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Rietveld refinement of TOF diffraction with multiple pulse overlap

Götz Schuck 1Uwe Stuhr 1Juan Rodriguez-Carvajal 2

1. Paul Scherrer Institut (PSI), WLGA, Villigen PSI 5232, Switzerland
2. Institut Laue Langevin (ILL), Avenue des martyrs, Grenoble 38042, France


Rietveld refinement of POLDI data time-of-flight diffraction with multiple pulse overlap is enabled through the development of a POLDI data extension implemented in FullProf.1,2

POLDI is a multiple pulse-overlap (MP-O) diffractometer at PSI, which is designed mainly for strain-scanning experiments. The MP-O method made it possible to build a TOF diffractometer at the continuous spallation neutron source SINQ with short flight path, high resolution (1×10-3) and high intensity together with a spatial resolution down to 0.6×0.6×1 mm3. In comparison with a similar conventional TOF diffractometer at a continuous source, an enhancement of the intensity of up to two orders of magnitude is possible when the MP-O technique is applied. Typical POLDI data consists of 200000 data points (400 2theta channels x 500 time channels). As a matter of routine, calculation of the correlation diffraction pattern (CDP) or individual peak fitting (based on CDP results) is used to analyse POLDI data.3

The weakness of the CDP method is that the determination of the peak intensities is not precise. To overcome this and other disadvantages Rietveld refinement on the 2D (2theta - time) diffraction data can be performed using the MP-O POLDI data adaptation for FullProf (based on FullProf 3.5 Fortran code). The FullProf-POLDI data implementation has the advantage that various inherent features of FullProf (e.g.: multiphase refinement and modelling of the peak shape) can be applied for the POLDI data analysis.

The FullProf-POLDI data implementation considers the wavelength distribution of the incident beam, Bragg reflections time shifts caused due to the applied chopper speed, Bragg reflections multiplication due to the eight-chopper-slit-sequences as well as special treatment of the statistical uncertainty sigmai (average number of neutrons per data point can be lower than 1).

(1) J.K. Cockcroft, G.J. Kearley, J. Appl. Crystallogr. 17 (1984) 464

(2) J. Rodriguez-Carvajal, Physica B. 192 (1993) 55

(3) U. Stuhr, Nuclear Instruments and Methods in Physics Research A 545 (2005) 319


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Submitted: 2008-04-30 16:12
Revised:   2009-06-07 00:48