J. E. Sadleir

Close-packed arrays of transition-edge x-ray microcalorimeters with high spectral resolution at 5.9 keV

We present measurements of high fill-factor arrays of superconducting transition-edge x-ray microcalorimeters designed to provide rapid thermalization of the x-ray energy. We designed an x-ray absorber that is cantilevered over the sensitive part of the thermometer itself, making contact only at normal-metal features. With absorbers made of electroplated gold, we have demonstrated an energy resolution between 2.4 and 3.1 eV at 5.9 keV on 13 separate pixels. We have determined the thermal and electrical parameters of the devices throughout the superconducting transition and, using these parameters, have modeled all aspects of the detector performance.

Advances in Small Pixel TES-Based X-Ray Microcalorimeter Arrays for Solar Physics and Astrophysics

We are developing small-pixel transition-edge sensor microcalorimeters for solar physics and astrophysics applications. These large format close-packed arrays are fabricated on solid silicon substrates and are designed to have high energy resolution, and also accommodate count-rates of up to a few hundred counts per second per pixel for X-ray photon energies up to ~ 8 keV. We have fabricated kilo-pixel versions that utilize narrow-line planar and stripline wiring. These arrays have a low superconducting transition temperature, which results in a low heat capacity and low thermal conductance to the heat sink. We present measurements of the performance of pixels with single 65-μm absorbers on a 75-μm pitch. With individual single pixels of this type, we have achieved a full-width at half-maximum energy resolution of 0.9 eV with 1.5 keV Al K X-rays, to our knowledge the first X-ray microcalorimeter with sub-eV energy resolution. We will discuss the properties of these arrays and their application to new solar and astrophysics mission concepts.

Transition-Edge Sensor Pixel Parameter Design of the Microcalorimeter Array for the X-Ray Integral Field Unit on Athena

The focal plane of the X-ray integral field unit (X-IFU) for ESA’s Athena X-ray observatory will consist of ~ 4000 transition edge sensor (TES) x-ray microcalorimeters optimized for the energy range of 0.2 to 12 keV. The instrument will provide unprecedented spectral resolution of ~ 2.5 eV at energies of up to 7 keV and will accommodate photon fluxes of 1 mCrab (90 cps) for point source observations. The baseline configuration is a uniform large pixel array (LPA) of 4.28” pixels that is read out using frequency domain multiplexing (FDM). However, an alternative configuration under study incorporates an 18 × 18 small pixel array (SPA) of 2” pixels in the central ~ 36” region. This hybrid array configuration could be designed to accommodate higher fluxes of up to 10 mCrab (900 cps) or alternately for improved spectral performance (< 1.5 eV) at low count-rates. In this paper we report on the TES pixel designs that are being optimized to meet these proposed LPA and SPA configurations. In particular we describe details of how important TES parameters are chosen to meet the specific mission criteria such as energy resolution, count-rate and quantum efficiency, and highlight performance trade-offs between designs. The basis of the pixel parameter selection is discussed in the context of existing TES arrays that are being developed for solar and x-ray astronomy applications. We describe the latest results on DC biased diagnostic arrays as well as large format kilo-pixel arrays and discuss the technical challenges associated with integrating different array types on to a single detector die.

Fine pitch transition-edge sensor X-ray microcalorimeters with sub-eV energy resolution at 1.5 keV

We are developing arrays of X-ray microcalorimeters on a 50-µm pitch that utilize transition-edge sensors as the sensor to measure the temperature rise when X-rays are absorbed. An array of this type of pixel has great potential for the study of point sources in future X-ray observatory missions. The pixels have gold absorbers with dimensions 45 × 45 × 4.2 µm3. We measured an energy resolution of 0.72 ± 0.03 eV full width at half maximum for the Al Kα complex in a subset of pixels within the array, which is the best resolution to date using a non-dispersive detector at this energy. We describe our characterization of this device including measurements of the heat capacity, thermal conductance to the heat bath, and the temperature and current sensitivity of the detector, and discuss its potential for improved performance.

Development of Embedded Heatsinking Layers for Compact Arrays of X-Ray TES Microcalorimeters

Transition-edge sensor microcalorimeter arrays in compact geometries and large formats experience local heating from bias power and x-ray hits that must be dissipated in the frame. For devices on solid, non-perforated silicon substrates, we have introduced an underlying embedded copper heatsinking layer to enhance the ability of the frame to remove this heat. In particular, such a layer can mitigate thermal crosstalk between nearby pixels within the array. Further improvements in array performance, such as decreased magnetic field sensitivity and stray inductance, are possible by turning the heatsinking layer into a superconducting ground plane. In this presentation, we report on the development of heatsinking layers consisting of a 1-2 μm thick high-quality copper layer which is sandwiched between two thin refractory metal-based diffusion barriers. These diffusion barriers are designed to avoid copper migration into the surrounding material over time, especially during our high temperature TES fabrication process which takes place in excess of 400°C . A 0.3-0.5 μm thick PECVD SiO2 cover layer isolates the heatsinking layer from the detector circuit. We present first results on our attempt to tailor the materials forming the diffusion barrier to fabricate both well defined superconducting ground planes and non-superconducting layers with the desired barrier characteristics.

