3.12

AnvredCorrection v1

../_images/AnvredCorrection-v1_dlg.png

AnvredCorrection dialog.

Summary

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

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 set with SetSampleMaterial
LinearAbsorptionCoef Input number Optional Linear absorption coefficient at 1.8 Angstroms in 1/cm if not set with SetSampleMaterial
Radius Input number Optional Radius of the sample in centimeters
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:

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: 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 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 calulation when done for each event.

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)

Categories: Algorithms | Crystal\Corrections | CorrectionFunctions\AbsorptionCorrections

Source

C++ source: AnvredCorrection.cpp (last modified: 2017-11-29)

C++ header: AnvredCorrection.h (last modified: 2017-01-25)