D20 is a medium to high resolution two-axis diffractometer capable of producing a neutron flux of 108 s−1 cm−2 at the sample position. The 1536 detection cells of its curved linear position sensitive detector (PSD) cover a continuous 2θ range of 153.6° over a total solid angle of 0.27 sr. This combination of a high incident neutron flux and a large detector solid angle provides D20 with the fastest counting rate, at a given resolution, of any reactor-based neutron diffractometer. Different monochromators and take-off angles, plus optional Soller collimators and secondary slits, permit a wide choice in the Q-space range, wavelength, resolution and flux. A high-resolution configuration offers Δd/d ~ 2 × 10−3. Fast modern counting electronics allow in situ time-resolved experiments at the timescale of a few tens of milliseconds. In addition, a variety of sample environments, including an optional radial oscillating collimator for suppressing parasitic scattering, contribute to a rich scientific programme.

https://iopscience.iop.org/article/10.1088/0957-0233/19/3/034001/pdf

The D20 instrument at the ILL: a versatile high-intensity two-axis neutron diffractometer

Neutron diffraction runs and gas-consumption experiments based on pressure−volume−temperature measurements are conducted to study the kinetics of methane hydrate formation from hydrogenated and deuterated ice powder samples in the temperature range of 245−270 K up to high degrees of transformation. An improved theory of the hydrate growth in a polydisperse ensemble of randomly packed ice spheres is developed to provide a quantitative interpretation of the data in terms of kinetic model parameters. This paper continues the research line of our earlier study which was limited to the monodisperse case and shorter reaction times (Staykova et al., 2003).1 As before, we distinguish the process of initial hydrate film spreading over the ice particle surface (stage I) and the subsequent hydrate shell growth (stage II) which includes two steps, i.e., an interfacial clathration reaction and the gas and water transport (diffusion) through the hydrate layer surrounding the shrinking ice cores. Although kinetics of hy...

Formation of methane hydrate from polydisperse ice powders.

Formation of methane hydrate from polydisperse ice powders

The magnetic ordering and thermal lattice expansion of nanocrystaline chromium (n-Cr) are studied by neutron powder diffraction. Nanocrystalline Cr with a volume-averaged crystallite size of 27 nm has been obtained by electrodeposition. Between room temperature and 240 K there is a simple antiferromagnetic ordering. At 240 K the magnetic ordering changes and a part of the material adopts a transverse spin-density-wave (TSDW) ordering. The spin-flip transition from the TSDW to the longitudinal LSDW ordering is supressed and about 40% of the volume has the TSDW ordering stabilized down to 2 K. The thermal lattice expansion of the material becomes negative around the magnetic phase transition near 240 K. The values of the magnetic moments of the simple antiferromagnetic and TSDW orderings observed at 2 K are in agreement with the magnetic moments observed in single crystals and polycrystalline Cr.

Magnetic ordering in electrodeposited nanocrystalline chromium particles

High-intensity neutron powder diffraction offers opportunities in timeand parameterresolved diffraction, in investigation of small samples, or the precise intensity measurement to determine structure factors of disordered samples or weak peak intensities by differential diffraction. One-shot experiments show the structural evolution of solids in situ during a chemical reaction. Faster, cyclic phenomena can be observed with a stroboscopic data acquisition mode. At ILL, D20 provides the highest flux in constant-wavelength neutron powder diffraction, a stationary, curved linear position sensitive detector (PSD) and a large choice in Q-space, resolution, wavelength and flux. A new diffractometer for rapid acquisition could provide an enormous gain in detected intensity by covering a larger solid angle with a two-dimensional PSD. GEM at ISIS sets the standards in time-of-flight (TOF) instrumentation. At a future European Spallation Source (ESS) a single-pulse diffractometer could allow for acquisitions down to the length of one pulse (20 ms) and below.

Future Trends in High Intensity Neutron Powder Diffraction

The High Intensity Two Axis Neutron Diffractometer D20 First Results

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The High Intensity Two Axis Neutron Diffractometer D20 First Results