4.2

ModeratorTzeroLinear v1

../_images/ModeratorTzeroLinear-v1_dlg.png

ModeratorTzeroLinear dialog.

Summary

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.

Properties

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

Description

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

  • t_0: emission time from the moderator
  • t_i: time from moderator to sample
  • t_f: time from sample to detector

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:

  • t_f = L_f/v_f, with L_f: distance from sample to detector, v_f: final velocity derived from E_f
  • t_i = L_i/v_i, with L_i: distance from moderator to sample, v_i: initial velocity unknown
  • t_0 = a'/v_i+b', with a' and b' constants derived from the aforementioned empirical formula a' = a \cdot 3.956 \cdot 10^{-3} with a' in units of meters

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

Source

C++ source: ModeratorTzeroLinear.cpp (last modified: 2019-07-17)

C++ header: ModeratorTzeroLinear.h (last modified: 2018-10-05)