TOF-ND Time-of-flight neutron diffractometer
Instrument responsible: György Káli
The TOF is a general-purpose high-resolution time-of-flight powder diffractometer. It covers the d-spacing ranging from 0.5 to 2.5Å (2.5 to 12.5 Å-1 Q-range) at variable band-with and resolution (Δd=0.0015-0.15Å). It can be applied for structure determination and refinement, for peak profile analysis, for phase and texture analysis of crystalline matarials and for diffraction on liquids as well.
The instrument is installed on a radial thermal neutron beam and is placed in a separated measuring hall. The monochromator system consists of a fast double and two single choppers and a straight neutron guide with 2.5x10 cm2 cross section at the end. The double chopper is designed for a maximum speed of 12000 rpm. On the one hand, in high resolution mode the very short - 10µs – neutron pulse and the 25m total flight path allows for obtaining a diffractogram with an accuracy of 1.5x10-3Å (at back scattering mode) in a single measurement on polycrystalline materials, on the other hand, in low resolution mode liquid diffraction can be performed at a good neutron intensity up to a scattering vector of 12.5Å-1. The beam is filtered by single crystal silicon 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). They can be operated in parallel and 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 using a 4.5m compressor neutron guide section upstream and an identical decompressor downstream the position of the pulse choppers (see Figure 1). Ch3 limits the crosstalk between different pulses and Ch4 prevents frame overlap.
The instrument has recently been equipped with a large-surface back-scattering detector bank consisting of 88 pieces of squashed 3He tube (Figure 3). It cover 20° of scattering angle (2θ=145°-165°). The data are acquired in th eso called "list" or "time stamping" mode: all events on the detector, the chopper signals and - optionally - all changes in the sample environment are registered together with the time elapsed since the start time of the experiment. In this mode many uncertainties can be filtered out during the treatment and re-treatments and allows one to perform time-dependent in-situ experiment in a single measurement.
Figure 1. Instrument Layout
|Total flight path from chopper 1||L=25m|
|Wavelength range||0.08-0.5 nm|
|Bandwidth in single experiment Δλ||from 0.4 nm to 0.08 nm (200 Hz)|
|Straight neutron guide cross section||25x100 mm2|
|Coating||supermirror NiTi, m=2|
|Beam flux at opened windows||4x107 neutron/s/cm2|
|Pulse length||10-1000 µs|
|Max. speed for the double chopper||12000 rpm|
Table 1. The main experimental parameters of TOF-ND
Figure 2. Medium resolution wide-band diffractogram of sintered alumina illustrates the d-spacing range available for high resolution experiemnt in back scattering geometry
Figure 3. Detector bank