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The Athena mission and its X-ray Integral Field Unit (X-IFU) instrument will be positioned at the Sun-Earth Lagrange point L1, where it will be subject to particles from the solar wind with energy below 0.1 MeV, and from galactic cosmic rays and solar flares with energies up to hundreds of MeV for protons and GeVs for heavier ions. Some of these particles will go through the satellite and hit the focal plane assembly and hence the detectors. These detectors will be TES (Transition-edge sensor) microcalorimeters, flown for the first time in such an environment. In order to ensure the performance of this type of detector throughout the duration of such a mission, it is critical to study the impact of the radiation on their behavior. Indeed, although a lot of reference material exist for semiconductor-based photodetectors such as CCDs and CISs, little is currently known about the impact of radiation on TES detectors. These energetic events could cause local heating or damage to the detectors and affect their performance. In this work, we describe how we designed a test campaign to assess the impact of L1 radiation on TES detectors for Athena/X-IFU-like missions and present the results of the tests, for a maximum dose of 4.3 krad(Si)). Analysis includes assessing changes in the pulse shapes and energy resolution of the detectors measured at 55 mK after several radiation dose steps performed at 4 K.

At NASA Goddard Space Flight Center, we have previously demonstrated a kilo-pixel array of transition-edge sensor (TES) microcalorimeters capable of meeting the energy resolution requirements of the future X-ray Integral Field Unit (X-IFU) instrument that is being developed for the Advanced Telescope for High ENergy Astrophysics (ATHENA) observatory satellite. The TES design in this array was a square device with side length of 50 μm. Here, we describe studies of TES designs with small variations of the dimensions, exploring lengths, parallel to the current direction, ranging from 75 μm to 50 μm and widths, perpendicular to the current direction, ranging from 50 μm to 15 μm. We describe how these changes impact transition properties, thermal conductance and magnetic field sensitivity. In particular, we show that using a TES with a length of 50 μm and width of 30 μm may be a promising route to reduce the maximum time-derivative of the TES current in an X-ray pulse and reduce the sensitivity of the TES to magnetic field.

It is now possible to fabricate stacks of buried wires with extremely high density, perfect yield, and many layers using fine-line lithography and chemical mechanical polish. Such wiring in principle will enable large format arrays of fine pitch sensors. Buried wiring has the distinct advantage that additional metallization – such as metallic links in a thermal multiplexer (“hydra”) design and attachment points between absorbers and the substrate – are not limited in their placement. In close-packed arrays, thermal crosstalk occurs when a heat pulse in one device transmits through the dielectric connection and is absorbed in a neighbor. We seek to integrate Mo/Au transition edge sensors with such wiring but encounter limitations of fabrication options by the restriction of materials permitted to be used, including noble metals and certain dielectrics. In this paper we show measurement results of X-ray microcalorimeters with multi-stack buried wiring on a silicon nitride dielectric layer exploring new methods of integrating heat sinking. We attempt to mitigate thermal crosstalk channels by enhancing coupling of the emitted heat to a thick metallic heatsinking layer. We measure the thermal response in close packed hydra arrays in a variety of heatsinking configurations and show energy resolution and pulse response in these devices.

We have developed fine-pitch, multilayer, superconducting wiring for routing around a ninety-degree corner terminated with wirebonding interfaces. The component-level testbed for the Advanced Telescope for High Energy Astrophysics (ATHENA) X-Ray Integral Field Unit (X-IFU) focal plane requires compact, high-density, low-crosstalk wiring fanout to connect the detectors in the focal plane array with NIST-fabricated SQUID time domain multiplexing (TDM) readout chips. The full assembly baselines two interface chips: a flexible interface chip bending around the corner and a planar silicon carrier chip. The TDM readout is indium bump-bonded to the silicon carrier and afterward the flexible chip is clipped in place and wirebonded to the detector and fanout wiring on the carrier. This assembly is repeated for each side of the hexagonal focal plane structure. As conventional commercial cables are not able to achieve the fine-pitch, low-crosstalk, superconducting wiring required, we fabricate these flexible interface chips in-house via lithographic patterning and etching of sputter deposited thin films to create broadside-coupled superconducting niobium microstrips. Within the chip, the wiring on the flexible polyimide region transitions to silicon substrate for the closely spaced wirebond pads. The Nb microstrip wiring climbing a thick polyimide sidewall presents a fabrication challenge which we shall discuss in this paper. We describe the function of these components to build an effective engineering testbed for the ATHENA X-IFU.

