TOF

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 (=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

  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)
Resolution Δd/d 1.5x10-3
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            TOF spectra

Figure 2. The medium resolution wide-band spectra from sintered alumina illustrates the d-spacing range available for high resolution at back scattering

TOF Det

Figure 3. Detector bank