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SRON (Netherlands Institute for Space Research) is developing the focal plane assembly (FPA) for Athena X-IFU, whose demonstration model (DM) will use for the first time a time domain multiplexing (TDM)-based readout system for the on-board transition-edge sensors (TES). We report on the characterization activities on a TDM setup provided by NASA goddard space flight center (GSFC) and national institute for standards and technology (NIST) and tested in SRON cryogenic test facilities. The goal of these activities is to study the impact of the longer harness, closer to X-IFU specs, in a different EMI environment and switching from a single-ended to a differential readout scheme. In this contribution we describe the advancement in the debugging of the system in the SRON cryostat, which led to the demonstration of the nominal spectral performance of 2.8 eV at 5.9 keV with 16-row multiplexing, as well as an outlook for the future endeavors for the TDM readout integration on X-IFU's FPA-DM at SRON. [ABSTRACT FROM AUTHOR]

The nonlinear electrical transport properties of metal-insulator-metal tunnel diodes based on a barrier of naturally grown niobium oxide have been measured at room temperature and analyzed. In most cases excellent agreement is found between the measured current-voltage characteristics and fits to the trapezoidal barrier model, over large ranges in conductance (up to several times the zero-bias value), and including the asymmetry induced by the differing electrode materials, niobium and silver. Moreover, we find that an in situ, Ar plasma etch may be used to modify the tunnel barrier in a controlled fashion. Specifically, as the etch time is increased from 0 to 120 s, the barrier thickness is continuously reduced from ∼2.8 to 1.1 nm, while the barrier height at the base (Nb) electrode remains roughly constant at 300±40 mV. Simultaneously, the barrier height at the Ag counterelectrode is lowered from an initial value of 800 mV to the asymptotic value implied by the difference in work functions of the electrode materials, namely, 230±40 mV. [ABSTRACT FROM AUTHOR]

Micro-X is an X-ray sounding rocket payload that had its first flight on July 22, 2018. The goals of the first flight were to operate a transition edge sensor X-ray microcalorimeter array in space and take a high-resolution spectrum of the Cassiopeia A supernova remnant. The first flight was considered a partial success. The array and its time-division multiplexing readout system were successfully operated in space, but due to a failure in the attitude control system, no time on-target was acquired. A re-flight has been scheduled for summer 2022. Since the first flight, modifications have been made to the detector systems to improve noise and reduce the susceptibility to magnetic fields. The three-stage SQUID circuit, NIST MUX06a, has been replaced by a two-stage SQUID circuit, NIST MUX18b. The initial laboratory results for the new detector system will be presented in this paper. [ABSTRACT FROM AUTHOR]

The Micro-X sounding rocket is a NASA funded X-ray telescope payload that completed its first flight on July 22, 2018. This event marked the first operation of Transition Edge Sensors (TESs) and their SQUID-based multiplexing readout system in space. Unfortunately, due to an attitude control system pointing failure, the rocket was spinning during its 5 min observation period and no scientific data was collected. However, data collected from the internal calibration source marked a partial success for the payload and offers a unique opportunity to study the response of TESs and SQUIDs in space. Of particular interest is the magnetic field response of the NIST MUX06a SQUID readout system to tumbling through Earth's magnetic field. We present a model to explain the baseline response of the SQUIDs, which lead to a subset of pixels failing to "lock" for the full observational period. Future flights of the Micro-X rocket will include the NIST MUX18b SQUID system with dramatically reduced sensitivity to magnetic fields. [ABSTRACT FROM AUTHOR]

We are developing lab-based readout electronics for Transition-edge sensors (TES) using commercial-off-the-shelf (COTS) modules. These COTS modules are advantageous since they increase development speed and keep the cost low. We have developed these electronics to support both non-multiplexed and time-division multiplexing (TDM) readout systems. The system utilizes remote control via Ethernet, and the interface allows many types of measurements to be automated. With the TDM readout system, we have achieved 2.05 eV at 6 keV, 2.1 eV at 7 keV, 2.3 eV at 8 keV, and 2.8 eV at 12 keV with 2-column × 32-row multiplexing. We will be using this system in the characterization of detectors for the X-Ray Integral Field Unit (X-IFU) instrument on Athena. In this paper, we present an overview of the design and their performance. [ABSTRACT FROM AUTHOR]

