Time-division SQUID multiplexed readout for Athena X-IFU and the LEM Microcalorimeter Spectrometer

Time-division-multiplexed (TDM) readout uses superconducting quantum-interference devices (SQUIDs) to measure the current signals from transition-edge sensors (TESs). In TDM, each dc-biased TES is coupled to its own first-stage SQUID; these are turned on and off sequentially such that one TES at a time is read out per TDM amplifier column. Multiple TDM columns, operated in parallel, read out a full array. Multiplexed readout of TES arrays is generally needed to reduce the heat load on the coldest cryogenic stage and the complexity of the cryogenic wiring. TDM has been used successfully in many bolometric TES-array instruments, each of the scale of many thousands of pixels, for millimeter and submillimeter-wave astronomy, such as SCUBA-2 and the many generations of ACT and ACTPol. In addition, many X-ray and gamma-ray microcalorimeter spectrometers have used TDM readout for TES arrays of the scale of hundreds of pixels; these include multiple synchrotron-based X-ray spectrometers for emission and absorption spectroscopy, several gamma-ray spectrometers for the analysis of special nuclear materials, and the Micro-X sounding rocket that operated a 128-pixel soft-X-ray TES array in space in 2022. Here we present some details of the TDM readout planned for two TES imaging-spectrometers for X-ray astrophysics. For the first, the X-ray Integral Field Unit (X-IFU) for ESA’s Athena mission, TDM is at an advanced stage of development. Before the 2022–2023 Athena rescope, X-IFU was planned as an array of 2,376 TES pixels, with an energy range of 0.2 to 12 keV and energy resolution of 2.5 eV (FWHM) at 7 keV. The readout consisted of 72 TDM columns, with 33 TESs and one dark row per column; rescoped specifications should be defined by early 2023. Recent developments in TDM for X-IFU have included the design and testing of modular 4-column x 34-row TDM SQUID chips that are cold-indium bump bonded to a wiring substrate. Testing of these 4-column chips indicates that TDM meets all X-IFU requirements in X-IFU-appropriate packaging. The second imaging spectrometer is the LMS (LEM Microcalorimeter Spectrometer) for NASA’s proposed Line Emission Mapper (LEM) mission. The LMS plans an array of 4,248 TESs, with an energy range of 0.1 to 2 keV and energy resolution of ~0.9 eV (FWHM) in the inner array and ~2 eV in the outer array. The TDM readout is 72 columns, each with a multiplexing factor of 60 (59 TESs and one dark row). A strategy similar strategy to X-IFU’s, of modular and bumped TDM chips, is planned for the LMS.