Lynx x-ray microcalorimeter

Abstract

Lynx is an x-ray telescope, one of four large satellite mission concepts currently being studied by NASA to be a flagship mission. One of Lynx's three instruments is an imaging spectrometer called the Lynx x-ray microcalorimeter (LXM), an x-ray microcalorimeter behind an x-ray optic with an angular resolution of 0.5 arc sec and ∼2 m² of area at 1 keV. The LXM will provide unparalleled diagnostics of distant extended structures and, in particular, will allow the detailed study of the role of cosmic feedback in the evolution of the Universe. We discuss the baseline design of LXM and some parallel approaches for some of the key technologies. The baseline sensor technology uses transition-edge sensors, but we also consider an alternative approach using metallic magnetic calorimeters. We discuss the requirements for the instrument, the pixel layout, and the baseline readout design, which uses microwave superconducting quantum interference devices and high-electron mobility transistor amplifiers and the cryogenic cooling requirements and strategy for meeting these requirements. For each of these technologies, we discuss the current technology readiness level and our strategy for advancing them to be ready for flight. We also describe the current system design, including the block diagram, and our estimate for the mass, power, and data rate of the instrument.

Keywords: Lynx; cryogenics; microcalorimeters; telescope; x-ray.

Fig. 1

This image shows the baseline layout of the LXM focal plane array. It consists of three different styles of pixels in three different regions. The Main Array consists of 1 arc-sec pixels over a 5 arc-min FoV with a 7-keV energy range, and the Enhanced Main Array 0.5 arc-sec pixels in the innermost central 1 arc-min region. Off to one side is the Ultra-Hi-Res Array consisting of 1 arc-min pixels over a separate 1 arc-min region, optimized for high-energy resolution below 1 keV, but will still have high energy resolution up to 2 keV.

Fig. 2

This table summarizes the main requirements of each of the different subarrays, and the main science drivers for these requirements. A thorough list of requirements is given in Sec. 9.

Fig. 3

(a) Schematic showing components of a TES calorimeter. This design uses the thermal boundary between the TES film and the substrate for the necessary decoupling of the TES from the substrate. (b) Schematic representation of the TES Hydra. Inset top right: Thermal model of a multipixel TES consisting of four x-ray absorbers, connected to a single TES via varied thermal conductance Gi (where i = 1,…,4). The TES is weakly thermally coupled to a heat sink via conductance Gb. The measured average x-ray pulse shapes for a 4-pixel Hydra at an energy of 6 keV are shown. The differences between pulse shapes before equilibration are used to discriminate which pixel the x-ray photon was absorbed in.

Fig. 4

Photographs of prototype LXM Hydras for the Main Array prior to attaching absorbers to the arrays. The TESs are the larger square gold-colored regions and the small gold dots are the locations of the 1.5-μm diameter stems for the different pixel absorbers attached to the TES. The narrow gold-colored lines between the pixels and TESs are the Hydra thermal links, the length of the lines determining the strength of the thermal conductances. The lines are 1-μm wide and 300-nm thick. The pitch of the TESs is 250 μm. (a) This photograph shows two different Hydras in an array. (b) This photograph shows a more zoomed in view of one of the Hydras.

Fig. 5

(a) An exploded cartoon of the second lead attachment process (with two sets of vias through the oxide) showing how the wiring mates to the GSFC TES fabrication process. (b) Photograph of a chip with an attachment between buried superconducting wiring and a TES through a process in which Nb oxide is removed from MIT/LL wires, where the surface of the top layer Nb is planarized.

Fig. 6

Prototype LXM arrays. The dimensions of this chip are 1.9 cm × 1.5 cm. (a) Photograph of an overhead view of a prototype LXM array with nearly 50,000 pixels. (b) Mask layout showing different regions of the array and the internal wiring for a subset of the pixels. The labels show the regions and number and pitches of pixels for the three different subarrays. (c) Angled photograph of the completed entire array. (d) Zoomed-in view of a region of the array showing the interface between the Main Array and the Enhanced Main Array.

Fig. 7

Photograph of the prototype UHR array. This is a 40 × 40 array of single pixels on a 50-μm pitch. Each absorber is 1-μm thick.

Fig. 8

Results from the UHR array fabricated. (a) Measured resistance versus temperature curve. (b) Measured α and β for this pixel type as a function of the TES bias level (as a percent of the normal resistance), as determined from impedance measurements. (c) Temperature as a function of TES bias level. (d) Measured baseline resolution as a function of TES bias level, as determined by the TES responsivity (α/β from impedance measurements) and the measured noise at the corresponding bias points.