Development of Position-Sensitive Transition-Edge Sensor X-Ray Detectors

We report on the development of position-sensitive transition-edge sensors (PoSTs) for future X-ray astronomy missions such as the International X-ray Observatory (IXO), under study by NASA and ESA. PoSTs consist of multiple absorbers each with a different thermal coupling to one or more transition-edge sensors (TESs). This results in a characteristic pulse shape for each absorber element and allows position discrimination. PoST development is motivated by a desire to achieve maximum focal-plane area with the fewest number of readout channels. We report detailed characterization of our single TES PoSTs or Hydras, which consist of four electroplated Au/Bi absorbers coupled to a low noise Mo/Au TES. Using a numerical model of the Hydra we fit to measured complex impedance curves and determine device parameters that allow us to accurately reproduce the measured pulse shapes and noise spectra. Results from Hydras with different internal thermal conductances reveal the trade-offs in optimizing for energy resolution or position-sensitivity. We report a best achievable energy resolution of < 6.0 eV across all pixels for a device with transition temperature of 86 mK, coupled with straightforward position discrimination by rise-time.

Detectors and cooling technology for direct spectroscopic biosignature characterization

Abstract. Direct spectroscopic biosignature characterization (hereafter “biosignature characterization”) will be a major focus for future space observatories equipped with coronagraphs or starshades. Our aim in this article is to provide an introduction to potential detector and cooling technologies for biosignature characterization. We begin by reviewing the needs. These include nearly noiseless photon detection at flux levels as low as <0.001  photons s−1 pixel−1 in the visible and near-infrared. We then discuss potential areas for further testing and/or development to meet these needs using noncryogenic detectors (electron multiplying charge coupled devices, HgCdTe array, HgCdTe APD array), and cryogenic single-photon detectors (microwave kinetic inductance device arrays and transition-edge sensor microcalorimeter arrays). Noncryogenic detectors are compatible with the passive cooling that is strongly preferred by coronagraphic missions but would add nonnegligible noise. Cryogenic detectors would require active cooling, but in return, deliver nearly quantum-limited performance. Based on the flight dynamics of past NASA missions, we discuss reasonable vibration expectations for a large UV-Optical-IR space telescope (LUVOIR) and preliminary cooling concepts that could potentially fit into a vibration budget without being the largest element. We believe that a cooler that meets the stringent vibration needs of a LUVOIR is also likely to meet those of a starshade-based Habitable Exoplanet Imaging Mission.

Magnetically Tuned Superconducting Transition-Edge Sensors

In this work we present a detector model for superconducting transition-edge sensors (TESs) that includes for the first time the magnetic field dependence of the resistive transition. By writing the resistance R as a function of temperature T current I and magnetic field B we present a general result requiring few assumptions that offers a new strategy to improve TES performance. Application of our TES models that agree with measurements of the critical current on TES sensors predicts that it is possible to design and operate a TES in a new regime by magnetically tuning the resistive transition surface R(T, I, B) . We show using all realizable device parameter values that this new magnetically tuned transition surface is predicted to give a sensor with larger signal size, faster speed capability, reduced performance limiting Johnson noise, and improved energy resolution; and do so over the entire pulse trajectory in R(T, I, B) space. We emphasize that our result is robust in that the performance benefits listed do not hinge on a precise functional form of the resistive transition. This magnetic tuning technique can improve performance for TESs governed by a wide range of resistive mechanisms such as weakly coupled to strongly coupled superconductors or nonequilibrium superconductivity.

Single Pixel Characterization of X-Ray TES Microcalorimeter Under AC Bias at MHz Frequencies

In this paper, we present the progress made at SRON in the read-out of X-ray Transition Edge Sensor (TES) microcalorimeters under AC bias. The experiments reported so far, whose aim was to demonstrate an energy resolution of 2 eV at 6 keV with a TES acting as a modulator, were carried out at frequencies below 700 kHz using a standard flux locked loop SQUID read-out scheme. The TES read-out suffered from the use of suboptimal circuit components, large parasitic inductances, low quality factor resonators, and poor magnetic field shielding. We have developed a novel experimental set-up that allows us to test several read-out schemes in a single cryogenic run. In this set-up, the TES pixels from a GSFC array are coupled via superconducting transformers to 18 high-Q lithographic LC filters with resonant frequencies ranging between 2 and 5 MHz. The signal is amplified by a two-stage SQUID current sensor and baseband feedback is used to overcome the limited SQUID dynamic range. We measured an X-ray energy resolution of 3.6 eV at 1.4 MHz, which is consistent with the measured integrated Noise Equivalent Power.

Uniformity of Kilo-Pixel Arrays of Transition-Edge Sensors for X-ray Astronomy

We are developing kilo-pixel arrays of transition-edge sensor (TES) microcalorimeters for use in future laboratory and space based X-ray astrophysics experiments. These arrays are required to achieve an energy resolution of ΔE_FWHM <; 3 eV full-width-half-maximum (FWHM) in the soft X-ray energy range. In this contribution we report on the development of 32 × 32 arrays of Mo/Au TESs with Bi/Au X-ray absorbers. We present measurements from 8 × 8 test arrays and 32 × 32 uniform arrays. Measurements of the bias point resistance and magnetic field dependence of the transition properties show that by carefully tuning the applied magnetic field the requirements on T_C and magnetic field uniformity can be reduced whilst maintaining the target energy resolution. Results show ΔE_FWHM ≈ 2.0 - 2.5 eV FWHM at a photon energy of 6 keV, measured across several pixels. Despite larger than typical T_C variation across the detector, time division multiplexed readout of 30 common electrically biased pixels in the 32 × 32 array (in a 2 column × 16 row configuration) show an average ΔE_FWHM = 3.0 ± 0.3 eV.