James A. Chervenak

Impedance measurements and modeling of a transition-edge-sensor calorimeter

We describe a method for measuring the complex impedance of transition-edge-sensor (TES) calorimeters. Using this technique, we measured the impedance of a Mo/Au superconducting transition-edge-sensor calorimeter. The impedance data are in good agreement with our linear calorimeter model. From these measurements, we obtained measurements of unprecedented accuracy of the heat capacity and the gradient of resistance with respect to temperature and current of a TES calorimeter throughout the phase transition. The measurements probe the internal state of the superconductor in the phase transition and are useful for characterizing the calorimeter.

Longitudinal Proximity Effects in Superconducting Transition-Edge Sensors

We have found experimentally that the critical current of a square thin-film superconducting transition-edge sensor (TES) depends exponentially upon the side length L and the square root of the temperature T, a behavior that has a natural theoretical explanation in terms of longitudinal proximity effects if the TES is regarded as a weak link between superconducting leads. As a consequence, the effective transition temperature T{c} of the TES is current dependent and at fixed current scales as 1/L{2}. We have also found that the critical current can show clear Fraunhofer-like oscillations in an applied magnetic field, similar to those found in Josephson junctions. We have observed the longitudinal proximity effect in these devices over extraordinarily long lengths up to 290 microm, 1450 times the mean-free path.

Proximity effects and nonequilibrium superconductivity in transition-edge sensors

We have recently shown that normal-metal/superconductor (N/S) bilayer superconducting transition-edge sensors (TESs) exhibit weak-link behavior.(1) Here, we extend our understanding to include TESs with added noise-mitigating normal-metal structures (N structures). We find that TESs with added Au structures also exhibit weak-link behavior as evidenced by the exponential temperature dependence of the critical current and Josephson-like oscillations of the critical current with applied magnetic field. We explain our results in terms of an effect converse to the longitudinal proximity effect (LoPE),(1) the lateral inverse proximity effect (LaiPE), for which the order parameter in the N/S bilayer is reduced due to the neighboring N structures. Resistance and critical current measurements are presented as a function of temperature and magnetic field taken on square Mo/Au bilayer TESs with lengths ranging from 8 to 130 mu m with and without added N structures. We observe the inverse proximity effect on the bilayer over in-plane distances many tens of microns and find the transition shifts to lower temperatures scale approximately as the inverse square of the in-plane N-structure separation distance, without appreciable broadening of the transition width. We also present evidence for nonequilbrium superconductivity and estimate a quasiparticle lifetime of 1.8 x 10(-10) s for the bilayer. The LoPE model is also used to explain the increased conductivity at temperatures above the bilayers steep resistive transition.

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.

Implications of weak-link behavior on the performance of Mo/Au bilayer transition-edge sensors

Understanding the physical properties of the superconducting-to-normal transition is fundamental for optimizing the design and performance of transition-edge sensors (TESs). Recent critical current IC measurements of square Mo/Au bilayer structures show that they act as weak superconducting links, exhibiting oscillatory, Fraunhofer-like behavior with applied magnetic field. In this paper, we investigate the implications of this behavior for TES x-ray detectors operated in the resistive transition. These devices include normal metal features used for absorber attachment and suppression of detector noise. We present extensive measurements of IC as a function of temperature T and field B, which show a complex temperature and current evolution when compared with the behavior expected from a simple geometry. We introduce a resistively shunted junction model for describing the TES resistive transition as a function of current I, temperature T, and magnetic field B. From this model, we calculate the R(T,I,B) tra...

The design, implementation, and performance of the Atro-H SXS calorimeter array and anti-coincidence detector

The calorimeter array of the JAXA Astro-H (renamed Hitomi) Soft X-ray Spectrometer (SXS) was designed to provide unprecedented spectral resolution of spatially extended cosmic x-ray sources and of all cosmic x-ray sources in the Fe-K band around 6 keV, enabling essential plasma diagnostics. The SXS has a square array of 36 microcalorimeters at the focal plane. These calorimeters consist of ion-implanted silicon thermistors and HgTe thermalizing x-ray absorbers. These devices have demonstrated a resolution of better than 4.5 eV at 6 keV when operated at a heat-sink temperature of 50 mK. We will discuss the basic physical parameters of this array, including the array layout, thermal conductance of the link to the heat sink, resistance function, absorber details, and means of attaching the absorber to the thermistorbearing element. We will also present the thermal characterization of the whole array, including thermal conductance and crosstalk measurements and the results of pulsing the frame temperature via alpha particles, heat pulses, and the environmental background. A silicon ionization detector is located behind the calorimeter array and serves to reject events due to cosmic rays. We will briefly describe this anti-coincidence detector and its performance.

