Davide Vaccaro

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.

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.

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.

The X-ray Integral Field Unit (X-IFU) is an instrument of European Space Agency’s future NewAthena space observatory, with the goal to provide high-energy resolution (<4 eV at X-ray energies up to 7 keV) and high-spatial resolution (9 in.) spectroscopic imaging over the X-ray energy range from 200 eV to 12 keV, by means of an array of ∼1500 transition-edge sensors (TESs) read out via superconducting quantum interference device time-division multiplexing (TDM). A TDM-based laboratory test bed has been assembled at Netherlands Institute for Space Research, hosting an array of 75×75 μm2 TESs that are read out via 2-column × 32-row TDM. A system component that is critical to high-performance operation is the wiring harness that connects the room-temperature electronics to the cryogenic readout componentry. We report here on our characterization of such a test bed, whose harness has a length close to what was envisioned for X-IFU, which allowed us to achieve a co-added energy resolution at a level of 2.7-eV full width half maximum at 6 keV via 32-row readout. In addition, we provide an outlook on the integration of TDM readout into the X-IFU focal plane assembly development model.

SRON (Netherlands Institute for Space Research) is developing the Focal Plane Assembly (FPA) for Athena X-IFU, whose Demonstration Model (DM) will use for the first time a time domain multiplexing (TDM)-based readout system for the on-board transition-edge sensors (TES). We report on the characterization activities on a TDM setup provided by NASA Goddard Space Flight Center (GSFC) and National Institute for Standards and Technology (NIST) and tested in SRON cryogenic test facilities. The goal of these activities is to study the impact of the longer harness, closer to X-IFU specs, in a different EMI environment and switching from a single-ended to a differential readout scheme. In this contribution we describe the advancement in the debugging of the system in the SRON cryostat, which led to the demonstration of the nominal spectral performance of 2.8 eV at 5.9~keV with 16-row multiplexing, as well as an outlook for the future endeavours for the TDM readout integration on X-IFU's FPA-DM at SRON.

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.