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PDCalibration v1

Summary

This algorithm determines the diffractometer constants that convert diffraction spectra from time-of-flight (TOF) to d-spacing. The results are output to a calibration table.

Properties

Name	Direction	Type	Default	Description
InputWorkspace	Input	MatrixWorkspace	Mandatory	Input workspace containing spectra as a function of TOF measured on a standard sample.
StartWorkspaceIndex	Input	number	0	Starting workspace index for fit
StopWorkspaceIndex	Input	number	Optional	Last workspace index for fit is the smaller of this value and the workspace index of last spectrum.
TofBinning	Input	dbl list	Mandatory	Min, Step, and Max of TOF bins. Logarithmic binning is used if Step is negative. Chosen binning should ensure sufficient datapoints across the peaks to be fitted, considering the number of parameters required by PeakFunction and BackgroundType.
PreviousCalibrationFile	Input	string		An existent file to be loaded into a calibration table, which is used for converting PeakPositions from d-spacing to TOF. Allowed extensions: [‘.h5’, ‘.cal’]
PreviousCalibrationTable	Input	TableWorkspace		An existent calibration table used for converting PeakPositions from d-spacing to TOF. This property has precedence over PreviousCalibrationFile.
PeakFunction	Input	string	Gaussian	Function to fit input peaks. Allowed values: [‘BackToBackExponential’, ‘Gaussian’, ‘Lorentzian’, ‘PseudoVoigt’, ‘IkedaCarpenterPV’]
BackgroundType	Input	string	Linear	Function to fit input peaks background. Allowed values: [‘Flat’, ‘Linear’, ‘Quadratic’]
PeakPositions	Input	dbl list	Mandatory	Comma-delimited positions (d-spacing) of reference peaks. Care should be taken to avoid using peaks whose predicted positions are too close considering their peak windows.
PeakWindow	Input	dbl list	0.1	Width of the window (d-spacing) over which to fit a peak. If a single value is supplied, it will be used as half the window width for all peaks. Otherwise, the expected input is a comma-delimited list of 2*N window boundaries, where N is the number of values in PeakPositions. The order of the window boundaries should match the order in PeakPositions.
PeakWidthPercent	Input	number	Optional	Used for estimating peak width (an initial parameter for peak fitting) by multiplying peak’s value in PeakPositions by this factor.
MinimumPeakHeight	Input	number	2	Used for validating peaks before and after fitting. If a peak’s observed/estimated or fitted height is under this value, the peak will be marked as an error.
MaxChiSq	Input	number	100	Used for validating peaks after fitting. If the chi-squared value is higher than this value, the peak will be excluded from calibration. The recommended value is between 2 and 10.
ConstrainPeakPositions	Input	boolean	False	If true, a peak center being fit will be constrained by the estimated peak position +/- 0.5 * “estimated peak width”, where the estimated peak position is the highest Y-value position in the window, and the estimated peak width is FWHM calculated over the window data.
HighBackground	Input	boolean	False	Flag whether the input data has high background compared to peaks’ heights. This option is recommended for data with peak-to-background ratios under ~5.
MinimumSignalToNoiseRatio	Input	number	0	Used for validating peaks before fitting. If the signal-to-noise ratio is under this value, the peak will be excluded from fitting and calibration. This check does not apply to peaks for which the noise cannot be estimated. The minimum recommended value is 12.
CalibrationParameters	Input	string	DIFC	Select which diffractometer constants (GSAS convention) to determine. Allowed values: [‘DIFC’, ‘DIFC+TZERO’, ‘DIFC+TZERO+DIFA’]
TZEROrange	Input	dbl list		Range for allowable calibrated TZERO value. Default: no restriction.
DIFArange	Input	dbl list		Range for allowable calibrated DIFA value. Default: no restriction.
UseChiSq	Input	boolean	False	Defines the weighting scheme used in the least-squares fit of the extracted peak centers that determines the diffractometer constants. If true, the peak weight will be the inverse square of the error on the fitted peak center. If false, the peak weight will be the square of the fitted peak height.
OutputCalibrationTable	Output	TableWorkspace	Mandatory	Output table workspace containing the calibration.
MaskWorkspace	Output	MaskWorkspace	_empty_	Mask workspace (optional input / output workspace): when specified, if the workspace already exists, any incoming masked detectors will be combined with any additional outgoing masked detectors detected by the algorithm
DiagnosticWorkspaces	Output	WorkspaceGroup	Mandatory	Auxiliary workspaces containing extended information on the calibration results.
MinimumPeakTotalCount	Input	number	Optional	Used for validating peaks before fitting. If the total peak window Y-value count is under this value, the peak will be excluded from fitting and calibration.

Description

This algorithm takes an InputWorkspace containing powder diffraction spectra from a standard sample with x-units of time-of-flight (TOF). These spectra are fitted to extract the Bragg peak positions. The extracted peak positions are then used to determine the diffractometer constants that convert diffraction spectra from TOF to d-spacing. These constants are described in detail in AlignDetectors v1. The number of the constants being determined (1,2,or 3) is controlled by CalibrationParameters. The results are output to a diffraction calibration workspace.

Unlike other calibration algorithms (see Time-of-Flight Powder Diffraction Calibration), peak fitting in PDCalibration is done in TOF, not d-spacing. Correspondingly, the input PeakPositions and PeakWindow values are converted from d-spacing to TOF. The conversion is based on either the old calibration (see PreviousCalibrationFile and PreviousCalibrationTable) or, if not provided, on the instrument geometry contained in the InputWorkspace.

