EnggFitDIFCFromPeaks dialog.
Table of Contents
The algorithm fits an expected diffraction pattern to a workpace spectrum by performing single peak fits.
| Name | Direction | Type | Default | Description |
|---|---|---|---|---|
| FittedPeaks | Input | TableWorkspace | Mandatory | Information on fitted peaks, in the format produced by EnggFitPeaks. The table must contain, for every peak fitted the expected peak value (in d-spacing), and the parameters fitted. The expected values are given in the column labelled ‘dSpacing’. When using the back-to-back exponential peak function, the ‘X0’ column must have the fitted peak center. |
| OutParametersTable | Input | string | Name for a table workspace with the fitted values calculated by this algorithm (DIFC and TZERO calibration parameters) for GSAS. These two parameters are added as two columns in a single row. If not given, the table workspace is not created. | |
| DIFA | Output | number | Fitted DIFA value. This parameter is not effectively considered and it is always zero in this version of the algorithm. | |
| DIFC | Output | number | Fitted DIFC calibration parameter | |
| TZERO | Output | number | Fitted TZERO calibration parameter |
Warning
This algorithm is being developed for a specific instrument. It might undergo significant changes, should instrument scientists decide to do so.
Finds the calibration parameters DIFC and TZERO (as defined in GSAS) from a list of peaks fitted to a diffraction pattern. The peaks can be fitted to a Mantid workspace spectrum using the algorithm EnggFitPeaks which produces a table with parameter values for peaks in time-of-flight (TOF, see UnitFactory). The table is the essential input to this algorithm.
This algorithm fits the adjusted peak time-of-fligth value positions that are fitted against expected dSpacing values (d) according to the expression:
The calibration parameters TZERO and DIFC can then be used within the GSAS program or in other Mantid algorithms (see EnggCalibrate and EnggCalibrateFull). These parameters are returned and can be retrieved as output properties as well. The DIFA coefficient (quadratic term on d) is not considered and is fixed to zero in this version of the algorithm.
If a name is given in OutParametersTable this algorithm also produces a table workspace with that name, containing the parameters fitted (DIFA, DIFC, TZERO).
The parameters DIFA, DIFC, TZERO are also used in other Mantid algorithms. For example see AlignDetectors where these parameters are used to convert units from time-of-flight to d-spacing.
Example - Fitting DIFC parameter with two peaks:
# Two BackB2Back exponential peaks
peak1 = "name=BackToBackExponential,I=6000,A=1,B=0.5,X0=15000,S=250"
peak2 = "name=BackToBackExponential,I=5000,A=1,B=0.7,X0=35000,S=300"
# Create workpsace with the above peaks and a single detector pixel
ws = CreateSampleWorkspace(Function="User Defined",
UserDefinedFunction=peak1 + ";" + peak2,
NumBanks=1,
BankPixelWidth=1,
XMin=6000,
XMax=45000,
BinWidth=10)
# Update instrument geometry to something that would allow converting to some sane dSpacing values
EditInstrumentGeometry(Workspace = ws, L2 = [1.5], Polar = [90], PrimaryFlightPath = 50)
# Run the algorithm. Defaults are shown below. Files entered must be in .csv format and if both ExpectedPeaks and ExpectedPeaksFromFile are entered, the latter will be used.
peaks_tbl = EnggFitPeaks(ws, 0, [0.8, 1.9])
out_tbl_name = 'difc_from_peaks'
difa, difc, tzero = EnggFitDIFCFromPeaks(FittedPeaks=peaks_tbl, OutParametersTable=out_tbl_name)
# Print the results
print "DIFA: %.1f" % difa
print "DIFC: %.0f" % round(difc,-1)
print "TZERO: %.0f" %round(tzero,-1)
tbl = mtd[out_tbl_name]
print "The output table has %d row(s)" % tbl.rowCount()
print "Parameters from the table, DIFA: %.1f, DIFC: %.0f, TZERO: %.0f" % (tbl.cell(0,0), round(tbl.cell(0,1),-1), round(tbl.cell(0,2),-1))
print "Number of peaks fitted: {0}".format(peaks_tbl.rowCount())
print "First peak expected (dSpacing): {0}".format(peaks_tbl.column('dSpacing')[0])
print "First fitted peak center (ToF): {0:.1f}".format(peaks_tbl.column('X0')[0])
print "Second peak expected (dSpacing): {0}".format(peaks_tbl.column('dSpacing')[1])
print "Second fitted peak center (ToF): {0:.0f}".format(round(peaks_tbl.column('X0')[1],-1))
Output:
DIFA: 0.0
DIFC: 18180
TZERO: 460
The output table has 1 row(s)
Parameters from the table, DIFA: 0.0, DIFC: 18180, TZERO: 460
Number of peaks fitted: 2
First peak expected (dSpacing): 0.8
First fitted peak center (ToF): 15006.0
Second peak expected (dSpacing): 1.9
Second fitted peak center (ToF): 35010
Categories: Algorithms | Diffraction\Engineering | Diffraction\Fitting
Python: EnggFitDIFCFromPeaks.py