Luciano Gottardi

Accurate laboratory transition energies for the Ni K α − Fe K β blend are urgently needed both for atomic-structure benchmarking and interpretation of astrophysical x-ray spectra observed with space telescopes. Crucially, inferred abundances of these elements, which are essential for the diagnostics of hot plasmas, depend on the treatment of those overlapping transitions. By combining a transition-edge-sensor x-ray microcalorimeter array and a superconducting electron beam ion trap, we distinguish with line widths between 3.5 eV and 5.5 eV the key transitions from Ni to Ni , including previously unmeasured ones, from the K β lines of Fe to Fe . Collisional-radiative calculations using the Flexible Atomic Code agree well with our results. By calibrating all sensors individually and correcting for their temporal drifts we achieve an average uncertainty below 20 km / s for their transition energies, easing the interpretation of astrophysical spectra. Furthermore, our charge-state-resolved results benchmark theory for complex open-shell ions in the intermediate coupling regime, for which we compare the results from pure configuration interaction and calculations including many-body perturbation theory (MBPT) and find a significant improvement when including MBPT.

Transition-edge sensor (TES) microcalorimeters for future X-ray space observatories require accurate low-energy calibration, a regime where conventional X-ray sources are limited by intrinsic linewidth, count rate, or experimental complexity. In this work, we investigate fiber-coupled optical photons as a compact calibration tool for an Athena-like TES array. Optical pulses at 405 nm were generated with an acousto-optic modulator and delivered through an optical fiber onto a selected TES pixel inside a dilution refrigerator, via a dedicated cryogenic alignment interface. Externally triggered pulses were processed with an optimal-filter pipeline, including baseline subtraction, drift correction, and multi-Gaussian reconstruction of the pulse-height spectrum. The reconstructed spectra show resolved photon-number peaks consistent with the Poissonian statistics expected from an attenuated coherent source, reaching equivalent energies above 400 eV with a mean photon number exceeding 100 per pulse. The detector response was further tested for linearity by comparing the measured photon-number distributions against those predicted from the independently calibrated number of photons sent to the detector and a single, globally estimated detection efficiency, confirming Poissonian, linear behaviour across nearly two orders of magnitude in photon number. These results demonstrate that fiber-delivered optical photons can probe the few-eV to sub-keV response of Athena-like TES microcalorimeters, offering a practical and extensible framework for future low-energy calibration studies.

Transition-edge sensor (TES) microcalorimeters for future X-ray space observatories require accurate low-energy calibration, a regime where conventional X-ray sources are limited by intrinsic linewidth, count rate, or experimental complexity. In this work, we investigate fiber-coupled optical photons as a compact calibration tool for an Athena-like TES array. Optical pulses at 405 nm were generated with an acousto-optic modulator and delivered through an optical fiber onto a selected TES pixel inside a dilution refrigerator, via a dedicated cryogenic alignment interface. Externally triggered pulses were processed with an optimal-filter pipeline, including baseline subtraction, drift correction, and multi-Gaussian reconstruction of the pulse-height spectrum. The reconstructed spectra show resolved photon-number peaks consistent with the Poissonian statistics expected from an attenuated coherent source, reaching equivalent energies above 400 eV with a mean photon number exceeding 100 per pulse. The detector response was further tested for linearity by comparing the measured photon-number distributions against those predicted from the independently calibrated number of photons sent to the detector and a single, globally estimated detection efficiency, confirming Poissonian, linear behaviour across nearly two orders of magnitude in photon number. These results demonstrate that fiber-delivered optical photons can probe the few-eV to sub-keV response of Athena-like TES microcalorimeters, offering a practical and extensible framework for future low-energy calibration studies.

High-resolution x-ray spectroscopy is a key diagnostic tool for the hot plasma core of fusion reactors, since it delivers crucial information on temperature, density and concentrations of heavy element impurities. Originally developed for astrophysical applications, cryogenic x-ray instruments based on superconducting transition-edge sensor (TES) microcalorimeters are non-dispersive spectrometers with high resolving power over a broad energy range of 100 eV to 12 keV. They reach over 90% quantum efficiency and offer extremely low background counts. In this study, we explore the advantages of our TES microcalorimeter for a specific burning plasma scenario of the International Thermonuclear Experimental Reactor (ITER) using a realistic end-to-end simulator developed for future x-ray space instrumentation. We compare the performance of existing diagnostic instruments with that of our TES microcalorimeter, which can simultaneously register spectra from the soft to the hard x-ray range at a fast rate, resolving closely spaced lines from heavy ions such as iron (Fe) and tungsten (W). This provides detailed diagnostics of ionization balance and impurity content for ITER and other Tokamaks as well as stellarators and reversed-field pinches. The TES spectrometer is capable of passively detecting x-ray emissions without interfering with the plasma. It can function from a considerable distance, minimizing neutron hazards, which makes it ideal for future fusion reactor such as DEMO, where diagnostic access is limited.

