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.
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.
Four astrophysics missions are currently being studied by NASA as candidate large missions to be chosen in the 2020 astrophysics decadal survey.1 One of these missions is the “X-Ray Surveyor” (XRS), and possible configurations of this mission are currently under study by a science and technology definition team (STDT). One of the key instruments under study is an X-ray microcalorimeter, and the requirements for such an instrument are currently under discussion. In this paper we review some different detector options that exist for this instrument, and discuss what array formats might be possible. We have developed one design option that utilizes either transition-edge sensor (TES) or magnetically coupled calorimeters (MCC) in pixel array-sizes approaching 100 kilo-pixels. To reduce the number of sensors read out to a plausible scale, we have assumed detector geometries in which a thermal sensor such a TES or MCC can read out a sub-array of 20-25 individual 1” pixels. In this paper we describe the development status of these detectors, and also discuss the different options that exist for reading out the very large number of pixels.
High spectral resolution, high cadence, imaging x-ray spectroscopy has the potential to revolutionize the study of
the solar corona. To that end we have been developing transition-edge-sensor (TES) based x-ray microcalorimeter
arrays for future solar physics missions where imaging and high energy resolution spectroscopy will enable
previously impossible studies of the dynamics and energetics of the solar corona. The characteristics of these xray
microcalorimeters are significantly different from conventional microcalorimeters developed for astrophysics
because they need to accommodate much higher count rates (300-1000 cps) while maintaining high energy
resolution of less than 4 eV FWHM in the X-ray energy band of 0.2-10 keV. The other main difference is a
smaller pixel size (less than 75 x 75 square microns) than is typical for x-ray microcalorimeters in order to
provide angular resolution less than 1 arcsecond. We have achieved at energy resolution of 2.15 eV at 6 keV in a
pixel with a 12 x 12 square micron TES sensor and 34 x 34 x 9.1 micron gold absorber, and a resolution of 2.30
eV at 6 keV in a pixel with a 35 x 35 micron TES and a 57 x 57 x 9.1 micron gold absorber. This performance
has been achieved in pixels that are fabricated directly onto solid substrates, ie. they are not supported by silicon
nitride membranes. We present the results from these detectors, the expected performance at high count-rates,
and prospects for the use of this technology for future Solar missions.
We are developing arrays of position-sensitive transition-edge sensor (PoST) X-ray detectors for future astronomy missions such as NASA's Constellation-X. The PoST consists of multiple absorbers thermally coupled to one or more transition-edge sensor (TES). Each absorber element has a different thermal coupling to the TES. This results in a distribution of different pulse shapes and enables position discrimination between the absorber elements. PoST's are motivated by the desire to achieve the largest possible focal plane area with the fewest number of readout channels and are ideally suited to increasing the Constellation-X focal plane area, without comprising on spatial sampling. Optimizing the performance of PoST's requires careful design of key parameters such as the thermal conductances between the absorbers, TES and the heat sink, as well as the absorber heat capacities. Our new generation of PoST's utilizes technology successfully developed on high resolution (~ 2.5 eV) single pixels arrays of Mo/Au TESs, also under development for Constellation-X. This includes noise mitigation features on the TES and low resistivity electroplated absorbers. We report on the first experimental results from new one-channel, four-pixel, PoST's or 'Hydras', consisting of composite Au/Bi absorbers. We have achieved full-width-at-half-maximum energy resolution of between 5-6 eV on all four Hydra pixels with an exponential decay time constant of 620 μs. Straightforward position discrimination by means of rise time is also demonstrated.
We report on our studies of possible configurations for the focal plane of the Constellation-X mission. Taking
advantage of new developments in both SQUID multiplexing technology and position-sensitive detectors, we
present a viable focal plane intrument design that would greatly enhance the reference Constellation-X configuration
of a 32 × 32 array. An order of magnitude increase in the number of pixels of the focal plane array from
the current 1024-pixel reference design is achievable.
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.
We present our latest results from our development of Position-Sensitive Transition-Edge Sensors (PoSTs). Our devices work as one-dimensional imaging spectrometers. They consist of a long absorber (segmented or solid) with a transition-edge sensor (TES) on each end. When X-rays hit the absorber, the comparison of the signals sensed in the two TESs determine the position of the TES, while the addition of the signals gives the energy of the X-ray. We obtained impedance curves for three different devices and obtained reasonable fits with our theoretical PoST model.
We have investigated the thermal, electrical, and structural properties of Bi and BiCu films that are being developed as X-ray absorbers for transition-edge sensor (TES) microcalorimeter arrays for imaging X-ray spectroscopy. Bi could be an ideal material for an X-ray absorber due to its high X-ray stopping power and low specific heat capacity, but it has a low thermal conductivity, which can result in position dependence of the pulses in the absorber. In order to improve the thermal conductivity, we added Cu layers in between the Bi layers. We measured electrical and thermal conductivities of the films around 0.1 K, the operating temperature of the TES calorimeter, to examine the films and to determine the optimal thickness of the Cu layer. From the electrical conductivity measurements, we found that the Cu is more resistive on the Bi than on a Si substrate. Together with a SEM picture of the Bi surface, we concluded that the rough surface of the Bi film makes the Cu layer resistive when the Cu layer is not thick enough to fill in the roughness. From the thermal conductivity measurements, we determined the thermal diffusion constant to be 2 x 103 μm2μs-1 in a film that consists of 2.25 μm of Bi and 0.1 μm of Cu. We measured the position dependence in the film and found that its thermal diffusion constant is too low to get good energy resolution, because of the resistive Cu layer and/or possibly a very high heat capacity of our Bi films. We show plans to improve the thermal diffusion constant in our BiCu absorber.