Frequency-division multiplexing (FDM) technologies are being developed for HUBS, which contains over 3000 transition-edge sensor (TES) microcalorimeters with an energy resolution of 2 eV (@0.6 keV). As a first step, an FDM system is designed and implemented for its pathfinder (DIXE), which employs a 10x10 TES microcalorimeter array, achieving an energy resolution of 6 eV or better over an energy range from 0.1 to 10 keV. The system has a multiplexing factor of 40 within the 1-5 MHz bandwidth. The warm electronics features a Kintex-7 FPGA and Magnicon Low-Noise Amplifier (LNA), coupled with baseband feedback software. Substantial progress has also been made on the cold electronics, with LC filters fabricated to achieve a 2 μm line width of the superconducting inductor and a dielectric constant of 11 for the capacitor. Superconducting Quantum Interference Devices (SQUIDs) have been fabricated, with the readout noise measured to be less than 6 pA/ √ Hz. This report presents the initial design both on the warm electronics and the superconducting circuit, offering an overview of the progress made. The findings support the conceptual viability of employing FDM for the multiplexed readout of TES microcalorimeters in the context of HUBS.
The Hot Universe Baryon Surveyor (HUBS) mission is proposed to study “missing” baryons in the universe. Unlike dark matter, baryonic matter is made of elements in the periodic table, and can be directly observed through the electromagnetic signals that it produces. Stars contain only a tiny fraction of the baryonic matter known to be present in the universe. Additional baryons are found to be in diffuse (gaseous) form, in or between galaxies, but a significant fraction has not yet been seen. The latter (“missing” baryons) are thought to be hiding in low-density warm-hot ionized medium (WHIM), based on results from theoretical studies and recent observations, and be distributed in the vicinity of galaxies (i.e., circumgalactic medium) and between galaxies (i.e., intergalactic medium). Such gas would radiate mainly in the soft X-ray band and the emission would be very weak, due to its very low density. HUBS is optimized to detect the X-ray emission from the hot baryons in the circumgalactic medium, and thus fill a void in observational astronomy. The goal is not only to detect the “missing” baryons, but to characterize their physical and chemical properties, as well as to measure their spatial distribution. The results would establish the boundary conditions for understanding galaxy evolution. Though highly challenging, detecting “missing” baryons in the intergalactic medium could be attempted, perhaps in the outskirts of galaxy clusters, and could shed significant light on the large-scale structures of the universe. The current design of HUBS will be presented, along with the status of technology development.
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