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diftreat

The Computing for Science (CS) group supports ILL scientists, students and visitors in a number of activities including data analysis, instrument simulation and sample simulation.

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Data treatment

Treatment of Neutron Diffraction Data

Version 1.3, R. Ghosh, October 2003


A brief survey of the principal programs used at ILL is presented here. Typically treatment follows an initial reduction to an instrument independent form of intensity as a function of angle, for powder and liquids work, intensities as a function of (hkl) for single crystals, and flipping ratios as a function of (hkl) for polarised neutron work. Subsequent analysis yields the detailed structural information which is the aim of the studies.

Powder Diffraction D1A, D2B, D1B, D16, D20

Primary data reduction:

The programs used at ILL were developed in-house to treat specific instruments, and, in general, each can write out the reduced data in one of the several common reduced data formats needed by the following programs.
For the multi-detector instrument D16 red16 performs basic data reduction from a set of scans to a simple data file.

Structure Refinement using Rietveld profile refinement

powder, prof1, prof2 A.W. Hewat, Harwell Report, AERE-R7350,1973

strap, fullprof J. Rodriquez-Carvajal, FULLPROF, a program for Rietveld refinement and pattern-matching analysis, q.v. Abstracts of the satellite meeting on powder diffraction at the XV Congress of the IUCr, p127 (Toulouse,1990)

Pattern Analysis

diffit, pkfit, abfitare programs often used to analyse simple patterns and obtain integrated intensities of component peaks. (abfit - A. Antoniadis, J. Berruyer & A. Filhol, Acta Cryst...)
f24s, sasfit4 are programs using the fitfun package to fit up to four selected peaks simultaneously. Sequences of fits can be performed allowing analysis of interesting peaks for example where these merge as, say, temperature is changed. An output file can be treated with Excel etc.

Single Crystal Diffraction D9, D10, D19, DB21

Primary Data Reduction to integrated intensities

coll5n M.S. Lehmann & F.K. Larsen, J. Appl. Cryst. is the standard program for reducing data from instruments with a conventional detector.

retreat, racer C. Wilkinson, H. Khamis, R.F.D. Stansfield & G.J. McIntyre, J. Appl. Cryst. is used for reducing data from instruments with area detectors.

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Data Correction programs

datap, d15abs, d19abs - absorption corrections

dsort - sorting and merging datasets

tdes, tds2 - correction of intensities for thermal diffuse scattering

mulref - analysis of multiple reflections

Structure Refinement Packages

Most of the principal standard packages are available at ILL. These include:

shelx, upals, prolsq (+protin), sflsq(CCSL)

Complete Suites for powder and single crystal refinements:

xtal 3.2 , prometheus, gsas, CCSL

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Polarised Single Crystal Diffraction, D3

Data Reduction

d3op94 (P.J. Brown) Data for this instrument are a log of the sequence of measurements which automatically optimise the polarisation flipping ratio obtained for specific reflections by progressively modifying counting times for measurement of each component. This program extracts these optimised results from the log.

arrang (P.J. Brown) treats the data from d3op94, and places equivalents together.

Data Analysis

sorgam (P.J. Brown) derives magnetic structure factors from the polarised neutron data.

Commercial packages for modelling and visualisation

Cerius 2-1.6 with modules for visualisation, model building, optimisation, with calculations of diffraction patterns, powder indexing and refinement is available. Other modules include molecular dynamics simulations.

Data inspection and display

LAMP based on IDL can be used for data inspection of the area and linear multi-detector data.

Other programs, some portable, some using licensed plotting packages, are used for data inspection. These include:

qsclt - summarises single crystal scans

qscplt - plots 2-D scans in q-space or in angular coordinates

plot3d, gdyplt - plots linear scans versus temperature, pressure, etc.

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The Cambridge Crystallography Subroutine Library

The CCSL is a primary constituent of a number of ILL programs, and contains a wide range of coordinated routines for resolving crystallographic problems and displaying results.

Scattering from Liquids, D4 (H. Fischer)

Precise measurement of scattering from liquids over a large range of momentum transfer, especially using isotope substitution, reveals the pair correlation functions in multi-component liquids.

Initial Data reduction

d4reg reduces data from the two area detectors used in these experiments, taking account of the current detector efficiencies.

d4eff produces the file of detector efficiency normalisations which can be measured in calibration runs.

d4opr will operate on two regrouped files, performing arithmetic functions.

d4get extracts a choice of several parameters from a sequence of numor files, e.g. time, temperature etc., which may be plotted using gnuplot.

d4nifit fits a (*.reg) file from a nickel sample to determine the neutron wavelength and zero angles.

lorgaun fits up to 9 pseudo-voigt functions plus a linear background. It includes scale transformation between 2theta and Q and plots results.

polyfit is available for fitting polynomials of positive and negativ powers.

d4fou performs a (normalisable) sine-integral Fourier transform of *.q files.

d4pro calculates the D4 resolution profile function for a peak at a given scattering angle, 2theta, including the umbrella effect at low-angles.

Results are examined using gnuplot, with the aid of script files.

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