Joel C. Weber

We present a new instrument for soft x-ray absorption and emission spectroscopy based on the latest generation of transition edge sensor array optimized for soft x-ray detection, built on microwave superconducting quantum interference device multiplexing readout, and cooled with a dilution refrigerator. Its extreme collecting efficiency enables spectroscopy measurements on dilute systems. It also enables shorter acquisition time and large energy-range spectra. We describe the design and operation of the spectrometer and characterize its performance in terms of energy resolution, photon collecting efficiency, and stability. A resolution of 0.7–1.8 eV in the energy range 260–900 eV with a high detection rate of up to 10 000 photons per second across the array is achieved with minimal performance degradation. In addition, the use of the dilution refrigerator to cool the detector results in a robust and stable energy calibration over time. The spectrometer is attached to a dedicated ultra-high vacuum sample chamber equipped with a fully motorized sample cryostat for experiments at temperatures between 10 and 300 K, multi-sample mounting, and electric field-gated devices. The spectrometer-sample chamber is installed at a beamline providing full polarization control. We demonstrate the capabilities of the setup using two representative examples of XES on extremely low-concentration systems, namely monolayer hexagonal boron nitride and the K3[Fe(CN)6] molecular system at sub-millimolar concentration.

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.

The development of large-scale, high-resolution transition-edge sensor (TES) arrays requires precise, wafer-scale control of the superconducting critical temperature (T_c). For microcalorimeters, this is often achieved with a superconductor-normal metal bilayer where the thickness ratio between the two materials determines T_c through the proximity effect. Reliably achieving a specific T_c across multiple wafers with good uniformity remains challenging as the fabrication process is highly sensitive to substrate quality, deposition parameters, and deposited thin-film thickness. Missing a T_c target results in an array with degraded or unusable performance and wastes fabrication/measurement resources. To help solve these limitations with TES microcalorimeter fabrication, we have developed a method of measuring 76.2 mm wafers after bilayer deposition to screen for T_c accuracy and uniformity. Four-wire resistance measurements of test structures across the wafer provide rapid feedback early in the fabrication process at temperatures down to 20 mK. We leverage the hybrid additive-subtractive TES process to screen TES arrays with multiple T_c values on a single wafer comprised of a single bilayer. After measurement, the bilayer normal metal thickness can be increased with an additional evaporation step to lower T_c or decreased with an ion mill step to raise T_c as needed. We also present a design to expand measurement capability to 150 mm wafers to screen both larger microcalorimeter arrays as well as NIST cosmic microwave background TES bolometer arrays.

We discuss the development of molybdenum-gold bilayer transition edge sensors on solid silicon substrate microcalorimeter designs suitable for real-time measurement of gamma-ray sources in the keV to MeV range. These novel detector designs incorporate tunable thermal conductance over a range from 1x10^-9 W/K to 4x10^-7 W/K and are mechanically robust. We describe the micro-fabrication process and the gamma-ray absorber attachment in detail. Initial prototypes obtain detector response times twice as fast as our previous design while maintaining an energy resolution of less than 100 eV full-width-half-maximum at 100 keV.

PAX (antiProtonic Atom X-ray spectroscopy) is an experiment that aims to test strong-field quantum elec trodynamics (QED) effects by performing high-precision X ray spectroscopy of antiprotonic atoms. PAX will use a low energy antiproton beam provided by the Extra Low Energy Antiproton (ELENA) ring at the European Organization for Nuclear Research (CERN) to create antiprotonic atoms. A superconducting transition-edge sensor (TES) spectrometer will be used to measure the energy of transitions between circular Rydberg states in these atoms. The energy range of interest for the experiment spans 50 keV to 250 keV, and the desired precision for measuring the centroids of the emission lines is 10−5. The spectrometer for PAX is intended to have four 96-pixel TES arrays and will be read out with a microwave superconducting quantum interference device (SQUID) multiplexer. As a step towards building the full instrument, we built a scaled-down version of the spectrometer that was installed at the TEst Line for Machine And Antimatter eXperiments (TELMAX) facility at ELENA in April 2025. The purpose of this deployment was to make an observation of X-ray emission by antiprotonic atoms and to better understand the effect of the pionic charged particle background due to antiproton annihilation on the per formance of the TES array. This pilot spectrometer had an array of 60 TES pixels in a compact adiabatic demagnetization refrigerator (ADR) cryostat. The sensors were read out with a microwave SQUID multiplexer. Each pixel consisted of a molybdenum/gold bilayer TES with a co-planar gold “landing pad” for a bulk tin absorber that was attached by an epoxy joint. We discuss the design of the TES pixels, the microwave SQUID readout, and the cryogenic platform. Finally, we present calibration data obtained at TELMAX using radioactive sources to assess the performance of the spectrometer in the antiproton beam-off condition.

