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SetScalingPSD v1¶

Summary ¶

For an instrument with Position Sensitive Detectors (PSDs) the ‘engineering’ positions of individual detectors may not match the true areas where neutrons are detected. This algorithm reads data on the calibrated location of the detectors and adjusts the parametrized instrument geometry accordingly.

Properties ¶

Name	Direction	Type	Default	Description
ScalingFilename	Input	string	Mandatory	The name of the scaling calibrations file to read, including its full or relative path. The file extension must be either .sca or .raw (filenames are case sensitive on linux). Allowed extensions: [‘.sca’, ‘.raw’]
Workspace	InOut	MatrixWorkspace	Mandatory	The name of the workspace to apply the scaling to. This must be associated with an instrument appropriate for the scaling file
ScalingOption	Input	number	0	Control scaling calculation - 0 => use average of left and right scaling (default). 1 => use maximum scaling. 2 => maximum + 5%

Description ¶

This algorithm was developed for the Merlin instrument but may be used with other instruments if appropriate scaling data is available. The scaling data should give the true centre point location of each pixel detector in the instrument. This may be obtained by a calibration run and post-processing of the results. Since the calibration data may vary with time, it is not convenient to store it in the instrument XML definition file. Instead it can be stored as an ACSII file with the extension “.sca” or within the “.raw” file associated with the data, as data on the position of each detector (r,theta,phi).

A scaling file (extension .sca) is expected to be an ASCII file with three header lines. Of these, only the second line is actual read and the first item on this line should give the number of detectors described by the file as an integer value. Each subsequent line after the first three will give the information for one detector with at least the five ordered values detector_ID, detector_offset, l2, code, theta and phi. Of these values only the detector_ID and the new position (l2, theta, phi) are used. The latter three values are taken as defining the true position of the detector in spherical polar coordinates relative to the origin (sample position). If a raw file is given the true positions are taken from this instead.

This algorithm creates a parameter map for the instrument that applies a shift to each detector so that is at the correct position. Monitors are not moved. Because the shift of detector locations can alter the effective width of the pixel it is necessary to apply a scaling factor. While each shift can have components in all three primary axes (X,Y,Z), it is assumed that a single PSD will maintain the co-linear nature of pixel centres. The width scaling factor for a pixel i is approximated as average of the left and right side scalings cased by the change in relative spacings with respect to neighbour pixels. End of detector pixels only have one scaling value to use. It is assumed that the scaling is both small and smooth so that the approximate scaling is reasonable.

Scaling and position correction will be reflected in properties of the detector objects including values such as the solid angle, bounding box, etc. The detector numbering in Merlin uses sequential numbers for pixels within a PSD and non-sequential jumps between PSDs. This algorithm uses these jumps to identify individual PSDs.

To apply this algorithm to instruments other than Merlin it may be necessary to modify the code depending on the type of detectors present and how they are numbered.

If the tube detector performance enhancement is used the results of the algorithm will not be visible in the instrument viewer, however all subsequent calclations on the workspace will be performed correctly.

Optional properties ¶

ScalingOpt - this integer value controls the way in which the scaling is calculated for pixels that have both left and right values for the scaling. The default is to just average the two together. Setting this to 1 causes the maximum scaling to be used and setting it to 2 uses the maximum scaling plus 5% to be used.