RefinePowderInstrumentParameters v3

../_images/RefinePowderInstrumentParameters-v3_dlg.png

RefinePowderInstrumentParameters dialog.

Summary

Parameters include Dtt1, Dtt1t, Dtt2t, Zero, Zerot.

Properties

Name Direction Type Default Description
InputPeakPositionWorkspace Input Workspace2D Mandatory Data workspace containing workspace positions in TOF agains dSpacing.
WorkspaceIndex Input number 0 Workspace Index of the peak positions in PeakPositionWorkspace.
OutputPeakPositionWorkspace Output Workspace2D   Output data workspace containing refined workspace positions in TOF agains dSpacing.
InputInstrumentParameterWorkspace Input TableWorkspace Mandatory INput tableWorkspace containg instrument’s parameters.
OutputInstrumentParameterWorkspace Output TableWorkspace   Output tableworkspace containing instrument’s fitted parameters.
RefinementAlgorithm Input string MonteCarlo Algorithm to refine the instrument parameters. Allowed values: [‘OneStepFit’, ‘MonteCarlo’]
RandomWalkSteps Input number 10000 Number of Monte Carlo random walk steps.
MonteCarloRandomSeed Input number 0 Random seed for Monte Carlo simulation.
StandardError Input string ConstantValue Algorithm to calculate the standard error of peak positions. Allowed values: [‘ConstantValue’, ‘UseInputValue’]
Damping Input number 1 Damping factor for (1) minimizer ‘Damped Gauss-Newton’. (2) Monte Carlo.
AnnealingTemperature Input number 1 Starting annealing temperature.
MonteCarloIterations Input number 100 Number of iterations in Monte Carlo random walk.
ChiSquare Output number    

Description

This algorithm refines the instrumental geometry parameters for powder diffractomers. The parameters that can be refined are Dtt1, Zero, Dtt1t, Dtt2t, Zerot, Width and Tcross.

It serves as the second step to fit/refine instrumental parameters that will be introduced in Le Bail Fit. It uses the outcome from algorithm FitPowderDiffPeaks().

Introduction

In order to do Rietveld refinement to experimental data, the diffractometer’s profile should be calibrated by the standards, such as LaB6 or Ni, with known crystal structure and lattice parameters.

For POWGEN and NOMAD, the type of the instrument profile is back-to-back exponential function convoluted with pseudo voigt of thermal neutron and epithermal neutron. It means that each diffraction peak is a back-to-back exponential,

I\frac{AB}{2(A+B)}\left[ \exp \left( \frac{A[AS^2+2(x-X0)]}{2}\right) \mbox{erfc}\left( \frac{AS^2+(x-X0)}{S\sqrt{2}} \right) + \exp \left( \frac{B[BS^2-2(x-X0)]}{2} \right) \mbox{erfc} \left( \frac{[BS^2-(x-X0)]}{S\sqrt{2}} \right) \right].

with peak parameter A, B, X_0 and S

And their corresponding peak parameters are functions described as .. math:

n_{cross} = \frac{1}{2} erfc(Width(xcross\cdot d^{-1}))

TOF_e = Zero + Dtt1\cdot d

TOF_t = Zerot + Dtt1t\cdot d - Dtt2t \cdot d^{-1}

Final Time-of-flight is calculated as:

TOF = n_{cross} TOF_e + (1-n_{cross}) TOF_t

Formula for calculating A(d), B(d), \sigma(d) and \gamma(d)

\alpha(d):

\alpha^e(d) = \alpha_0^e + \alpha_1^e d_h

\alpha^t(d) = \alpha_0^t - \frac{\alpha_1^t}{d_h}

\alpha(d)   = \frac{1}{n\alpha^e + (1-n)\alpha^t}

\beta(d):

\beta^e(d) = \beta_0^e + \beta_1^e d_h

\beta^t(d) = \beta_0^t - \frac{\beta_1^t}{d_h}

\beta(d)   = \frac{1}{n\alpha^e + (1-n)\beta^t}

For \sigma_G and \gamma_L, which represent the standard deviation for pseudo-voigt

\sigma_G^2(d_h) = \sigma_0^2 + (\sigma_1^2 + DST2(1-\zeta)^2)d_h^2 + (\sigma_2^2 + Gsize)d_h^4

\gamma_L(d_h) = \gamma_0 + (\gamma_1 + \zeta\sqrt{8\ln2DST2})d_h + (\gamma_2+F(SZ))d_h^2

The analysis formula for the convoluted peak at d_h

\Omega(TOF(d_h)) = (1-\eta(d_h))N\{e^uerfc(y)+e^verfc(z)\} - \frac{2N\eta}{\pi}\{\Im[e^pE_1(p)]+\Im[e^qE_1(q)]\}

where

erfc(x) = 1-erf(x) = 1-\frac{2}{\sqrt{\pi}}\int_0^xe^{-u^2}du

E_1(z) = \int_z^{\infty}\frac{e^{-t}}{t}dt

u = \frac{1}{2}\alpha(d_h)(\alpha(d_h)\sigma^2(d_h)+2x)

y = \frac{\alpha(d_h)\sigma^2(d_h)+x}{\sqrt{2\sigma^2(d_h)}}

p = \alpha(d_h)x + \frac{i\alpha(d_h)H(d_h)}{2}

v = \frac{1}{2}\beta(d_h)(\beta(d_h)\sigma^2(d_h)-2x)

z = \frac{\beta(d_h)\sigma^2(d_h)-x}{\sqrt{2\sigma^2(d_h)}}

q = -\beta(d_h)x + \frac{i\beta(d_h)H(d_h)}{2}

erfc(x) and E_1(z) will be calculated numerically.

Break down the problem

If we can do the single peak fitting on each single diffraction peak in a certain range, then we can divide the optimization problem into 4 sub problems for X_0, A, B and S, with the constraint on n, the ratio between thermal and epi thermal neutrons.

The function to fit is

X_0:

TOF\_h = n(Zero + Dtt1\cdot d) + (1-n)(Zerot + Dtt1t\cdot d + Dtt2t/d)

A:

\alpha(d)   = \frac{1}{n\alpha^e + (1-n)\alpha^t}

B:

\beta(d)   = \frac{1}{n\alpha^e + (1-n)\beta^t}

S:

\sigma_G^2(d_h) = \sigma_0^2 + (\sigma_1^2 + DST2(1-\zeta)^2)d_h^2 + (\sigma_2^2 + Gsize)d_h^4

with constraint:

n = 1/2 erfc(W\cdot (1-Tcross/d))

The coefficients in this function are strongly correlated to each other.

Current Implementation

Only the parameters of the function for X_0 are fitted in present implementation.

Refinement Algorithm

Two refinement algorithms, DirectFit and MonteCarlo, are provided.

DirectFit

This is a simple one step fitting. If there is one parameter to fit, Levenberg Marquart minimizer is chosen. As its coefficients are strongly correlated to each other, Simplex minimizer is used if there are more than 1 parameter to fit.

MonteCarlo

This adopts the concept of Monte Carlo random walk in the parameter space. In each MC step, one parameter will be chosen, and a new value is proposed for it. A constraint fitting by Simplex minimizer is used to fit the coefficients in new configuration.

Simulated annealing will be tried as soon as it is implemented in Mantid.

Constraint

In future, constaint will be considered.

How to use algorithm with other algorithms

This algorithm is designed to work with other algorithms to do Le Bail fit. The introduction can be found in the wiki page of LeBailFit v1.

Source

C++ source: RefinePowderInstrumentParameters3.cpp (last modified: 2018-10-05)

C++ header: RefinePowderInstrumentParameters3.h (last modified: 2018-10-05)