TOF – High resolution time-of-flight powder diffractometer
Instrument responsible: György Káli
The TOF is a general purpose high resolution time-of-flight powder diffractometer. It can cover 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 (Δd=0.0015-0.15Å). It is applicable for structure determination and refinement, peak profile analyses, phase and texture analyses of crystalline matarials and for liquid diffraction as well.
The instrument is installed to a radial thermal neutron beam and placed in a separated measuring hall. The monochromator system consists of a fast double and the 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. While in high resolution mode the very short - 10µs – neutron pulse and the 25m total flight path allows us to obtain a diffractogram with an accuracy of 1.5x10-3Å (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 12.5Å-1 scattering vector. 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 operate 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 at the position of the pulse choppers using a 4.5m compressor neutron guide section before and a same decompressor after them (see Figure 1). Ch3 limits crosstalk between different pulses and Ch4 prevents frame overlap.
The instrument is recently equipped with a large surface back-scattering detector bank consist of 88 pieces of squashed 3He tube (Figure 3). It cover 20° scattering angle (2θ=145°-165°). The data are acquisited in so called "list" or "time stamping" mode: all the event on the detector, the chopper signals and optionally changes in the sample environment are registered with the time passed since the starting of the experiment. In this mode many uncertainties can be filtered out during the treatment and re-treatments and allows 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. Main parameters
Figure 2. The medium resolution wide-band spectra from sintered alumina illustrates the d-spacing range available for high resolution at back scattering
Figure 3. Detector bank