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Overview

The TOF-ND instrument is a general purpose high resolution time-of-flight powder diffractometer. It covers a d-spacing range from 0.5 to 2.5Å (2.5 to 12.5 Å-1 in Q-range) at variable band-with and resolution (d=0.0015–0.15Å). It is applicable for structure determination and refinement, peak profile analyses, phase and texture analyses of solid state materials and for liquid diffraction as well. TOF provides information of the fine phase changes inside the materials, without producing any type of destruction in the sample.

Technical details

The instrument is installed at a radial thermal neutron beam. The monochromator system consists of a fast double and two single choppers and a straight neutron guide with 2.5Å~10 cm2 cross section at the exit. The double chopper is designed for a maximum speed of 12000 rpm. While in high resolution mode the very short – 10μs – neutron pulse and the 25 m total flight path allows to obtain a diffractogram with an accuracy of 1.5Å~10-3Å (at back scattering mode) in a single measurement on polycrystalline materials, in low resolution mode liquid diffraction can be performed at good neutron intensity up to 12.5 Å-1 scattering vector value. The beam is filtered by a silicon single crystal against fast neutrons. The double disk chopper (Ch1 and Ch2) has two windows: a 1.5° opening for short pulses (10 μs) and a 15° window for long variable pulses (20–200 μs), that can operate in parallel and in counter rotating mode. The latter option is used to produce very short pulses at high speed. To minimize the opening time, the neutron beam is reduced from 25 to 10 mm width at the position of the pulse choppers using a 4.5 m compressor neutron guide section before and a same decompressor after them. Ch3 defines the bandwidth and Ch4 prevents frame overlap. The instrument is recently equipped with a large surface back-scattering detector bank consisting of 88 pieces of squashed 3He tubes. It covers 20° scattering angle (2=145°–165°). The data are collected in the so called event recording mode: all events on the detector, the chopper signals and optionally changes in the sample environment are registered versus time. If this mode is used, many uncertainties can be filtered out during the data treatment, allow to perform time-dependent in-situ experiments in a single measurement.

Further readings
  1. http://www.bnc.hu/?q=node/10
Providers

BNC-WIGNER (HU)