Instrument Description

guide hall n 1, thermal guide H22
 
guide
divergence < 1mrad
monochromators
Laue white thermal beam, 0.8 < λ <5.2 Å
filtered, position 1 0.8 < λ < 3.0 Å
filtered, position 2 3.0 < λ < 4.5 Å
collimation
pinholes 0.5 to 10 mm in diameter
sample
beam size up to 10 mm in diameter
flux (unfiltered) 1.2 108 cm-2s-1
sample space 120 mm in diameter
detector
neutron image plate Gd2O3 doped BaFBr:Eu2+

configuration

. radius

. length

. active area

. angle subtended

vertical cylinder

159.7 mm

400 mm

800 x 400 mm2

+/-144° in 2θ, +/-52° in ν

pixel size 100 x 100, 200 x 200, or 400 x 400 µm2
readout time ~3.5 min
time-of-flight spectrum analyser
chopper & 3He detector
chopper frequency 6000 rpm
path length 2.0 m
resolution Δλ = 0.04 Å

Background

The instrument LADI had demonstrated that good-quality macromolecular diffraction data could be rapidly obtained using Laue diffraction on a quasi-white cold-neutron beam with a large-angle cylindrical detector based on neutron image plates . The same technique has been shown in trials on a thermal-neutron beam to be very well suited to fast data collection from the smaller unit cells of interest to physicists, chemists and materials scientists. These trials inspired the design and construction of a new image-plate Laue diffractometer specially adapted to bulky sample environments as part of the ILL Millennium Programme. (Click here for the full scientific case for VIVALDI.)

VIVALDI received its first neutron beam on Friday, November 23, 2001. A few hours later a strong diffraction pattern was observed from a crystal of La1.7Co2, qualitatively similar to those observed on LADI in earlier tests on H22. The first scheduled experiments were performed just two weeks into the first cycle of 2002. Since then VIVALDI has gone from strength to strength...

VIVALDI (Very-Intense, Vertical-Axis Laue DIffractometer) provides a tool for development of new diffraction experiments, and is complementary to other ILL single-crystal diffractometers. Fields of interest for experiments on VIVALDI include magnetism, charge density waves, high-pressure studies and structural phase transitions. VIVALDI allows rapid preliminary investigation of new materials, even when only small single crystals are available. The detector is also suitable for some types of diffuse scattering experiments on a monochromatic beam.

The Instrument

The VIVALDI image-plate detector was constructed by SICN Veurey, in consultation with the EMBL Grenoble Outstation. The key differences compared to the LADI detector are that the detector cylinder axis is vertical to accept an 'Orange' cryostat, and the image plates are mounted and read on the inside of the cylinder to improve the detection efficiency. As on LADI the neutron-sensitive plates are based on the same storage-phosphor (BaFBr doped with Eu2+ ions) which is used for X-ray image plates, but with Gd2O3 added. The Gd nuclei act as neutron scintillators by creating a cascade of rays and conversion electrons.

 
VIVALDI is usually located at the end of the thermal guide H22. The full thermal spectrum can be accepted without detrimental overlap of reflections for primitive unit cells up to 25 Å on edge. A filter based on multilayer supermirrors can be placed upstream to deflect wavelengths longer than 3 Å which would otherwise contribute primarily to the background. A time-of-flight spectrum analyzer lies downstream to monitor coarse changes in the wavelength spectrum due to other instruments on the H22 guide. The entire instrument pivots on air cushions around the filter to allow selection of just wavelengths shorter than 3 Å, just wavelengths longer than 3 Å, or the unfiltered beam. Beam heights between 1100 mm and 1600 mm are acceptable to allow the detector to be used on other guides or monochromatic beams.