3.8

UnwrapMonitor v1

../_images/UnwrapMonitor-v1_dlg.png

UnwrapMonitor dialog.

Summary

Takes an input workspace that contains ‘raw’ data, unwraps the data according to the reference flightpath provided and converts the units to wavelength.The output workspace will have common bins in the maximum theoretical wavelength range.

Properties

Name Direction Type Default Description
InputWorkspace Input MatrixWorkspace Mandatory A workspace with x values in units of TOF and y values in counts
OutputWorkspace Output MatrixWorkspace Mandatory The name of the workspace to be created as the output of the algorithm
LRef Input number Mandatory The length of the reference flight path (in metres)
JoinWavelength Output number    

Description

This algorithm is for use with white-beam instruments with choppers. The chopper cuts the range of wavelengths, so all detectors (including monitors) should be reduced to the same wavelength range. This is done using a “reference” flightpath, L_{ref}, which is (usually, see below) the flightpath of the farthest detectors.

If T_{min} and T_{max} are the beginning and end of the frame, for each detector D at total flightpath L_d the following times are defined:

T_1 = T_{max} - \left ( T_{min} \times \left ( 1 - \frac{L_d}{L_{ref}} \right ) \right )

T_2 = T_{max} \times \left ( \frac{L_d}{L_{ref}} \right )

Thus if L_d < L_{ref} then T_1 > T_2

Neutron velocities (and hence wavelengths) for the detector D are calculated in the following way:

  • For T_{min} < T < T_2, velocities are calculated in the usual way, i.e. v = \frac{L_d}{T}
  • Data in the range T_2 < T < T_1 are ignored (i.e. they are not used in the wavelength-converted histograms)
  • For T_1 < T < T_{max}, velocities are calculated using the formula v = \frac{L_d}{T - T_{max} + T_{min}}

Note that the minimum and maximum velocities for the points that are actually used are:

v_{max} = \frac{L_d}{T - T_{max} + T_{min}} = \frac{L_{ref}}{T_{min}}

and v_{min} = \frac{L_d}{T_2} = \frac{L_{ref}}{T_{max}}

In other words, these velocities are the same for all detectors, so the wavelength range in the transformed histogram will correspondingly be the same and this algorithm rebins the data into common bins in this range.

Occasionally, it may be that some detectors (typically downstream monitors) may be at a *longer* flightpath than L_{ref}. This depends entirely on the chopper aperture/setting. These detectors are “frame-overlapped” - in other words, there is an ambiguity in the definition of the wavelength for certain points, which should therefore be excluded. These points are at the very beginning and at the very end of the frame for a range Dt (equal on both sides) given by

D_t = (T_{max} - T_{min}) \times \left (1 - \frac{L_{ref}}{L_d} \right)

In other words, points between T_{min} and T_{min} + D_t and between T_{max} - D_t and T_{max} should be excluded. For all other points, velocities and wavelengths are calculated in the normal way.

Note that since we are dealing with histogrammed data, the cut-off values above will almost certainly not fall exactly on a bin boundary. The approach taken by this algorithm is that if any part of a bin has a value that should be excluded, then the entire bin is excluded. What this means in practice is that the edge bins will possibly have a reduced number of counts.

Restrictions on the Input Workspace

The input workspace must contain histogram 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.

Child Algorithms Used

If the input workspace contains more than a single spectrum, Unwrap makes use of the rebin algorithm to set the bins on the output workspace to common values which cover the maximum theoretically accessible wavelength range.

Usage

Example - unwrapping a raw workspace.

# Create a raw data workspace.
ws = CreateSampleWorkspace()

# The result variable will contain a tuple: (OutputWorkspace, JoinWaveLength)
# To access individual outputs use result[i] where i is the index of the required output.
result = UnwrapMonitor(InputWorkspace=ws,LRef=11)

print "JoinWaveLength is: " + str(result[1])

Output:

JoinWaveLength is: 1.42417223264

Categories: Algorithms | CorrectionFunctions\InstrumentCorrections

Source

C++ source: UnwrapMonitor.cpp

C++ header: UnwrapMonitor.h