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AnvredCorrection v1

Summary

Calculates anvred correction factors for attenuation due to absorption and scattering in a spherical sample

See Also

LorentzCorrection

Properties

Name	Direction	Type	Default	Description
InputWorkspace	Input	MatrixWorkspace	Mandatory	The X values for the input workspace must be in units of wavelength or TOF
OutputWorkspace	Output	MatrixWorkspace	Mandatory	Output workspace name
LinearScatteringCoef	Input	number	Optional	Linear scattering coefficient in 1/cm. If not provided this will be calculated from the material cross-section if present (set with SetSampleMaterial)
LinearAbsorptionCoef	Input	number	Optional	Linear absorption coefficient at 1.8 Angstroms in 1/cm. If not provided this will be calculated from the material cross-section if present (set with SetSampleMaterial)
Radius	Input	number	Optional	Radius of the sample in centimeters. f not provided the radius will be taken from the sample shape if it is a sphere (set with SetSample).
PreserveEvents	Input	boolean	True	Keep the output workspace as an EventWorkspace, if the input has events (default). If false, then the workspace gets converted to a Workspace2D histogram.
OnlySphericalAbsorption	Input	boolean	False	All corrections done if false (default). If true, only the spherical absorption correction.
ReturnTransmissionOnly	Input	boolean	False	Corrections applied to data if false (default). If true, only return the transmission coefficient.
PowerLambda	Input	number	4	Power of lamda
DetectorBankScaleFactors	Input	boolean	False	No scale factors if false (default). If true, use scale factors from instrument parameter map.

Description

Following A.J.Schultz’s anvred, the weight factors should be:

\(\text{sin}^2(theta) / (lambda^4 * spec * eff * trans)\)

where

• theta = scattering_angle/2

• lamda = wavelength (in angstroms?)

• spec = incident spectrum correction

• eff = pixel efficiency

• trans = absorption correction

The quantity: \(\text{sin}^2(theta) / eff\) depends only on the pixel and can be pre-calculated for each pixel. It could be saved in array pix_weight[].

For now, pix_weight[] is calculated by the method BuildPixWeights() and just holds the \(\text{sin}^2(theta)\) values. The wavelength dependent portion of the correction is saved in the array lamda_weight[].

The time-of-flight is converted to wave length by multiplying by tof_to_lamda[id], then (int)STEPS_PER_ANGSTROM * lamda gives an index into the table lamda_weight[]. The lamda_weight[] array contains values like: 1/(lamda^power * spec(lamda)) which are pre-calculated for each lamda. These values are saved in the array lamda_weight[]. The optimal value to use for the power should be determined when a good incident spectrum has been determined. Currently, power=3 when used with an incident spectrum and power=2.4 when used without an incident spectrum.

The pixel efficiency and incident spectrum correction are NOT CURRENTLY USED. The absorption correction, trans, depends on both lamda and the pixel, Which is a fairly expensive calculation when done for each event. The transmission is calculated for a spherical sample using the fits to the tabulated values of \(A^* = 1/\text{transmission}\) in [1] using the functional form set out in [2].

Also see LorentzCorrection v1

Usage

Example:

ws = CreateSampleWorkspace("Event",XMin=5000)
wsOut = AnvredCorrection(ws,LinearScatteringCoef=1.302,
    LinearAbsorptionCoef=1.686,Radius=0.170,PowerLambda=3)

References

[1]

Weber, K. Acta Crystallographica Section B, 25.6 (1969): 1174-1178. doi: 10.1107/S0567740869003682

[2]

Dwiggins, C. W. Acta Crystallographica Section A 31.3 (1975): 395-396. doi: 10.1107/S0567739475000873

Categories: AlgorithmIndex | Crystal\Corrections | CorrectionFunctions\AbsorptionCorrections

Source

C++ header: AnvredCorrection.h

C++ source: AnvredCorrection.cpp