It is often desirable to steer light within a cryostat and have a well focussed laser spot incident upon a detector. We demonstrate the operation of superconducting transitionedge sensor microcalorimeters with a fibre-coupled laser scanning device co-located on the 20mK mixing plate of a dilution refrigerator. The scanner directs a 515nm laser beam with spot size $< 40\mu$m over a range of $\pm$ 14mm in two orthogonal axes (covering 784mm${}^{2}$). We present the optical elements of the system, show that the detector noise spectrum is unchanged when the scanner is operating or off, and observe less than 340 nW power dissipation in our use-case. This device will allow detector calibration up to 10 keV on a pixel-by-pixel basis with simultaneous X-ray measurements. Further, this device will enable more effective characterisation of certain detector artifacts including cross-talk, position dependences, and drift corrections.

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.

In recent years, the implementation of thin-film Ta has led to improved coherence times in superconducting circuits. Efforts to further optimize this materials set have become a focus of the subfield of materials for superconducting quantum computing. It has been previously hypothesized that grain size could be correlated with device performance. In this work, we perform a comparative grain size experiment with α-Ta on c axis sapphire. Our evaluation methods include both room-temperature chemical and structural characterization and cryogenic microwave measurements, and we report no statistical difference in device performance between smaller- and larger-grain-size devices with grain sizes of 924 and 1700 nm2, respectively. These findings suggest that grain size is not correlated with loss in the parameter regime of interest for Ta grown on c axis sapphire, narrowing the parameter space for optimization of this materials set.

Arrays of superconducting transition-edge sensor (TES) microcalorimeters are attractive for some X-ray experiments where the combination of higher collection efficiency than crystal spectrometers and much better energy resolution than semiconductor detectors is enabling. For hard X-ray (>2 keV) applications, a TES is often integrated with an extra absorber to extend its dynamic range, quantum efficiency and collecting area. For >100 keV applications, millimeter size absorbers have been manually attached to TES pixels after fabrication. At lower energies, the absorber size needs to be much smaller, because a large heat capacity degrades energy resolution. In that case, the absorber is directly electroplated or evaporated on the TES during the fabrication process. In this work, we present TES microcalorimeters optimized for the intermediate hard X-ray energy region (20 keV ∼ 100 keV) that have thick absorbers directly deposited during the TES fabrication process. This microcalorimeter is designed to measure high-resolution muonic atom transition X-ray spectra as a method to study quantum electrodynamics (QED) effects. The absorber is comprised of evaporated gold and electroplated bismuth layers and is designed to optimize dynamic range, collecting efficiency, and energy resolution in the 20 keV to 45 keV energy range. A prototype with pure gold absorbers has been fabricated and characterized, in order to obtain a first experimental reference in the detector development process.

Transition-edge sensors (TESs) are thermal detectors in which a superconducting film that is electrically biased in the superconducting-to-normal transition is used as a thermometer. In most TESs, the film is a superconductor-normal metal bilayer where the two materials and their thicknesses are chosen to achieve various specifications including the transition temperature T _c . Traditionally, the materials in the bilayer are deposited in sequence without breaking vacuum in order to achieve a clean, uniform bilayer interface at the wafer-scale. This approach leads to constraints in material properties, fabrication techniques and, ultimately, TES designs. To overcome these constraints, we have developed a bilayer fabrication process that allows the layers to be deposited and patterned separately with an exposure to atmosphere between the deposition steps. We demonstrate better than 6% transition-temperature uniformity across a 7.6 cm (3 in) substrate and present satisfactory spectra from TES x-ray detectors fabricated in this fashion. We show how the new hybrid additive-subtractive TES fabrication process creates new design possibilities, including broad tuning of T _c across a substrate with a single bilayer thickness.