We developed a wafer-scale 1504-pixel Ti/Au TES X-ray microcalorimeter array at SRON as a backup technology for the NewAthena X-IFU. The improved fabrication process yields of 99% wiring integrity and uniform absorber thickness within $\sim$10%, corresponding to minimal performance variation. These results demonstrate a high level of process maturity and support the feasibility of Ti/Au TES arrays for future space-based X-ray spectrometers.

The new generation of Cosmic Microwave Background (CMB) experiments exploits the properties of low-temperature detectors, particularly superconducting Transition Edge Sensor (TES) bolometers, whose characteristics make them ideal for present and future CMB measurements. Frequency Domain Multiplexing (FDM) configuration is performed by connecting an LC filter in series with each TES. The presence of the LC filters and the SQUID input coil and wiring in series with all the LC channels cause systematic electrical crosstalk.In this work we present the tests and characterization performed on an LC chip produced by SRON (Space Research Organization Netherlands) for the experiment SWIPE. These LC chips are composed of lithographically fabricated inductors and by parallel plate capacitors. The measurements were focused on retrieving the resonance frequencies to be compared to the expected design values, the quality factor and the parasitic resistance. The results were consistent with previous measurements performed on similar setups, showing Q-factors above $\mathbf {10^{4}}$ and parasitic resistance on the order of $\mathbf {10^{-2}\;\Omega }$. The dependence of the properties of the LC filters and the SQUID on the temperature and the bias settings was studied. Measurements to define the level and the major contributors to electrical crosstalk were performed. Both modeling and experimental results indicated an electrical crosstalk level on the order of 10^-4.

We present finite element simulations of the transition-edge sensor (TES) microcalorimeter behavior using COMSOL Multiphysics. The simulated detector has a large absorber with a size of 990×990μm2. The simulation calculates the TES response after x-ray impact for a single pixel with a known, realistic, current-dependent resistive transition R(T,I). The simulation includes the full electrothermal feedback effects and heat conduction through the multiple contact points of the absorber to the TES thermometer and supporting membrane. The presented model, especially the 2D model, has been optimized for high accuracy, making it suitable for simulating the detector response when incident photons hit different positions on the absorber. We study the effects of position dependence using Principal Component Analysis with the aim of extracting correct photon energies from the pulses. The simulations show that the degradation, i.e., the FWHM broadening, due to the pulse variations will be less than 0.7 eV at below 2 keV, while it can be significant (>1 eV) at a higher energy band. The simulation result for position dependence can guide the design of large-absorber detectors for future x-ray missions, such as the Hot Universe Baryon Surveyor.

The X-ray Integral Field Unit (X-IFU) is an imaging spectrometer based on a large array of Transition Edge Sensors (TES) measured using Time Domain Multiplexing (TDM). For the development of a backup detector array, we have designed and realized a cryogenic test setup capable of measuring 9 detectors in a single cooldown under DC bias. We have used this setup to study a small selection of low aspect ratio TES designs, intended to have a low normal resistance suitable for TDM readout. In this work we show how the different designs are affected by magnetic fields. We do this by presenting the impact on the transition shape, detector integrated Noise Equivalent Power (NEP), and sensitivity of the energy scale calibration. We find, in agreement with previous studies, that reducing the width of the TES bilayer greatly improves the detector resilience to magnetic fields, potentially by several orders of magnitude.

We report on the x-ray background rate measured with transition-edge sensors (TES) micro-calorimeters under frequency-domain multiplexing (FDM) readout as a possible technology for future experiments aiming at a direct detection of axion-like particles. Future axion helioscopes will make use of large magnets to convert axions into photons in the keV range and x-ray detectors to observe them. To achieve this, a detector array with high spectral performance and extremely low background is necessary. TES are single-photon, non-dispersive, high-resolution micro-calorimeters and represent a possible candidate for this application. We have been developing x-ray TES micro-calorimeters and an FDM readout technology in the framework of the space-borne x-ray astronomical observatories. We show that the current generation of our detectors is already a promising technology for a possible axion search experiment, having measured an x-ray background rate of 2.2(2) × 10−4 cm−2 s−1 keV−1 with a cryogenic demonstrator not optimized for this specific application. We then make a prospect to further improve the background rate down to the required value (<10−7 cm−2 s−1 keV−1) for an axion-search experiment, identifying no fundamental limits to reach such a level.