A microwave superconducting quantum interference device multiplexer has been optimized for reading out large arrays of superconducting transition-edge sensor (TES) bolometers. We present the scalable cryogenic multiplexer chip design that may be used to construct an 1820-channel multiplexer for the 4–8 GHz rf band. The key metrics of yield, sensitivity, and crosstalk are determined through measurements of 455 readout channels, which span 4–5 GHz. The median white-noise level is 45 pA/ Hz , evaluated at 2 Hz, with a 1/f knee ≤ 20 mHz after common-mode subtraction. The white-noise level decreases the sensitivity of a TES bolometer optimized for detection of the cosmic microwave background at 150 GHz by only 3%. The measured crosstalk between any channel pair is ≤ 0.3%. [ABSTRACT FROM AUTHOR]

Exotic atom experiments place transition-edge sensor (TES) microcalorimeter arrays in a high-energy charged particle-rich environment. When a high-energy charged particle passes through the silicon substrate of a TES array, a large amount of energy is deposited and small pulses are generated across multiple pixels in the TES array due to thermal crosstalk. We have developed analysis techniques to assess and reduce the effects of charged particle events on exotic atom X-ray measurements. Using this technique, the high-energy and low-energy components of the X-ray peaks due to pileup are eliminated, improving the energy resolution from 6.6 to 5.7 eV at 6.9 keV. [ABSTRACT FROM AUTHOR]

A novel triggering function developed for 240-pixel Transition-Edge Sensors is demonstrated under the high rate of particle background. The function is integrated into the standard data acquisition system in the NIST TES framework. It enables any type of combination of trigger pattern when a pixel is triggered, which is called "group trigger." As a practical implementation, the primary trigger is distributed to the four physically nearest pixels. The group trigger function was utilized throughout the entire one-month J-PARC experiment for the measurement of the Kaonic-atom X-rays. This trigger allowed us to confirm that the increased background and degraded energy resolution we observed when operating the TES array in the presence of an ion beam are the result of thermal crosstalk from charged particles. We show that the maximum of the average of the signals among the neighboring four pixels is useful for event selection. We use cuts based on this parameter to improve the peak-to-background level in a measured x-ray energy spectrum by a factor of ∼ 2, while keeping 95% of measured events. This flexible group triggering technique allows us to improve the signal to noise on the very faint Kaonic Helium x-ray lines we are measuring, better understand our experiment environment, and we believe this technique may prove useful in other ground and space-based TES applications. [ABSTRACT FROM AUTHOR]

We are developing an imaging capability ("Hyperspectral X-ray Imaging") for mapping chemical information (molecular formula, phase, oxidation state, hydration) that is based on ultra-high-resolution X-ray emission spectroscopy with large transition-edge sensor microcalorimeter arrays in the scanning electron microscope. By combining microcalorimeter arrays with hundreds of pixels, high-bandwidth microwave frequency-division multiplexing, and fast digital electronics for near real-time data processing, our goal is to enable measurements using laboratory-scale instrumentation rather than synchrotron beamlines. Our application focus here is on mapping the chemical form of uranium compounds on the nanoscale. We will present our approach to developing the Hyperspectral X-ray Imaging capability, progress toward a 128-pixel microwave multiplexed X-ray fluorescence instrument at LANL, and the path to high-throughput nanoscale chemical mapping. [ABSTRACT FROM AUTHOR]

A useful function, called group trigger, is implemented in a data acquisition system of a 240-pixel X-ray transition-edge sensor array to store simultaneous waveforms from any desired set of pixels. It works as a diagnostic tool which can record both primary pulses and associated crosstalk events with enough freedom to optimize the data output. Under a high rate of charged particle background, such as in an accelerator, investigating signals from any combination of trigger patterns for a particular event is of high importance. We utilized this function throughout an entire experiment at J-PARC, measuring kaonic atom X-rays in 2018. This was the first time a group trigger was applied to exotic-atom spectroscopy. In this experiment, the primary trigger was distributed to the four physical nearest neighbor pixels for practical purposes. The function clarified the effect of thermal and electrical cross talk from X-rays and charged particles, resulting in a better understanding of the behavior of the TES array in such a high-background environment. [ABSTRACT FROM AUTHOR]