We are developing superconducting transition-edge sensor (TES) microcalorimeter arrays for a variety of applications such as ground-based laboratory astrophysics experiments and next generation space-based X-ray missions. These detectors can provide X-ray spectral information with an unprecedent resolution of ∼2 eV at 6 keV and have for instance been selected for the X-ray Integral Field Unit (X-IFU) instrument of ESA's large flagship mission Athena. To maintain detector performance over the lifetime of the mission, it is important to understand whether environmental conditions that the detector may be exposed to will affect its properties over time. This “aging” begins right after the array leaves the fabrication environment, with potential exposure to humidity, oxygen, or elevated temperatures which may affect the detector performance. In a few prior arrays we have observed increased fall times in the pulse shape and/or the introduction of anomalous low energy tails on the X-ray spectrum. This is thought to be an indication of “aging” on chips exposed to such conditions, causing e.g., changes in the absorber properties. In this contribution, we report on a systematic characterization of TES properties, before and after exposing the chip to various controlled temperature and humidity levels and assess the changes in the measured transition and pulse shapes, energy resolution, and spectral redistribution.

We have characterized a full-scale transition-edge sensor (TES) microcalorimeter array as a milestone towards the demonstration of technology readiness level (TRL) 5 for the detector array for the X-ray Integral Field Unit (X-IFU) instrument on ESA's future flagship X-Ray Observatory called Athena. We fabricated a 90 mm full-scale prototype TES array and measured the properties and the performance in a newly developed platform in which up to 960 out of ∼3,200 pixels can be read out. In this paper, we report on measurements of the uniformity of the transition temperature and shape, the uniformity of the spectral energy resolutions at 7 keV and 10 keV, the thermal crosstalk for the first, the diagonal, and the second nearest neighbors, and the energy sensitivities to environment perturbations for the magnetic field, the TES bias voltage, and the heat bath temperature.

The X-ray Integral Field Unit on Athena will be subject to a cosmic-ray induced thermal background on orbit, with energy depositions into the detector wafer leading to thermal bath fluctuations. Such fluctuations have the potential to degrade energy resolution performance of the transition-edge sensor based microcalorimeter. This problem was previously studied in simulations that modeled thermal bath fluctuations induced by cosmic-ray events and evaluated the resulting energy resolution degradation due to a simulated timeline of such events. Now taking an experimental approach, we present results using a collimated Am-241 alpha particle source to deposit a known energy to specific locations on the detector wafer. Thermal pulses induced by the alpha particle energy depositions are measured at various detector pixels for several different experimental configurations, including for energy deposited into the inter-pixel structure of the wafer, as well as the frame area outside the pixel array. Further, we also test both with and without a thick backside heatsinking metallization layer that is baselined for the instrument. In each case results are compared to expectations based on the thermal model developed for the previous study.

We report measurements of the intensity ratio of the 1s²2s22p1/253d3/2→1s²2s22p6 resonance line to the 1s²2s22p1/253d3/2→1s²2s22p6 intercombination line in neonlike Kr26+ and Mo32+. The measurements were performed at the EBIT-I electron beam ion trap facility at the Lawrence Livermore National Laboratory and utilized an x-ray microcalorimeter. The measured ratio for Mo32+ is in four times closer agreement with theoretical predictions than earlier measurements of ions with lower atomic number. Our measurement thus suggests a narrowing of the disagreement with atomic number, which had not been observed in the previously existing data. This implies that the disagreement with theory may be localized to ions within a range of atomic numbers in which intermediate coupling dominates. [ABSTRACT FROM AUTHOR]