Multiplexed readout of uniform arrays of TES x-ray microcalorimeters suitable for Constellation-X

Following our development of a superconducting transition-edge-sensor (TES) microcalorimeter design that en- ables reproducible, high performance (routinely better than 3 eV FWHM energy resolution at 6 keV) and is compatible with high-fill-factor arrays, we have directed our efforts towards demonstrating arrays of identical pixels using the multiplexed read-out concept needed for instrumenting the Constellation-X X-ray Microcalorime- ter Spectrometer (XMS) focal plane array. We have used a state-of-the-art, time-division SQUID multiplexer system to demonstrate 2 ×8 multiplexing (16 pixels read out with two signal channels) with an acceptably modest level of degradation in the energy resolution. The average resolution for the 16 multiplexed pixels was 2.9 eV, and the distribution of resolution values had a relative standard deviation of 5%. The performance of the array while multiplexed is well understood. The technical path to realizing multiplexing for the XMS instrument on the scale of 32 pixels per signal channel includes increasing the system bandwidth by a factor of four and reducing the non-multiplexed SQUID noise by a factor of two. In this paper we discuss the characteristics of a uniform 8 ×8 array and its performance when read out non- multiplexed and with various degrees of multiplexing. We present data acquired through the readout chain from the multiplexer electronics, through the real-time demultiplexer software, to storage for later signal processing. We also report on a demonstration of real-time data processing. Finally, because the multiplexer provides unprecedented simultaneous access to the pixels of the array, we were able to measure the array-scale uniformity of TES calorimeter parameters such as the individual thermal conductances and superconducting transition temperatures of the pixels. Detector uniformity is essential for optimal operation of a multiplexed array, and we found that the distributions of thermal conductances, transition temperatures, and transition slopes were sufficiently tight to avoid significant compromises in the operation of any pixel.

A planar two-dimensional superconducting bolometer array for the Green Bank Telescope

In order to provide high sensitivity rapid imaging at 3.3 mm (90 GHz) for the Green Bank Telescope - the worlds largest steerable aperture - a camera is being built by the University of Pennsylvania, NASA/GSFC, and NRAO. The heart of this camera is an 8x8 close-packed, Nyquist-sampled detector array. We have designed and are fabricating a functional superconducting bolometer array system using a monolithic planar architecture. Read out by SQUID multiplexers, the superconducting transition edge sensors will provide fast, linear, sensitive response for high performance imaging. This will provide the first ever superconducting bolometer array on a facility instrument.

High-density arrays of x-ray microcalorimeters for Constellation-X

We have been developing x-ray microcalorimeters for the Constellation-X mission. Devices based on superconducting transition-edge sensors (TES) have demonstrated the potential to meet the Constellation-X requirements for spectral resolution, speed, and array scale (> 1000 pixels) in a close-packed geometry. In our part of the GSFC/NIST collaboration on this technology development, we have been concentrating on the fabrication of arrays of pixels suitable for the Constellation-X reference configuration. We have fabricated 8x8 arrays with 0.25-mm pixels arranged with 92% fill factor. The pixels are based on Mo/Au TES and Bi/Cu or Au/Bi absorbers. We have achieved a resolution of 4.0 eV FWHM at 6 keV in such devices, which meets the Constellation-X resolution requirement at 6 keV. Studies of the thermal transport in our Bi/Cu absorbers have shown that, while there is room for improvement, for 0.25-mm pixels the standard absorber design is adequate to avoid unacceptable line-broadening from position dependence caused by thermal diffusion. In order to improve reproducibility and to push closer to the 2-eV goal at 6 keV, however, we are refining the design of the TES and the interface to the absorber. Recent efforts to introduce a barrier layer between the Bi and the Mo/Au to avoid variable interface chemistry and thus improve the reproducibility of device characteristics have thus far yielded unsatisfactory results. However, we have developed a new set of absorber designs with contacts to the TES engineered to allow contact only in regions that do not serve as the active thermometer. We have further constrained the design so that a low-resistance absorber will not electrically short the TES. It is with such a design that we have achieved 4.0 eV resolution at 6 keV.

Astronomical demonstration of superconducting bolometer arrays

We have built a prototype submillimeter spectrometer, FIBRE, which is based on a helium-cooled scanning Fabry-Perot and superconducting transition edge sensor bolometers (TES). SQUID multiplexers are used to read out the individual detector pixels. The spectral resolving power of the instrument is provided by the Fabry-Perot spectrometer. The outgoing light from the Fabry-Perot passes onto a low resolution grating for order sorting. A linear bolometer array consisting of 16 elements detects this dispersed light, capturing 5 orders simultaneously from one position on the sky. With tuning of the Fabry-Perot over one free spectral range, a spectrum covering Δλ/λ=1/7 at a resolution of ~1/1200 can be achieved. The spectral resolution is sufficient to resolve doppler broadened line emission from external galaxies. FIBRE operates in the 350 μm and 450 μm bands. These bands cover line emission from the important PDR tracers neutral carbon [CI] and carbon monoxide CO. The spectrometer was used at the Caltech Submillimeter Observatory to obtain the first ever astronomical observations using multiplexed arrays of superconducting transition edge bolometers.