3.6

AlignDetectors v1

../_images/AlignDetectors-v1_dlg.png

AlignDetectors dialog.

Summary

Performs a unit change from TOF to dSpacing, correcting the X values to account for small errors in the detector positions.

Properties

Name Direction Type Default Description
InputWorkspace Input MatrixWorkspace Mandatory A workspace with units of TOF
OutputWorkspace Output MatrixWorkspace Mandatory The name to use for the output workspace
CalibrationFile Input string   Optional: The .cal file containing the position correction factors. Either this or OffsetsWorkspace needs to be specified. Allowed extensions: [‘.h5’, ‘.hd5’, ‘.hdf’, ‘.cal’]
CalibrationWorkspace Input TableWorkspace   Optional: A Workspace containing the calibration information. Either this or CalibrationFile needs to be specified.
OffsetsWorkspace Input OffsetsWorkspace   Optional: A OffsetsWorkspace containing the calibration offsets. Either this or CalibrationFile needs to be specified.

Description

This algorithm applies a calibration table to convert a workspace from time-of-flight to dSpacing as described below. The equation in GSAS converts from d-spacing (d) to time-of-flight (TOF) by the equation:

TOF = DIFC * d + DIFA * d^2 + TZERO

The manual describes these terms in more detail. Roughly, TZERO is related to the difference between the measured and actual time-of-flight base on emission time from the moderator, DIFA is an empirical term (ideally zero), and DIFC is

DIFC = \frac{2m_N}{h} L_{tot} sin \theta

Measuring peak positions using a crystal with a very well known lattice constant will give a good value for converting the data. The d-spacing of the data will be calculated using whichever equation below is appropriate for solving the quadratic.

When DIFA = 0 then the solution is just for a line and

d = \frac{TOF - TZERO}{DIFC}

For the case of needing to solve the actual quadratic equation

d = \frac{-DIFC}{2 DIFA} \pm \sqrt{\frac{TOF}{DIFA} + \left(\frac{DIFC}{2 DIFA}\right)^2 - \frac{TZERO}{DIFA}}

Here the positive root is used when DIFA > 0 and the negative when DIFA < 0.

This algorithm always uses a calibration table which it either reads from the CalibrationWorkspace property, or uses ConvertDiffCal or LoadCalFile to produce.

Note: the workspace that this algorithms outputs is a Ragged Workspace.

Restrictions on the input workspace

The input workspace must contain histogram or event data where the X unit is time-of-flight and the Y data is raw counts. The instrument associated with the workspace must be fully defined because detector, source & sample position are needed if an OffsetsWorkspace is provided.

Usage

Example: Use offset to move peak in Dspace

ws = CreateSampleWorkspace("Event",NumBanks=1,BankPixelWidth=1)
ws = MoveInstrumentComponent(Workspace='ws', ComponentName='bank1', X=0.5, RelativePosition=False)
wsD = ConvertUnits(InputWorkspace='ws',  Target='dSpacing')
maxD = Max(wsD)
offset = GetDetectorOffsets(InputWorkspace='wsD', DReference=2.5, XMin=2, XMax=3)
wsA = AlignDetectors(InputWorkspace='ws', OutputWorkspace='wsA', OffsetsWorkspace='offset')
maxA = Max(wsA)
print "Peak in dSpace", maxD.readX(0)[0]
print "Peak from calibration", maxA.readX(0)[0]

Output:

Peak in dSpace 2.66413186052
Peak from calibration 2.5596132087

Categories: Algorithms | Diffraction\Calibration

Source

C++ source: AlignDetectors.cpp

C++ header: AlignDetectors.h