with wrong TOF axis.
TOFMin = 0.0
TOFMax = 100.0
peakCentre = TOFMin + 2.0 * (TOFMax - TOFMin) / 3.0
# Build a Gaussian elastic peak in the first workspace.
spectrumDescription = '''name=Gaussian, PeakCentre={0},
Height=100, Sigma={1}'''.format(peakCentre, 0.03 * peakCentre)
ws1 = CreateSampleWorkspace(WorkspaceType='Histogram',
    NumBanks=1,
    BankPixelWidth=1,
    Function='User Defined',
    UserDefinedFunction=spectrumDescription,
    BankDistanceFromSample=L2,
    SourceDistanceFromSample=L1,
    XMin=TOFMin,
    XMax=TOFMax,
    BinWidth=0.5)
# Build a second workspace with slightly different Gaussian.
spectrumDescription = '''name=Gaussian, PeakCentre={0},
Height=100, Sigma={1}'''.format(peakCentre, 0.06 * peakCentre)
ws2 = CreateSampleWorkspace(WorkspaceType='Histogram',
NumBanks=1,
BankPixelWidth=1,
Function='User Defined',
UserDefinedFunction=spectrumDescription,
BankDistanceFromSample=L2,
SourceDistanceFromSample=L1,
XMin=TOFMin,
XMax=TOFMax,
BinWidth=0.5)

# Correct the first workspace using the EPP table method.
EPPTable = FindEPP(ws1)

# Do the correction.
correctedWs1 = CorrectTOFAxis(ws1,
    EPPTable=EPPTable,
    IndexType='Workspace Index',
    ReferenceSpectra='0',
    EFixed=Ei)

# Correct the second workspace by using the first as a reference.
correctedWs2 = CorrectTOFAxis(ws2,
    ReferenceWorkspace=correctedWs1)

# Check results
print('First workspace original TOF for the elastic peak: {0:0.1f}'.format(
    ws1.readX(0)[numpy.argmax(ws1.readY(0))]))
print('EPP table corrected TOF for the elastic peak: {0:0.1f}'.format(
    correctedWs1.readX(0)[numpy.argmax(correctedWs1.readY(0))]))
print('Elastic TOF for the corrected second workspace: {0:0.1f}'.format(
    correctedWs2.readX(0)[numpy.argmax(correctedWs2.readY(0))]))

Output:

First workspace original TOF for the elastic peak: 66.5
EPP table corrected TOF for the elastic peak: 1232.7
Elastic TOF for the corrected second workspace: 1232.7

Categories: Algorithms | Inelastic\Corrections