We integrated a TES X-ray spectrometer with a charged kaon beam line at J-PARC to perform X-ray spectroscopy of kaonic helium atoms. Limited beam intensity and a broad beam spot size made it crucial to increase the detector acceptance angle as much as possible, requiring significant development. To this end, our TES system shared the same vacuum with a cryogenic system of the liquid helium experimental target. We also specially developed a target cell for liquid helium and a thinned aperture array on top of the TES detector. Additionally, thermal and magnetic shields and infrared filters were optimized in terms of a larger acceptance angle and energy resolution of the detector. The scientific campaign was performed in June, 2018, where the whole system was stably operated for almost one month. [ABSTRACT FROM AUTHOR]

Advanced ACTPol is the second-generation polarization-sensitive upgrade to the 6 m aperture Atacama Cosmology Telescope (ACT), which increased detector count and frequency coverage compared to the previous ACTPol receiver. Advanced ACTPol utilizes a new two-stage time-division multiplexing readout architecture based on superconducting quantum interference devices to achieve a multiplexing factor as high as 64 (rows), fielding a 2012 detector camera at 150/220 GHz and two 90/150 GHz cameras containing 1716 detectors each. In a time-domain system, aliasing introduces noise to the readout. In this work we present a figure of merit to measure this noise contribution and present measurements of the aliased noise fraction of the Advanced ACTPol receiver as deployed. [ABSTRACT FROM AUTHOR]

Micro-X is a sounding rocket-borne instrument that uses a microcalorimeter array to perform high-resolution X-ray spectroscopy. This instrument flew for the first time on July 22nd, 2018, from the White Sands Missile Range, USA. This flight marks the first successful operation of a transition-edge sensor array and its time-division multiplexing readout system in space. This launch was dedicated to the observation of the supernova remnant Cassiopeia A. A failure in the attitude control system prevented the rocket from pointing and led to no time on target. The onboard calibration source provided X-rays in flight, and it is used to compare detector performance during preflight integration, flight, and after the successful post-flight recovery. This calibration data demonstrate the capabilities of the detector in a space environment as well as its potential for future flights. [ABSTRACT FROM AUTHOR]

We use narrow spectral lines from the X-ray spectra of various highly charged ions to measure low-energy tail-like deviations from a Gaussian response function in a microcalorimeter X-ray spectrometer with Au absorbers at energies from 650 to 3320 eV. We review the literature on low-energy tails in other microcalorimeter X-ray spectrometers and present a model that explains all the reviewed tail fraction measurements. In this model, a low-energy tail arises from the combination of electron escape and energy trapping associated with Bi X-ray absorbers. [ABSTRACT FROM AUTHOR]

Time division multiplexing (TDM) uses a digital flux-locked loop (DFLL) to linearize each first-stage SQUID amplifier. Presently, the dynamic range of our TDM systems is limited by the use of a proportional-integral controller to maintain the DFLL. In this paper, we use simulations to assess the improvements possible with a predictive control algorithm that anticipates rapid changes in transition-edge sensor current during the rising edge of an X-ray pulse. We calculate that the predictive control algorithm can improve our TDM architecture's dynamic range by 35%. This significant increase in multiplexing capabilities could be used to read out higher-energy X-rays, reduce readout noise, increase multiplexing factors, or reduce SQUID power output. [ABSTRACT FROM AUTHOR]

We report on the development of a polarization-sensitive dichroic (150/220 GHz) detector array for the Cosmology Large Angular Scale Surveyor (CLASS) delivered to the telescope site in June 2019. In concert with existing 40 and 90 GHz telescopes, the 150/220 GHz telescope will make observations of the cosmic microwave background over large angular scales aimed at measuring the primordial B-mode signal, the optical depth to reionization, and other fundamental physics and cosmology. The 150/220 GHz focal plane array consists of three detector modules with 1020 transition edge sensor bolometers in total. Each dual-polarization pixel on the focal plane contains four bolometers to measure the two linear polarization states at 150 and 220 GHz. Light is coupled through a planar orthomode transducer fed by a smooth-walled feedhorn array made from an aluminum–silicon alloy (CE7). In this work, we discuss the design, assembly, and in-laboratory characterization of the 150/220 GHz detector array. The detectors are photon-noise limited, and we estimate the total array noise-equivalent power to be 2.5 and 4 aW s for 150 and 220 GHz arrays, respectively. [ABSTRACT FROM AUTHOR]

