\(\renewcommand\AA{\unicode{x212B}}\)
Table of Contents
Corrects the time of flight of an indirect geometry instrument by a time offset that is linearly dependent on the wavelength of the neutron after passing through the moderator.
Name | Direction | Type | Default | Description |
---|---|---|---|---|
InputWorkspace | Input | MatrixWorkspace | Mandatory | The name of the input workspace, containing events and/or histogram data, in units of time-of-flight |
Gradient | Input | number | Optional | Wavelength dependent TOF shift, units in microsec/Angstrom. Overrides the value stored in the instrument object |
Intercept | Input | number | Optional | TOF shift, units in microseconds. Overrides the valuestored in the instrument object |
OutputWorkspace | Output | MatrixWorkspace | Mandatory | The name of the output workspace |
This algorithm Corrects the time of flight (TOF) of an indirect geometry instrument by substracting a time offset \(t_0\) linearly dependent on the wavelength of the neutron when emitted through the moderator. This algorithm is suitable to data reduction of indirect instruments featuring a neutron flux with a narrow distribution of wavelengths. A empirical formula for the correction, stored in the instrument definition file, is taken as linear on the initial neutron wavelength \(\lambda_i\): \(t_0 = a * \lambda_i + b\). Gradient \(a\) is in units of microsec/Angstrom and Intercept \(b\) is in units of microsec. Below is the example XML code included in BASIS beamline parameters file.
<!-- Moderator Tzero/LambdaZero Parameters -->
<parameter name="Moderator.TimeZero.Gradient">
<value val="11.967"/>
</parameter>
<parameter name="Moderator.TimeZero.Intercept">
<value val="-5.0"/>
</parameter>
The recorded TOF: \(TOF = t_0 + t_i + t_f\), with
This algorithm will replace TOF with \(TOF' = TOF-t_0 = t_i + t_f\)
For an indirect geometry instrument, \(\lambda_i\) is not known but the final energy, \(E_f\), selected by the analyzers is known. For this geometry:
and \(b' = b\) with \(b'\) in units of microseconds.
Putting all together: \(TOF' = \frac{L_i}{L_i+a'} \cdot (TOF-t_f-b') + t_f\), with [TOF’]=microsec
If the detector is a monitor, then we can treat it as both sample and detector. Thus, we use the previous formula inserting the time from sample to detector \(t_f = 0\) and with the initial fligh path \(L_i\) as the distance from source to monitor.
Categories: AlgorithmIndex | CorrectionFunctions\InstrumentCorrections
C++ header: ModeratorTzeroLinear.h (last modified: 2021-03-31)
C++ source: ModeratorTzeroLinear.cpp (last modified: 2021-03-31)