4.2

ModeratorTzero v1

../_images/ModeratorTzero-v1_dlg.png

ModeratorTzero dialog.

Summary

Corrects the time of flight of an indirect geometry instrument by a time offset that is dependent on the energy 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
OutputWorkspace Output MatrixWorkspace Mandatory The name of the output workspace
EMode Input string Indirect The energy mode (default: Indirect). Allowed values: [‘Indirect’, ‘Direct’, ‘Elastic’]
tolTOF Input number 0.1 Tolerance in the calculation of the emission time, in microseconds (default:1)
Niter Input m 1 Number of iterations (default:1)

Description

Corrects the time of flight (TOF) by a time offset that is dependent on the energy of the neutron after passing through the moderator. A heuristic formula for the correction is stored in the instrument definition file. Below is shown the entry in the instrument file for the VISION beamline:

<!-- formula for t0 calculation. See http://muparser.sourceforge.net/mup_features.html#idDef2 for available operators-->
<parameter name="t0_formula" type="string">
  <value val="(incidentEnergy < 34.7332) ? 37.011296*incidentEnergy^(-0.052874) : (incidentEnergy < 88.7556) ? 124.267307*incidentEnergy^(-0.394282) : (incidentEnergy < 252.471) ? 963.775145*incidentEnergy^(-0.850919) : (incidentEnergy < 420.145) ? 33.225834*incidentEnergy^(-0.242105) : (incidentEnergy < 100000.0) ? 120.569231*incidentEnergy^(-0.455477) : 0.0" />
</parameter>

The recorded TOF = t_0 + t_i + t_f with

  • t_0: emission time from the moderator
  • t_1: time from moderator to sample or monitor
  • t_2: time from sample to detector

This algorithm will replace TOF with TOF^* = TOF-t_0 = t_i+t_f

For a direct geometry instrument, the incident energy E_1 is the same for all neutrons. Hence, the moderator emission time is the same for all neutrons. For an indirect geometry instrument, E_1 is different for each neutron and is not known. However, the final energy E_2 selected by the analyzers is known.

  • t_0 = func(E_1) , a function of the incident energy
  • t_1 = L_1/v_1 with L_1 the distance from moderator to sample, and v_1 the initial unknown velocity (E_1=1/2*m*v_1^2)
  • t_2 = L_2/v_2 with L_2 the distance from sample to detector, and v_2 is the final fixed velocity (E_2=1/2*m*v_2^2)

Note

We obtain TOF^* as an iterative process, taking into account the fact that the correction t_0 is much smaller than t_i+t_f. Thus TOF-t_0^{(n)} = L_1/v_1^{(n)} + L_2/v_2 , n=0, 1, 2,.. Set t_0^{(0)}=0 and obtain v_1^{(0)} from the previous formula. From v_1^{(0)} we obtain E_1^{(0)} Set t_0^{(1)}=func( E_1^{(0)} ) and repeat the steps until |t_0^{(n+1)} - t_0^{(n+1)}| < tolTOF. With tolTOF=0.1 microsecond, only one iteration is needed for convergence.

Here’s the result of applying ModeratorTzero to both the event list and the histogrammed data of a run in the VISION beamline. The transformation of either events or histograms shifts the curve to smaller TOF’s. The transformed curves are not supposed to be identical, but similar and differentiated from the original curve.

Summed Histogram
Elastic Line
Inelastic Peaks

For indirect instruments featuring an incoming neutron flux having a sufficiently narrow distribution of energies, a linear relationship between t_0 and the wavelength of the incoming neutron can be established. This relation allows for coding of an algorithm with faster execution times. For indirect instruments that comply with these conditions, use of ModeratorTzeroLinear v1 is preferred.

Categories: AlgorithmIndex | CorrectionFunctions\InstrumentCorrections

Source

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

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