ALS-U is an ongoing upgrade of the Advanced Light Source (ALS) at Berkeley Lab that will endow the ALS with revolutionary x-ray capabilities. The new storage ring will enable the production of highly focused beams of soft x-ray light that are at least 100 times brighter than those of the existing ALS. Applying this technology at the ALS will help us better understand and develop new materials and chemical systems needed to advance our research needs in energy science, environmental systems research, and biosciences in the 21st century. This will create a world-leading platform for next-generation soft x-ray and tender x-ray instrumentation.

Four beamlines with new and upgraded capabilities will become available after the upgrade. The FLEXON beamline (FLuctuation and EXcitation of Orders in the Nanoscale), a high-brightness coherent soft x-ray beamline, will provide x-ray photon correlation spectroscopy and diffraction imaging techniques to study electronic, chemical, and magnetic fluctuations in quantum materials with nanosecond temporal and nanometer spatial resolution. A new ALS-U developed tender x-ray beamline is designed to address challenges at the frontiers of diverse scientific areas, ranging from soft condensed matter and biomaterials to energy science and Earth and environmental sciences. It will offer state-of-the-art nanometer-resolved chemical imaging and resonant scattering nanoprobes, enabling operando and in situ studies of materials of K-edges of elements (Na through Ca) and the M and L edges of lanthanides and actinides. These two beamlines will be complemented by two upgraded beamlines for nanometer 3D chemical tomography based on ptychography of light elements and first-row transition metals and for high-resolution ARPES.

The high coherent flux of the upgraded ALS will drastically improve the speed, sensitivity, and spatial resolution of nanoprobes, enabling the speciation and forensic study of nanoscale constituents and contaminants via x-ray absorption spectroscopy and fluorescence detection. Chemical signatures can be correlated with morphology and compared with chemical standards. Operando experiments, for example, of liquid phase systems and studies under ambient conditions, will be enabled by specially designed sample holders and liquid cells using the high penetration of tender x-rays.

Spectromicroscopy methods, combining the chemical insight of spectroscopy with microscopy imaging, can give a unique and informative view of a sample of interest. Scanning Transmission X-ray Microscope (STXM) spectromicroscopy is one such method, utilizing synchrotron radiation to probe electronic structure with a spatial resolution in the tens of nanometers. Specialized methods like ptychography can push the spatial resolution even lower into the single nanometer range.

Discussed here, STXM spectromicroscopy is applied to two actinide samples: spent nuclear fuel, which offers a unique insight into the complex environment of nuclear fuel undergoing burnup, and uranium oxide allowed to age in a humid environment, which serves as a demonstration of the power of this technique for nuclear forensics investigations. These samples are created by focused ion beam (FIB) sectioning to generate cross-sections of ideal thickness for soft x-ray measurements (100-200 nm). The FIB method of sample preparation also allows for the measurement of highly radioactive spent fuel without containment due to the extremely small amount of material present.

Data analysis is a key component to the understanding of spectromicroscopy results for varied samples like these. Non-negative matrix factorization (NMF) is employed to identify key components and recent efforts to improve NMF to work with noisy individual STXM spectra are outlined here. Reproducibility of the analysis is a concern (as with similar methods like multivariate curve regression) and methods to enhance both reproducibility and interpretability of the results are discussed.

Combining STXM spectromicroscopy, FIB sectioning, and NMF analysis has allowed for unique insights into actinide materials. Potential future developments utilizing this method for other samples and with advanced techniques like ptychography are also discussed.