3.9

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 the ‘temperature’ sample log is missing or needs to be overriden.

Description

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

  1. Calculate the Debye-Waller factor according to Sears and Shelley Acta Cryst. A 47, 441 (1991):

    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 ‘temperature’ entry in the sample logs. If the entry is missing, or incorrect, the Temperature input property can be used instead.

Warning

If the input Temperature is not specified or the sample log ‘temperature’ is not present in the given Vanadium workspace, or is set to an invalid value, T=293K will be taken for the Debye-Waller factor calculation. The algorithm will produce a warning in this case.

  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 coresponding to x Y value for i-th detector.

  2. 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 v1 algorithm.

Usage

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: ', 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: ', round(wsVana.readY(999)[0], 1)
print 'Spectrum 4 of the corrected workspace is filled with: ', round(wsCorr.readY(999)[0], 5)

Output:

Spectrum 4 of the output workspace is filled with:  6897.0
Spectrum 4 of the input workspace is filled with:  1.0
Spectrum 4 of the corrected workspace is filled with:  0.00014

Categories: Algorithms | CorrectionFunctions\EfficiencyCorrections