SANDALS

SANDALS, the Small Angle Neutron Diffractometer for Liquid and Amorphous Samples, is on the north side of ISIS Target Station 1 and views the liquid methane moderator, making use of neutrons with wavelengths ranging from 0.05 – 4.95 Å.

Three sets of B4C jaws are used to define the beam geometry, which most commonly has a 30 x 30 mm² square cross-section. The ‘small angle’ part of the SANDALS name reflects the forward scattering arrangement of its 660 detectors, which cover 2θ = 3 – 38°. This angular range comprises of 18 banks of 20 × 1 × 2 cm (height × width × depth) ZnS scintillator detectors providing a 0.3sr solid angle coverage with 85% efficiency for 1 Å neutrons and 0.1% stability over the timescales of a typical experiment. This arrangement of detectors means SANDALS is optimised for looking at samples containing light elements such as hydrogen or lithium as the contribution to the data by inelastic neutron scattering is minimised [1]. To help minimise backgrounds, the beam collimation and sample space are under vacuum during data collection to prevent air scattering.

The delivered wavelength bandpass combined with the detector angle coverage result in a practical operating Q-range for SANDALS of 0.1 Å^-1 ≤ Q ≤ 50 Å^-1. This delivers sub-Angstrom distance resolution (~0.1 Å) for pair distribution studies of liquids and disordered materials out to a maximum length scale of ~30 Å.

Owing to the forward scattering arrangement of detectors with a maximum 2θ of 38°, resolution of Bragg features in the measured diffraction patterns is limited in comparison to a traditional crystallography instrument. SANDALS can achieve a resolution of 2% ΔQ/Q across most of its operating Q-range. Instrument backgrounds have been minimised to very low levels, which, together with the detector stability, mean SANDALS can reliably perform isotopic difference measurements at the few % difference level. Typical measurement times are now between 3 and 4 hours for a hydrogen-containing sample and shorter for a deuterium or non-hydrogen containing sample.

[1] A. K. Soper, Inelasticity corrections for time-of-flight and fixed wavelength neutron diffraction experiments, Mol. Phys., 2009, 107, 1667-1684.

Isotopic substitution plays a major role in most experiments on the SANDALS instrument since the majority of samples contain hydrogen, and which can usually be substituted for its heavy analogue, deuterium.

Isotopic Substitution in Disordered Materials Experiments (pdf) – This is a short guide on the power of isotopic substitution in the context of total scattering experiments on the Disordered Materials instruments, as well as how to choose which isotopic systems to run to get the best out of your experiment.

Typical sample environments (non-exhaustive list):

• Flat plate sample changer, 15 samples, -15 to +90°C
• Cylindrical sample changer, 20 samples
• Oil bath superstick, -40 to +240°C
• CCR, 10 to 300 K
• Furnaces, up to 1600 K, or higher with non-V elements
• High-pressure fluids up to 0.5 GPa
• Gas panel

Gudrun is a the main data reduction program used for the processing and correction of total scattering data and is written and maintained by Alan Soper. It comes in two flavours:

• ​​GudrunN is used for correcting neutron diffraction data taken (primarily) on SANDALS, NIMROD and GEM, taking the raw data and generating the experimental structure factor.
• GudrunX is use​d for correcting x-ray diffraction data, for example that taken on the group’s XRD instruments​.

GudPy

GudPy is a replacement user interface ​for Gudrun_GUI written by Jared Swift and offers a modern user interface with integrated plotting and enhanced workflows for data processing.

Installers for GudPy can be found on GitHub​, along with the source code.

Data modelling

​​​Empirical Potential Structure Refinem​​ent (EPSR)

EPSR is a Monte Carlo based modelling program that allows the reconstruction of a 3D model of the system under study, inputting information from the diffraction data in an iterative way. Once a suitable fit to the experimental data has been obtained, the underlying atomic model can then be probed for the structural properties of interest, either within EPSR itself or via external programs such as those in dlputils.

Dissolve

Di​ssolve builds on the success of EPSR and employs similar techniques, but utilises a full classical forcefield and is extensible to multi-configuration, million-atom simulations.

The source code for the project is hosted on GitHub.

​MuSSIC

​The Multiscale Simulation Scattering Intensity Calculator (MuSSIC) is a tool to calculate neutron-weighted structure factors from coarse-grained simulation trajectories. The source code, user guide and documentation ​for the project is hosted on G​itHub.​