Microwave SQUID multiplexing has become a key technology for reading out large arrays of X-ray and gamma-ray microcalorimeters with mux factors of 100 or more. The desire for fast X-ray pulses that accommodate photon counting rates of hundreds or thousands of counts per second per sensor drives system design toward high sensor current slew rate. Typically, readout of high current slew rate events is accomplished by increasing the sampling rate, such that rates of order 1 MHz may be necessary for some experiments. In our microwave multiplexed readout scheme, the effective sampling rate is set by the frequency of the flux-ramp modulation ( f r ) used to linearize the SQUID response. The maximum current slew rate between samples is then nominally Φ 0 f r / 2 M in (where M in is the input coupling) because it is generally not possible to distinguish phase shifts of > π from negative phase shifts of < - π . However, during a pulse, we know which direction the current ought to be slewing, and this makes it possible to reconstruct a pulse where the magnitude of the phase shift between samples is > π . We describe a practical algorithm to identify and reconstruct pulses that exceed this nominal slew rate limit on the rising edge. Using pulses produced by X-ray transition-edge sensors, we find that the pulse reconstruction has a negligible impact on energy resolution compared to arrival time effects induced by under-sampling the rising edge. This technique can increase the effective slew rate limit by more than a factor of two, thereby either reducing the resonator bandwidth required or extending the energy range of measurable photons. The extra margin could also be used to improve crosstalk or to decrease readout noise. [ABSTRACT FROM AUTHOR]

We report on the design, commissioning, and initial measurements of a Transition-Edge Sensor (TES) x-ray spectrometer for the Electron Beam Ion Trap (EBIT) at the National Institute of Standards and Technology (NIST). Over the past few decades, the NIST EBIT has produced numerous studies of highly charged ions in diverse fields such as atomic physics, plasma spectroscopy, and laboratory astrophysics. The newly commissioned NIST EBIT TES Spectrometer (NETS) improves the measurement capabilities of the EBIT through a combination of high x-ray collection efficiency and resolving power. NETS utilizes 192 individual TES x-ray microcalorimeters (166/192 yield) to improve upon the collection area by a factor of ∼30 over the 4-pixel neutron transmutation doped germanium-based microcalorimeter spectrometer previously used at the NIST EBIT. The NETS microcalorimeters are optimized for the x-ray energies from roughly 500 eV to 8000 eV and achieve an energy resolution of 3.7 eV–5.0 eV over this range, a more modest (<2×) improvement over the previous microcalorimeters. Beyond this energy range, NETS can operate with various trade-offs, the most significant of which are reduced efficiency at lower energies and being limited to a subset of the pixels at higher energies. As an initial demonstration of the capabilities of NETS, we measured transitions in He-like and H-like O, Ne, and Ar as well as Ni-like W. We detail the energy calibration and data analysis techniques used to transform detector counts into x-ray spectra, a process that will be the basis for analyzing future data. [ABSTRACT FROM AUTHOR]

Micro-X is a sounding rocket-borne X-ray telescope that uses a transition edge sensor microcalorimeter array to provide high-energy-resolution spectroscopy. Micro-X is a versatile instrument with plans to observe the Puppis A supernova remnant during its first flight, as well as future observations of the Milky Way to search for X-ray signals from decaying dark matter. Commissioning and functionality testing are complete, and the thermal performance of the system has been validated. We are currently evaluating the detector performance in the flight cryostat with the flight multiplexing electronics. Operating in this setup has allowed us to characterize sources of detector and readout noise, as well as to implement mitigation techniques to improve performance in anticipation of the upcoming flight. We present an overview of important noise considerations in addition to an update on latest detector performance. [ABSTRACT FROM AUTHOR]

The readout requirements for instruments based on transition-edge sensors (TESs) have dramatically increased over the last decade as demand for systems with larger arrays and faster sensors has grown. Emerging systems are expected to contain many thousands of sensors and/or sensors with time constants as short as 100 ms. These requirements must be satisfied while maintaining low noise, high dynamic range, and low crosstalk. A promising readout candidate for future TES arrays is the microwave SQUID multiplexer, which offers several gigahertz of readout bandwidth per pair of coaxial cables. In microwave SQUID multiplexing, sensor signals are coupled to RF-SQUIDs embedded in superconducting microwave resonators, which are probed via a common microwave feedline and read out using gigahertz signals. This form of SQUID multiplexing moves complexity from the cryogenic stages to room temperature hardware and digital signal processing firmware which must synthesize the microwave tones and process the information contained within them. To demultiplex signals from the microwave SQUID multiplexer, we have implemented an FPGA-based firmware architecture that is flexible enough to read out a variety of differently optimized TESs. A gamma-ray spectrometer targeted at nuclear materials accounting applications, known as SLEDGEHAMMER, is an early adopter of microwave SQUID multiplexing and is driving our current firmware development effort. This instrument utilizes 300 kHz full-width half-maximum resonators with 256 channels in a one gigahertz wide band. We have recently demonstrated undegraded readout of 128 channels using two ROACH2s on a single pair of coaxial cables. This manuscript describes the firmware implementation for the readout electronics of these early array-scale demonstrations. [ABSTRACT FROM AUTHOR]

