4.2

ComputeCalibrationCoefVan v1

../_images/ComputeCalibrationCoefVan-v1_dlg.png

ComputeCalibrationCoefVan dialog.

Summary

Calculate coefficients for detector efficiency correction using the Vanadium data.

Properties

Name Direction Type Default Description
VanadiumWorkspace Input MatrixWorkspace Mandatory Input Vanadium workspace
EPPTable Input TableWorkspace Mandatory Input EPP table. May be produced by FindEPP algorithm.
OutputWorkspace Output MatrixWorkspace Mandatory Name the workspace that will contain the calibration coefficients
Temperature Input number Optional Temperature during the experiment (in Kelvins) if temperature is not given in the sample logs or needs to be overriden.
EnableDWF Input boolean True Enable or disable the Debye-Waller correction.

Description

Algorithm creates a workspace with detector sensitivity correction coefficients using the given Vanadium workspace. The correction coefficients are calculated as follows.

  1. Load the peak centre and sigma from the EPPTable. These values are used to calculate sum S_i as

    S_i = \sum_{x = x_C - 3\,\mathrm{fwhm}}^{x_C + 3\,\mathrm{fwhm}} Y_i(x)

    where x_C is the peak centre position and Y_i(x) is the corresponding to x Y value for i-th detector.

  2. (If EnableDWF is true) Calculate the Debye-Waller factor according to [1]:

    D_i = \exp\left[-B_i\cdot\left(\frac{4\pi\sin\theta_i}{\lambda}\right)^2\right]

    B_i = \frac{3\hbar^2\cdot 10^{20}}{2m_VkT_m}\cdot J(y)

    where J(y) = 0.5 if y < 10^{-3}, otherwise

    J(y) = \int_0^1 x\cdot\mathrm{coth}\left(\frac{x}{2y}\right)\,\mathrm{d}x

    where y=T/T_m is the ratio of the temperature during the experiment T to the Debye temperature T_m = 389K, m_V is the Vanadium atomic mass (in kg) and \theta_i is the polar angle of the i-th detector. By default, the temperature is read from the sample logs. The log entry can be given as the ‘temperature_sample_log’ in the IPF, otherwise ‘temperature’ entry is used. If the log is missing, or incorrect, the Temperature input property can be used instead.

Warning

If no temperature is available, or is set to an invalid value, T = 293K will be taken for the Debye-Waller factor calculation. The algorithm will log a warning in this case.

  1. (If EnableDWF is true) Finally, the correction coefficients K_i are calculated as

    K_i = \frac{S_i}{D_i}

Workspace containing these correction coefficients is created as an output and can be used as a RHS workspace in Divide v1 to apply correction to the LHS workspace.

Restrictions on the input workspaces

The valid input workspace:

  • must have an instrument set
  • must have a wavelength sample log

Restrictions for EPPTable:

  • number of rows of the table must match to the number of histograms of the input workspace.
  • table must have the PeakCentre and Sigma columns.

Note

The input EPPTable can be produced using the FindEPP v2 algorithm.

Usage

Note

To run these usage examples please first download the usage data, and add these to your path. In MantidPlot this is done using Manage User Directories.

Example

# load Vanadium data
wsVana = LoadMLZ(Filename='TOFTOFTestdata.nxs')
# find elastic peak positions
epptable = FindEPP(wsVana)
# calculate correction coefficients
wsCoefs = ComputeCalibrationCoefVan(wsVana, epptable)
print('Spectrum 4 of the output workspace is filled with:  {}'.format(round(wsCoefs.readY(999)[0])))

# wsCoefs can be used as rhs with Divide algorithm to apply correction to the data
wsCorr = wsVana/wsCoefs
print('Spectrum 4 of the input workspace is filled with:  {}'.format(round(wsVana.readY(999)[0], 1)))
print('Spectrum 4 of the corrected workspace is filled with:  {}'.format(round(wsCorr.readY(999)[0], 5)))

Output:

Spectrum 4 of the output workspace is filled with:  6895.0
Spectrum 4 of the input workspace is filled with:  1.0
Spectrum 4 of the corrected workspace is filled with:  0.00015

References

[1]Sears, V. F. and Shelley, S. A., Acta Cryst. A 47 441 (1991) doi: 10.1107/S0108767391002441

Categories: AlgorithmIndex | CorrectionFunctions\EfficiencyCorrections

Source

Python: ComputeCalibrationCoefVan.py (last modified: 2018-11-08)