Tejas Guruswamy

Soft X-ray resonant scattering is a sensitive probe of electronic, magnetic, and orbital order, but its weak signals are often embedded in substantial elastic, inelastic, and fluorescence backgrounds. In order to enable new science at the Advanced Photon Source (APS) X-ray synchrotron, we have developed a transition edge sensor array-based spectrometer instrument optimized for energy-resolved detection in the 0.2 to 2keV band, sustained operation at elevated photon count rates, and a mechanical design that allows for a small sensor–sample distance without interfering with existing hardware. The newly fabricated 256-pixel sensor array consists of sidecar Mo/Au transition edge sensors with Bi/Au absorbers, packaged onto a microsnout platform with frequency multiplexed $\mu$Mux readout. We report on laboratory characterization of this array prior to final instrument integration and beamline deployment. DC current–voltage measurements establish acceptable uniformity and yield but identify a higher than intended bilayer critical temperature. Readout characterization shows the majority of $\mu$Mux channels operating in a regime compatible with TESlimited energy resolution. Soft Xray spectral measurements using a lab X-ray source demonstrate a fullwidthathalfmaximum energy resolution median across the array of 2.3eV at Al K$\alpha$ (1.5keV), with almost 80% of all pixels producing spectra with resolution below 3eV. The larger than targeted energy resolution is primarily attributed to a reduced resistance sensitivity to temperature ($\alpha$) relative to the prototype designs. Overall the results indicate the spectrometer meets the minimum requirements for resonant soft X-ray scattering and related measurements at the APS. Immediate next steps focus on commissioning of the instrument at the beamline, validation of high countrate operation, and establishment of energy calibration and data analysis workflows tailored to soft X-ray science.

In this study, we conducted simulations and experimental characterization of thermal crosstalk in a hard X-ray transition-edge sensor (TES) array. TESs are low-temperature superconducting detectors with excellent energy resolution, making them ideal for synchrotron applications. To achieve the required count rate and collecting area, TES detectors are typically organized into multi-pixel arrays. However, tight pixel spacing and multiplexing can introduce thermal and electrical crosstalk that degrades spectral resolution at high photon rates. We characterized the crosstalk phenomena between X-ray TESs in a 24-pixel array. This TES array, fabricated at the Argonne National Laboratory (ANL), is designed to operate in the tens of keV regime. In prior work, we experimentally developed procedures and algorithms to identify and quantify crosstalk in various setups. To further explain the phenomenon, we created a multibody model to simulate the thermal crosstalk response between neighboring TES pixels. Simulations correlate the observed thermal crosstalk with key device properties: material composition, TES transition shape parameters ($\alpha$ and $\beta$), and intrinsic characteristics such as heat capacity and thermal conductance.

A combination of gold and bismuth is commonly used as absorber for X-ray transition-edge sensors, due to high atomic numbers and, consequently large cross sections, and to their complementary thermal properties. Since Au conductivity dominates, especially at cryogenic temperatures, electrodeposited bismuth films have been grown over sputtered gold, so that bismuth forms a bridge on the gold fingers and, therefore, the conductivity contribution from the underlying Au layer can be removed. Resistance as a function of the temperature has been investigated for pristine and annealed samples, both with and without a dc magnetic field. A general behavior has been observed, thus evidencing three different zones. At high temperatures a metal regime, described in terms of a Fermi liquid model, is identified. In an intermediate temperature region, a hysteretic and irreversible insulating-type resistance is observed. While, at low temperatures a resistance plateau appears. To better understand the electrical conduction properties, noise measurements have been also performed. Two possible models have been considered to interpret experimental data in the intermediate temperature region: Fluctuation-Induced Tunneling and Quantum Interference Effects. The analysis here reported gives indication on the granular structure of the investigated films, which could be represented as random networks of tunneling junctions, where the nodes are the conducting pathways connecting different junctions. In these regions, temperature-induced intergranular processes could be effectively detected giving additional information on the prediction of the ultimate energy resolving capabilities of detectors with bismuth absorbers.

