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The logo: Bragg vs Diffuse
I created the logo from the real experimental data collected at CORELLI, SNS. It contains two images plotted from the same dataset. The left half uses a linear scale with a gray map, which displays many isolated peak features sitting on the grid of the reciprocal lattice. They are the Bragg reflections, from the intensity of which researchers can determine the average arrangement of the atoms in the crystal. The right one uses a logarithmic scale with a color map (called a "jet" colormap in matplotlib), where many weak features connecting the Bragg peaks show up. These weak features are diffuse scattering. Such structured diffuse scattering patterns reflect the correlated disorders i.e., the short-range correlation, in the crystal. The short-range correlation, also known as the local structure deviated from the average structure, plays critical roles in many functional materials, such as high-temperature superconductors, colossal magnetoresistance (CMR) materials, and ionic conductors. Many molecular crystals and quantum materials also show rich diffuse scattering patterns, usually reflecting the highly degenerated ground state.
The logo reflects the beamline's dual purposes - researchers can get insights into both the average structure and the local structure from the same experiment. CORELLI at SNS has some unique features, both its optics and the high-flux source, to enable researchers to get volumetric diffuse scattering datasets with about a two-order improvement in the data collection rate and to have the energy discrimination capability at the same time.
Time-of-flight Laue Diffactometer
CORELLI, SNS is a time-of-fligth neutron scatttering instrument with a large detector array. CORELLI Detector view (top). A single spot on the detector is a trajectory in reciprocal space (bottom left). Data collected at a single sample angle correspond to a volumetric 3D data in the reciprocal space (using the elasltic assumption) An example of a 2D slice in the reciprocal space is shown here (bottom right).
How does the cross-correlation chopper work?
This cartoon gives a simplified explanation of how the cross-correlation (CC) chopper can help to determine the elastic scattering contribution. The event data acquisition mode enables all the neutron events on the detector to be recorded with a time-stamp. The phase information of the CC chopper, i.e., whether it is open or closed at a specific time during the experiment, is saved.
Assuming a neutron leaves the source at t = 0 with a certain speed, it will arrive at the CC chopper position at a specific time. If the CC chopper is closed at that time, it will absorb the neutron, and the detector will not see the neutron. If the CC chopper is open at that time, the neutron will pass the CC chopper and have a chance to be scattered by the sample. The detector may record the scattering neutron event at t_det.
If the recorded neutron is from an elastic scattering event, it does not change its speed after the scattering. One can calculate when the neutron passed the CC chopper from t_det from the simple relationship between the source-cc chopper distance and the source-detector distance.
If the recorded neutron is from an inelastic scattering event, it will change the speed after scattering from the sample, and thus we cannot tell when it passed the chopper. However, we can still calculate a t_cc using the equation. Now, at t_cc, the cc chopper could either in the open or closed states.
Let us rewind the story. Since the CC chopper's phase information has been recorded, we know the chopper status at t_cc. If the CC chopper was open at t_cc, the scattered neutron could be from either elastic or inelastic scattering. However, if the CC chopper was at a closed state, it must come from an inelastic scattering event. Therefore, we can group the scattering events based on the CC chopper's status at t_cc, and the weighted difference will then give us the elastic scattering signal - a handwaving explanation. It turns out the weighting factor is the ratio between the open and close (i.e., absorbing) areas of the CC chopper.
A tale of Bragg peaks
TOF diffraction measurements usually see long tails from strong Bragg peaks (bottom left), whcih is an artifact due to the emission tails (see the top). Cross-correlation analaysis at CORELLI can remove these artifacts (bottom right).
Role of the CC chopper for epitaxial-film diffraction
Here is a unique application of the cross-correlation application. In epitaxial-film diffraction, the volume ratio between the thin film of interest and the "boring" substrate is about 1: 10, 000 (i.e., 100 nm thin film on 1 mm substrate). Therefore, even the substrate's weak signals can dominate if they overlap with the film's Bragg peak. These weak signals could be the Bragg peak's tails or the phonon scattering. The CC chopper at CORELLI can effectively remove these contributions and reveal the thin-film Bragg peaks, giving rise to more accurate quantitive information on the peak intensity.