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 sigma_i (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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