Fig. 9

(a) Measured spectrum in prototype UHR array for Al K_α x-rays. The red line is a best fit through the data corresponding to a FWHM broadening on the Al K_α x-rays of 2.31 ± 0.05 eV. The dashed blue lines represents the four broadened Al K_α x-rays lines broadened by this same detector resolution. (b) Filtered pulse height as a function of energy for the observe Al and Si K_α x-rays.

Fig. 10

This plot shows how the modeled energy range and energy resolution change as a function of bath temperature as this temperature is increased closer to the superconducting transition temperature for a typical TES x-ray microcalorimeter. The transition temperature, in this case, is 65 mK. This modeled example assumed that the maximum allowable slew rate is constant and sufficient for a 7-keV range and 3-eV energy resolution for a bath temperature of 45 mK. It also assumes that the energy range is limited by the slew rate. In order to extend the energy range above 15 keV, the bath temperature needs to be biased just a few mK below the transition temperature, where the energy resolution is expected to degrade to ∼5 eV.

Fig. 11

(a) Schematic showing components of an MMC calorimeter. Like the TES, this design uses the thermal boundary between the MMC AuEr sensor film and the substrate for thermally decoupling of the MMC from the substrate. (b) Schematic of the basic circuit for reading out MMCs.

Fig. 12

Prototype LXM array using MMCs. (a) Design layout of the entire prototype array in which all of the pixels are wired within the array. The blue lines are the wiring to the Main Array, the purple lines are for the Enhanced Main Array and the red lines are for the UHR array. There are 112 pairs bond pads spread across two rows in along both the top and bottom edges. (b) Photograph of a completely fabricated prototype. (c) Scanning electron microscope image of a small region of the array that shows the interface between the Main Array pixels and the Enhanced Main Array pixels. The 2.8-μm-thick gold pixel absorbers are cantilevered above the substrate on small stems that make contact in the pixel centers, where the pixel surfaces are recessed. The gaps between pixels in this first prototype are 5 μm, designed conservatively to ensure high yield. (d) Photograph of Main Array Hydra before the 25-pixel absorbers was added (pixels on a 50-μm pitch). The raffle-shaped region is the MMC sensor, and the lengths and widths of the lines between the waffle and central pixel stem locations determine the thermal conductance to each pixel. (e) Photograph of an Enhanced Main Array Hydra before the 25 absorbers was attached, these pixels being on a 25-μm pitch.

Fig. 13

(a) Microwave SQUID multiplexer read-out circuit. (b) Photograph of a prototype set-up for testing arrays pixels with this read-out circuit. The set-up has one microwave read-out channel, capable of reading out 34 separate TESs.

Fig. 14

(a) Overview of the LXM cryostat and read-out electronics. The compressor is mounted independent of the cryostat and connected to the cryostat through a flexible transfer tube. All of the electronics boxes shown in the LXM block diagram (Fig. 18) are visible. (b) Side-on view of the LXM, including a cross-sectional view of the cryostat. The x-ray enter the cryostat from the bottom. The filter wheel and MXS (with its electronics) are located a small distance below the bottom of the main cryostat on a separate mounting plate, which is attached to the main cryostat. The height of the main cryostat shown is 1.43 m and the diameter is 60 cm.

Fig. 15

The LXM multistage ADR design that produces the required cooling power at 50 mK and 0.6 K continuously, assuming a 4.5-K heat sink temperature. It consists of five different salt pills and five heat switches. Its salt pill is surrounded by its own magnet.

Fig. 16

Preliminary LXM FPA design. (a) Cross-section of the FPA. The high-magnetic-permeability cryoperm shield is at 4.5 K and the superconducting niobium shield is at 0.6 K. (b) Angled view of the FPA 50 mK stage. The main, enhanced main, and UHR arrays are visible on the top surface through am infrared blocking filter, which is almost transparent in this figure. The read-out components are on each of the eight side panels.

Fig. 17

(a) Illustration showing how bump-bonded connections are made between the detector chip and the side panels containing the readout. (b) Illustration showing the geometry of the flex and bumps. (c) Scanning electron microscope image of some prototype indium bumps fabricated and bump-bonded at GSFC. (d) Photograph a hexagonal detector chip connected to several silicon chips with wire-bond pads using superconducting microstrip flex and bump-bond connections. (e) Set-up for testing the bump-bonded connections between two chips through flex with superconducting microstrip.

Fig. 18

Block diagram of the LXM. The cryocooler and MEB consist of two redundant boxes. The DEEP box is actually two separate segmented boxes.

Fig. 19

The schedule for the LXM development and a very brief summary of the plan for demonstrating the TRL advancements.