Modern X-ray detector systems urgently require compact, efficient, and fast data compression schemes to handle the transmission of big data from pixel arrays, enabling frame rates in the MHz regime. In this work, a data compression ASIC that implements a streaming fixed-length lossy compression scheme is introduced and analyzed, proving the feasibility and benefits of on-chip compression. The compression scheme utilizes a vector matrix product logic, which performs a number of floating-point multiplications, additions, and accumulations. The logic is verified, synthesized, and shown to fit in the area resource available for the X-ray detector under study, which comprises 192 × 168 pixels each of 12-bit width, and having a total area of 20 mm× 20 mm, about 2 mm× 20 mm of which are available for the digital logic. Several system architectures, precisions, and compression ratios ranging from 100 to 250 were analyzed to pave the way for on-chip fixed-length compression (e.g., principal component analysis, singular value decomposition) and data reduction (e.g., azimuthal integration) for X-ray and electron detectors.

The measurement of shifts in the energy of X-ray emission lines is important for understanding the electronic structure and physical properties of materials. In this study, we demonstrate a method using a synchrotron source to introduce controlled eV-scale shifts of a narrow line in between fixed-energy fluorescence lines. We use this to characterize the ability of a hard X-ray superconducting Transition Edge Sensor (TES) array to measure line shifts. Fixed fluorescence lines excited by higher harmonics of the monochromatic X-ray beam are used for online energy calibration, while elastic scattering from the primary harmonic acts as the variable energy emission line under study. We use this method to demonstrate the ability to track shifts in the energy of the elastic scattering line of magnitude smaller than the TES energy resolution, and find we are ultimately limited by our calibration procedure. The method can be applied over a wide X-ray energy range and provides a robust approach for the characterization of the ability of high-resolution detectors to detect X-ray emission line shifts, and the quantitative comparison of energy calibration procedures.

The measurement of shifts in the energy of X-ray emission lines is important for understanding the electronic structure and physical properties of materials. In this study, we demonstrate a method using a synchrotron source to introduce controlled eV-scale shifts of a narrow line in between fixed-energy fluorescence lines. We use this to characterize the ability of a hard X-ray superconducting Transition Edge Sensor (TES) array to measure line shifts. Fixed fluorescence lines excited by higher harmonics of the monochromatic X-ray beam are used for online energy calibration, while elastic scattering from the primary harmonic acts as the variable energy emission line under study. We use this method to demonstrate the ability to track shifts in the energy of the elastic scattering line of magnitude smaller than the TES energy resolution, and find we are ultimately limited by our calibration procedure. The method can be applied over a wide X-ray energy range and provides a robust approach for the characterization of the ability of high-resolution detectors to detect X-ray emission line shifts, and the quantitative comparison of energy calibration procedures.

This project explores the design and development of a transition edge sensor (TES) spectrometer for resonant soft X-ray scattering (RSXS) measurements developed in collaboration between Argonne National Laboratory (ANL) and the National Institute of Standards and Technology (NIST). Soft X-ray scattering is a powerful technique for studying the electronic and magnetic properties of materials on a microscopic level. However, the lack of high-performance soft X-ray spectrometers has limited the potential of this technique. TES spectrometers have the potential to overcome these limitations due to their high energy resolution, high efficiency, and broad energy range. This project aims to optimize the design of a TES spectrometer for RSXS measurements and more generally soft X-ray spectroscopy at the Advanced Photon Source (APS) 29-ID, leading to improved understanding of advanced materials. We will present a detailed description of the instrument design and implementation. The spectrometer consists of a large array of approximately 250 high-speed and high-resolution pixels. The pixels have saturation energies of approximately 1 keV, sub-ms pulse duration and energy resolution of approximately 1 eV. The array is read out using microwave multiplexing chips with MHz bandwidth per channel, enabling efficient data throughput. To facilitate measurement of samples in situ under ultra-high vacuum conditions at the beamline, the spectrometer is integrated with an approximately 1 m long snout.

Our Compton profile measurements of Ti and TiH2 using readily available hard X-ray radiation at 27.5 keV, detected by both a Hitachi Vortex silicon-drift detector and a high-resolution superconducting transition-edge sensor array, are found to be in excellent accord with state-of-the-art density functional theory based calculations. The spherically averaged difference between the Compton profiles of TiH2 and Ti is well described by an inverted parabola, supporting an itinerant behavior of the electron gas screening the protons in the Ti matrix. Our experimental approach, validated by two different detectors, extends the applicability of Compton scattering technique to the readily accessible hard x-ray regime (below 30 keV). Our study suggests possibilities for experiments at low-flux bending magnet synchrotron beamlines and paves the way for the development of tabletop Compton experiments with x-ray tubes.