The peak fitting properties are explained in more detail in FitPeaks v1. This is used to perform a least-squares fitting of peaks using as much information as is provided as possible. Each input spectrum is calibrated separately following the same basic steps:

1. The PeakPositions are used to determine fit windows in combination with PeakWindow. The windows are half the distance between the provided peak positions, with a maximum size of PeakWindow.

2. The positions and windows are converted to TOF for the spectrum using either the previous calibration information or the instrument geometry.

3. For each peak, the background is estimated from the first and last ten points in the fit window.

4. For each peak, the nominal center is selected by locating the highest point near the expected position. The height is used as the initial guess as well.

5. For each peak, the width is estimated by multiplying the peak center position with PeakWidthPercent or by calculating FWHM of the data in the window.

6. For each peak, parameters (such as center, height, and width) are fitted with least-squares.

7. All of the fitted peak centers, weighted according to UseChiSq, are used in a separate least-squares procedure to determine the diffractometer constants.

If more than one constant is requested, the result that has the lowest reduced chi-squared value is returned. This favors using less parameters.

When not specified using the MaskWorkspace parameter, the default name for the mask workspace will be the OutputCalibrationTable parameter + _mask. If the mask workspace already exists, its masked values will be combined with those from any uncalibrated pixels detected during the algorithm’s execution.

The resulting calibration table can be saved with SaveDiffCal v1, loaded with LoadDiffCal v1 and applied to a workspace with AlignDetectors v1. There are also three workspaces placed in the DiagnosticWorkspace group. They are:

• evaluated fit functions (_fitted) which is the OutputPeakParametersWorkspace from FitPeaks

• raw peak fit values (_fitparam) which is the FittedPeaksWorkspace from FitPeaks

• uncertainties in raw fit values (_fiterror) which is the OutputParameterFitErrorsWorkspace from FitPeaks when UseChisSq=True is set

• peak fitted positions in d-space ( _dspacing) derived from the effective peak parameters

• peak widths (_width) derived from the effective peak parameters

• peak heights (_height) derived from the effective peak parameters

• instrument resolution (delta-d/d _resolution) derived from the average of effective width/height of each peak. This is only correct for Gaussian and Lorentzian peak shapes

Since multiple peak shapes can be used, see the documentation for the individual fit functions to see how they relate to the effective values displayed in the diagnostic tables. For Gaussian and Lorentzian, the widths and resolution are converted to values that can be directly compared with the results of EstimateResolutionDiffraction v1.

Limiting Spectra Calibrated

Supplying StartWorkspaceIndex and/or StopWorkspaceIndex will limit the spectra that are fitted. Only those that are fitted will exist in the output table, OutputCalibrationTable. CombineDiffCal can accept input of partial instrument calibration as the GroupedCalibration and will copy all other values fom the PixelCalibration. In this mode, the CalibrationWorkspace supplied to CombineDiffCal should still be the InputWorkspace supplied to PDCalibration.

Usage

Example - PDCalibration

# If you have an old calibration it can be used as the starting point
oldCal = 'NOM_calibrate_d72460_2016_05_23.h5'

# list of d values for diamond
dvalues = (0.3117,0.3257,0.3499,0.4205,0.4645,0.4768,0.4996,0.5150,0.5441,0.5642,0.5947,0.6307,.6866,.7283,.8185,.8920,1.0758,1.2615,2.0599)

LoadEventNexus(Filename='NOM_72460', OutputWorkspace='NOM_72460')
PDCalibration(InputWorkspace='NOM_72460',
              TofBinning=[300,-.001,16666.7],
              PreviousCalibrationFile=oldCal,
              PeakPositions=dvalues,
              PeakWidthPercent=.008,
              OutputCalibrationTable='cal',
              DiagnosticWorkspaces='diag')

# Print the result
print("The calibrated difc at detid {detid} is {difc}".format(**mtd['cal'].row(40000)))

Output:

The calibrated difc at detid 40896 is 5523.060327692842

Example - PDCalibration with BackToBackExponential fit function

The following example shows how to use PDCalibration with the BackToBackExponential fit function. The fit works best if sensible initial values for the parameters are specified in an instrument definition or parameter file (for more details, see the fitting parameters documentation):

Load(Filename=r'ENGINX00193749.nxs', OutputWorkspace='193749')
dpks = (1.913220892, 1.631600313,
        1.562138267, 1.352851554, 1.104598643)

# initial values for GSAS parameters A, B, S are in ENGINX parameters .xml
# use log binning
PDCalibration(InputWorkspace='193749',
              TofBinning=[10000,-0.0005,46000],
              PeakPositions=dpks,
              PeakWindow = 0.03,
              MinimumPeakHeight = 0.5,
              PeakFunction = 'BackToBackExponential',
              CalibrationParameters = 'DIFC',
              OutputCalibrationTable='cal_B2B_DIFC_chisqTrue',
              DiagnosticWorkspaces = 'diag_B2B_DIFC_chisqTrue',
              UseChiSq = True)

# Print the result
print("The calibrated difc at detid {detid} is {difc}".format(**mtd['cal_B2B_DIFC_chisqTrue'].row(1000)))

Output:

The calibrated difc at detid 108041 is 16834.952770921267

Categories: AlgorithmIndex | Diffraction\Calibration

Source

C++ header: PDCalibration.h

C++ source: PDCalibration.cpp