HEXRD and HEXRDGUI: a flexible toolkit for X-ray diffraction analysis in dynamic compression experiments

To understand the material world, scientists often look at how X-rays bounce off the atoms inside a substance. When a beam of X-rays hits a crystal, the atoms act like a grating, scattering the light into a pattern of rings or spots that reveals the spacing and arrangement of the atoms. This technique, known as X-ray diffraction, is a standard tool for studying materials under normal conditions. However, when scientists want to see what happens to matter under extreme pressure and temperature—such as the conditions found deep inside planets or during a nuclear explosion—the rules change. In these dynamic compression experiments, materials are crushed by powerful lasers or gas guns in a fraction of a second. The equipment used to capture these fleeting moments is often custom-built and highly irregular, with detectors placed at odd angles, curved surfaces, and multiple X-ray beams firing at once. Standard software, designed for neat, laboratory setups, often cannot make sense of this chaotic data.

A team of researchers has developed a new, flexible toolkit called HEXRD and its companion interface, HEXRDGUI, to solve this problem. This open-source software allows scientists to calibrate their unique, non-standard instruments and analyze the resulting diffraction patterns with high precision. The researchers demonstrated that their tool can handle complex setups involving multiple detectors and X-ray sources simultaneously, correcting for the distortions caused by the physical layers of the target and the collimators used to shape the beam. By testing their software against independent methods and real-world data from major facilities like the National Ignition Facility and the European XFEL, they proved that it can accurately map the geometry of these difficult experiments. This capability is crucial because even tiny errors in measuring the angle of a scattered X-ray can lead to large mistakes in calculating the pressure inside a material. With this new toolkit, researchers can now reliably determine the structure of matter under extreme conditions, from the formation of diamonds from graphite to the behavior of liquid metals at pressures found in the cores of giant planets.

The core challenge in these experiments is that the detectors are rarely arranged in a simple, flat grid. Instead, they might be curved image plates wrapped around a target, or an array of flat panels pointing in different directions. Furthermore, the X-rays often pass through layers of material—such as a protective window or a pusher plate—before and after hitting the sample. These layers shift the apparent position of the diffraction rings, and if not corrected, they distort the final measurement. The new software builds a detailed geometric model of the entire setup, treating each detector panel as an independent object that can be positioned and tilted in three-dimensional space. It also accounts for the specific path the X-rays take through the target assembly, applying corrections that straighten out the distorted rings so they can be measured accurately.

One of the most significant advances of this toolkit is its ability to handle multiple X-ray sources at the same time. In some experiments, two different beams strike the sample from different angles to provide a more complete picture of the material's state. Older software struggled with this, often requiring scientists to calibrate the instrument for one beam and then hope the settings would work for the other, which frequently led to errors. The new system refines the geometry of the entire instrument by looking at the data from all beams simultaneously. In a test using data from a dual-source experiment, the software was able to fit the diffraction rings from both beams to a precision of about 0.18 degrees, which is roughly the size of a single pixel on the detector. When the researchers tried to calibrate for just one beam at a time, the fit for that beam looked good, but the prediction for the other beam was off by a much larger margin, demonstrating the necessity of the joint approach.

The software also includes tools to correct for the physical shape of the detectors themselves. Some detectors, particularly those used in high-energy facilities, suffer from optical distortions where the image appears pinched or stretched near the edges. The toolkit allows scientists to model these distortions mathematically and remove them during the analysis. In a comparison with other established software packages using a standard reference material, the new system, when combined with a distortion model, produced results that were consistent to within a tiny fraction of a degree. Without this distortion correction, the other software packages showed a systematic error that grew larger as the angle of the X-ray increased. This level of precision is vital for experiments where the goal is to measure changes in the atomic spacing of a material as it is compressed to pressures exceeding a million atmospheres.

Beyond just measuring angles, the toolkit helps scientists determine the physical state of the material. Once the diffraction pattern is calibrated, the software can fit the entire pattern to a theoretical model to extract details about the crystal structure, the size of the grains, and the amount of strain within the material. It can even distinguish between solid crystals and amorphous, glass-like states, which is important when studying materials that melt or become disordered under extreme stress. The researchers showed that the software works across a wide variety of facilities, including those using laser-driven shocks and those using powerful particle accelerators. Whether the data comes from a single flat detector or a complex array of curved plates, the same underlying logic applies, allowing scientists to use a single, unified workflow for their analysis.

The development of this toolkit represents a shift toward a more unified approach in the field of high-energy density physics. Previously, researchers at different facilities often relied on custom-written code specific to their own machines, which made it difficult to share methods or compare results directly. By providing a flexible, open-source solution that can be adapted to new instruments with minimal changes, the authors have created a common language for these experiments. The software is freely available and includes a graphical interface that allows researchers without programming skills to visualize the data, adjust the instrument settings, and refine the models interactively. This accessibility ensures that the complex mathematics behind the analysis is accessible to a broader community of experimentalists, enabling them to focus on the physics of the materials they are studying rather than the intricacies of the data processing.

In practice, this means that scientists can now take raw images from a dynamic compression experiment, import them into the toolkit, and quickly see if their instrument is aligned correctly. They can mask out unwanted features, correct for the target layers, and refine the geometry until the simulated rings match the observed data. If the material is unknown or in a new phase, the software can still determine the geometry of the experiment without needing to know the crystal structure in advance. This flexibility is essential for exploring the unknown, where the material might transform into a state that has never been seen before. The ability to accurately measure these transformations opens the door to understanding the fundamental properties of matter under the most extreme conditions imaginable, from the interiors of exoplanets to the behavior of materials in high-energy density environments.

The success of the toolkit was validated through rigorous testing against independent codes and synthetic data. The researchers showed that their geometric model produces results that match other established methods to a high degree of numerical precision, confirming that the underlying math is sound. They also demonstrated that the software can handle the specific challenges of dual-source experiments, where the consistency between different beams is critical. By refining the instrument geometry against all sources at once, they eliminated the errors that arise when trying to fit one beam at a time. This capability is particularly important for facilities like the National Ignition Facility, where multiple X-ray sources are used to probe the target from different angles. The software's ability to integrate these different views into a single, coherent picture ensures that the final measurements of pressure and temperature are as accurate as possible.

Ultimately, the paper describes a tool that bridges the gap between the messy reality of extreme physics experiments and the clean, precise data needed to understand them. It does not claim to have solved every problem in the field, but it provides a robust and flexible foundation for analyzing dynamic compression experiments. By making the software open and adaptable, the authors have invited the community to build upon it, ensuring that as new facilities come online and new experimental techniques emerge, the tools for analyzing the data will be ready. The result is a more reliable way to peer into the heart of matter under pressure, revealing the secrets of how materials behave when pushed to their limits.