LoadILLReflectometry v1¶

Summary¶

Loads an ILL reflectometry Nexus file (instrument D17 or Figaro).

Properties¶

Description¶

Loads data of a Nexus file obtained from an ILL reflectometry instrument D17 or Figaro into a Workspace2D. Both time-of-flight and monochromatic instrument configurations are supported. In general, this loader reads detector and monitor counts and adds x-axis and error values. The output workspace contains histogram data. The x-axis can have units in time-of-flight or wavelength with non-varying and varying bins, respectively. The conversion to wavelength uses the algorithm ConvertUnits v1. The sample logs associated to the output workspace contain two additional entries, Facility and stheta (unit radian). While Facility is the ILL, the variable stheta is the computed Bragg angle and can serve directly as input for the algorithm ConvertToReflectometryQ v1 if desired.

Time of flight axis¶

The chopper values are used for computing the time-of-flight values for the bin edges by the following equation:

with the following variables: channel width , time-of-flight delay , offset , phase of second chopper , phase of first chopper , open offset and velocity of first chopper .

Detector position¶

This loader will update the detector position from what is defined in the instrument definition files. The detector will be moved to the current sample-detector distance and rotated around the origin either on the horizontal or vertical plane.

The rotation angle can be one of the following:

• The detector angle in the sample logs. This is the default behavior if BraggAngle and BeamPosition are not given.
• User-specified angle given by the BraggAngle input property. This option will always take precedence over other options.
• A calibrated detector angle. The calibration is done using a direct beam measurement. This option is triggered when the BeamPosition property is specified.

For the direct beam calibration, a direct beam file should be loaded separately and the OutputBeamPosition output property used to obtain a special TableWorkspace containing information on the direct beam position. This workspace can be further given as the BeamPosition input to proceeding loads as exemplified in the following:

LoadILLReflectometry('directbeam.nxs', OutputWorkspace='direct_beam_ws', OutputBeamPosition='beam_position_ws')
LoadILLReflectometry('sample1.nxs', OutputWorkspace='sample1_ws', BeamPosition='beam_position_ws')
LoadILLReflectometry('sample2.nxs', OutputWorkspace='sample2_ws', BeamPosition='beam_position_ws')
# ...

Source position¶

In the case of D17, this loader will move the source position (‘chopper1’ in the instrument definition file) on the z-axis to the position

where is the VirtualChopper.dist_chop_samp sample log entry and the Distance.ChopperGap entry.

Description of Nexus file and corresponding workspace SampleLog entries¶

The following table summarizes the Nexus file entries partially required by the loader: the choice of the chopper is Chopper or VirtualChopper for D17 and two choppers are selected out of four existing choppers for Figaro.

Usage¶

Example - Load ILL D17 Nexus file:

# Optional: set facility and default instrument
config['default.facility'] = 'ILL'
config['default.instrument'] = 'D17'

# Load ILL D17 data file (TOF mode) into a workspace 2D using default input options:
ws1 = Load('ILL/D17/317370.nxs')

print("Workspace {} has {} dimensions and {} histograms.").format(ws1.name(), ws1.getNumDims(), ws1.getNumberHistograms())

Output:

Workspace ws1 has 2 dimensions and 258 histograms.

Example - Specify user angle:

import numpy

# Load ILL d17 data file (TOF mode) into a workspace 2D using a user-defined angle of 30 degrees:
ws2 = Load('ILL/D17/317370.nxs', BraggAngle = 5.5)

# The original detector angle can be found in the sample logs:
angleOrig = ws2.getRun().getProperty("dan.value").value

# The Sample Log entry stheta will be the user defined angle of 30 degrees:
angleBragg = ws2.getRun().getProperty("stheta").value * 180. / numpy.pi

print("The detector of workspace {} was rotated to {} degrees.").format(ws2.name(), 2. * angleBragg)
print("The nominal angle in the NeXus file was {:.2} degrees.".format(angleOrig))

Output:

The detector of workspace ws2 was rotated to 5.5 degrees.
The nominal angle in the NeXus file was 3.2 degrees.

Example - Calibration of detector angle by direct beam:

import numpy

directBeamWS = Load('ILL/D17/317369.nxs', OutputBeamPosition='beamPositionWS')

beamPosWS = mtd['beamPositionWS']
peakCentre = beamPosWS.cell('FittedPeakCentre', 0)
print('Fitted direct beam maximum (in workspace indices): {:.5}'.format(peakCentre))

reflectedBeamWS = Load('ILL/D17/317370.nxs', BeamPosition=beamPosWS)

# Lets load the data without detector angle calibration just for reference

refWS = Load('ILL/D17/317370.nxs')

detAngle = numpy.degrees(reflectedBeamWS.getRun().getProperty('stheta').value)
refAngle = numpy.degrees(refWS.getRun().getProperty('stheta').value)

print('Uncalibrated detector angle: {:.4} degrees.'.format(refAngle))
print('Detector angle after calibration using direct beam: {:.4} degrees.'.format(detAngle))

Output:

Fitted direct beam maximum (in workspace indices): 202.18
Uncalibrated detector angle: 1.591 degrees.
Detector angle after calibration using direct beam: 1.627 degrees.

Categories: Algorithms | DataHandling\Nexus

Source¶

C++ source: LoadILLReflectometry.cpp

C++ header: LoadILLReflectometry.h