ReflectometryILLPreprocess v1¶

ReflectometryILLPreprocess dialog.

Table of Contents

Summary
- See Also
Properties
Description
- Detector angles
- Foreground and backgrounds
Usage
Source

Summary ¶

Loads, merges, normalizes and subtracts background from ILL reflectometry data.

Properties ¶

Name	Direction	Type	Default	Description
Run	Input	list of str lists		A list of input run numbers/files. Allowed values: [‘nxs’]
InputWorkspace	Input	MatrixWorkspace		An input workspace (units TOF) if no Run is specified.
BeamPositionWorkspace	Input	TableWorkspace		A beam position table corresponding to InputWorkspace.
BraggAngle	Input	number	Optional	A user-defined beam angle (unit degrees).
BeamCentre	Input	number	Optional	A workspace index corresponding to the beam centre.
OutputWorkspace	Output	MatrixWorkspace	Mandatory	The preprocessed output workspace (unit wavelength), single histogram.
SubalgorithmLogging	Input	string	Logging OFF	Enable or disable child algorithm logging. Allowed values: [‘Logging OFF’, ‘Logging ON’]
Cleanup	Input	string	Cleanup ON	Enable or disable intermediate workspace cleanup. Allowed values: [‘Cleanup ON’, ‘Cleanup OFF’]
DirectBeamPositionWorkspace	Input	TableWorkspace		A beam position table from a direct beam measurement.
WaterWorkspace	Input	MatrixWorkspace		A (water) calibration workspace (unit TOF).
SlitNormalisation	Input	string	Slit Normalisation OFF	Enable or disable slit normalisation. Allowed values: [‘Slit Normalisation OFF’, ‘Slit Normalisation ON’]
FluxNormalisation	Input	string	Normalise To Time	Neutron flux normalisation method. Allowed values: [‘Normalise To Time’, ‘Normalise To Monitor’, ‘Normalisation OFF’]
ForegroundHalfWidth	Input	int list		Number of foreground pixels at lower and higher angles from the centre pixel.
FlatBackground	Input	string	Background Constant Fit	Flat background calculation method for background subtraction. Allowed values: [‘Background Constant Fit’, ‘Background Linear Fit’, ‘Background OFF’]
LowAngleBkgOffset	Input	number	7	Distance of flat background region towards smaller detector angles from the foreground centre, in pixels.
LowAngleBkgWidth	Input	number	5	Width of flat background region towards smaller detector angles from the foreground centre, in pixels.
HighAngleBkgOffset	Input	number	7	Distance of flat background region towards larger detector angles from the foreground centre, in pixels.
HighAngleBkgWidth	Input	number	5	Width of flat background region towards larger detector angles from the foreground centre, in pixels.
OutputBeamPositionWorkspace	Output	TableWorkspace		Output the beam position table.

Description ¶

This algorithm is the first step in the ILL reflectometry reduction workflow. It:

loads data from disk
merges the numors
normalizes to a (water) reference (optional)
normalizes to slit sizes (optional)
normalizes to experiment time or monitor counts (optional)
subtracts time-independent background (optional)
converts to wavelength

The algorithm can be thought as an ‘advanced loader’, and should be used to load both direct beam and reflected beam measurements.

The OutputWorkspace can be further fed to ReflectometryILLSumForeground.

The workflow diagram below gives an overview of the algorithm:

../_images/ReflectometryILLPreprocess-v1_wkflw.png

The properties BeamCentre, BraggAngle and DirectBeamPositionWorkspace affect the pixel $\theta$ angles. They map directly to the corresponding properties of LoadILLReflectometry. The BeamCentre property can be further used in determining the foreground pixels as discussed below.

Foreground and backgrounds ¶

Foreground is a set of pixels intensities of which will be summed in ReflectometryILLSumForeground. However, foreground needs to be defined already in this algorithm as the information is needed for the background pixels. The foreground pixel information is stored in the sample logs of OutputWorkspace under the entries starting with foreground..

Background, on the other hand, is a set of pixels which are be used for fitting a constant or linear background by CalculatePolynomialBackground.

The foreground pixels are defined by the foreground centre and ForegroundHalfWidth property. In normal use cases, the foreground center (workspace index) is taken from the fitting in LoadILLReflectometry. This can be overriden by giving the pixel as BeamCentre. Fractional values are rounded to nearest integer. The full process of deciding the foreground centre is as follows:

If Run is given then data is loaded using LoadILLReflectometry:
- If BeamCentre is set, it is passed over to LoadILLReflectometry.
- Otherwise, LoadILLReflectometry will fit the beam centre.
- Use the beam centre returned by the LoadILLReflectometry, rounded to nearest integer, as the foreground centre.
If InputWorkspace is given:
- If BeamPositionWorkspace is given, take the beam centre from there, round it to nearest integer and use as the foreground centre.
- If BeamCentre is given, round the value to nearest integer and use as the foreground centre.
- Otherwise fit the beam centre using similar method to LoadILLReflectometry and use the rounded result as the foreground centre.

ForegroundHalfWidth is a list of one or two values. If a single value is given, then this number of pixels on both sides of the centre pixel are included in the foreground. For example, ForegroundHalfWidth=[3] means three pixel on both sides are included, making the foreground seven pixels wide in total. ForegroundHalfWidth=[0] means that only the centre pixel is included. When two values are given, then the foreground is asymmetric around the centre. For instance, ForegroundHalfWidth[2,5] indicates that two pixel at lower $\theta$ and five pixels at higher $\theta$ are included in the foreground.

LowAngleBkgWidth and HighAngleBkgWidth define the number of the background fitting pixels at low and high $\theta$ . Either one or both widths can be defined. The distance between the background pixels and the foreground can in turn be given by LowAngleBkgOffset and HighAngleBkgOffset.

The following figure exemplifies the foreground and background for the D17 instrument at ILL. Note, that in this particular case, the pixel indices increase with decreasing $\theta$ .

(Source code, png, hires.png, pdf)

../_images/ReflectometryILLPreprocess-v1-1.png

Usage ¶

Example - Load direct and reflected beams

from directtools import SampleLogs

# Use same foreground and background settings for direct and reflected
# beams.
# Python dictionaries can be passed to algorithms as 'keyword arguments'.
settings = {
    'ForegroundHalfWidth':[5],
    'LowAngleBkgOffset': 10,
    'LowAngleBkgWidth': 20,
    'HighAngleBkgOffset': 10,
    'HighAngleBkgWidth': 50
}

direct = ReflectometryILLPreprocess(
    Run='ILL/D17/317369.nxs',
    OutputBeamPositionWorkspace='direct_beam_pos',  # For reflected angle calibration.
    **settings
)

reflected = ReflectometryILLPreprocess(
    Run='ILL/D17/317370.nxs',
    DirectBeamPositionWorkspace='direct_beam_pos',
    **settings
)

# Check foreground settings from sample logs
logs = SampleLogs(reflected)
print('Reflected beam centre: {}'.format(logs.foreground.centre_workspace_index))
# Half widths + centre pixel
width = logs.foreground.last_workspace_index - logs.foreground.first_workspace_index + 1
print('Foreground width: {}'.format(width))

Output:

Reflected beam centre: 202
Foreground width: 11

Categories: Algorithms | ILL\Reflectometry | Workflow\Reflectometry

Source ¶

Python: ReflectometryILLPreprocess.py (last modified: 2018-06-22)