Transition edge sensors (TESs) have proven to be highly sensitive and versatile X-ray spectrometers. Upcoming missions, including Athena X-IFU, will rely on highly multiplexed focal planes where more than 32 TES pixels are read out using a single SQUID amplifier channel. We have implemented an error correction algorithm for code-division multiplexed TES signals that can compensate for potential failures of individual SQUID readout channels and that is both scalable and easily implemented in hardware. We present this algorithm for error correction and show results of laboratory tests to assess algorithm performance at recovering TES channels after a SQUID failure. [ABSTRACT FROM AUTHOR]

In the next decade, new ground-based cosmic microwave background (CMB) experiments such as Simons Observatory, CCAT-prime, and CMB-S4 will increase the number of detectors observing the CMB by an order of magnitude or more, dramatically improving our understanding of cosmology and astrophysics. These projects will deploy receivers with as many as hundreds of thousands of transition edge sensor (TES) bolometers coupled to superconducting quantum interference device (SQUID)-based readout systems. It is well known that superconducting devices such as TESes and SQUIDs are sensitive to magnetic fields. However, the effects of magnetic fields on TESes are not easily predicted due to the complex behavior of the superconducting transition, which motivates direct measurements of the magnetic sensitivity of these devices. We present comparative four-lead measurements of the critical temperature versus applied magnetic field of AlMn TESes varying in geometry, doping, and leg length, including Advanced ACT and POLARBEAR-2/Simons Array bolometers. MoCu ACTPol TESes are also tested and are found to be more sensitive to magnetic fields than the AlMn devices. We present an observation of weak-link-like behavior in AlMn TESes at low critical currents. We also compare measurements of magnetic sensitivity for time division multiplexing SQUIDs and frequency division multiplexing microwave (μMUX) rf-SQUIDs. We discuss the implications of our measurements on the magnetic shielding required for future experiments that aim to map the CMB to near-fundamental limits. [ABSTRACT FROM AUTHOR]

We performed small-scale demonstrations at GSFC of high-resolution X-ray TES microcalorimeters read out using a microwave SQUID multiplexer. This work is part of our effort to develop detector and readout technologies for future space-based X-ray instruments such as the microcalorimeter spectrometer envisaged for Lynx, a large mission concept under development for the Astro 2020 Decadal Survey. In this paper we describe our experiment, including details of a recently designed, microwave-optimized low-temperature setup that is thermally anchored to the 55 mK stage of our laboratory ADR. Using a ROACH2 FPGA at room temperature, we read out pixels of a GSFC-built detector array via a NIST-built multiplexer chip with Nb coplanar waveguide resonators coupled to rf-SQUIDs. The resonators are spaced 6 MHz apart (at ∼ 5.9 GHz) and have quality factors of ∼ 15,000. In our initial demonstration, we used flux-ramp modulation frequencies of 125 kHz to read out 5 pixels simultaneously and achieved spectral resolutions of 2.8-3.1 eV FWHM at 5.9 keV. Our subsequent work is ongoing: to-date we have achieved a median spectral resolution of 3.4 eV FWHM at 5.9 keV while reading out 28 pixels simultaneously with flux-ramp frequencies of 160 kHz. We present the measured system-level noise and maximum slew rates and briefly describe our future development work. [ABSTRACT FROM AUTHOR]

Key performance characteristics are demonstrated for the microwave superconducting quantum interference device (SQUID) multiplexer (μmux) coupled to transition edge sensor (TES) bolometers that have been optimized for cosmic microwave background (CMB) observations. In a 64-channel demonstration, we show that the μmux produces a white, input referred current noise level of 29 pA at a microwave probe tone power of -77 dB, which is well below the expected fundamental detector and photon noise sources for a ground-based CMB-optimized bolometer. Operated with negligible photon loading, we measure 98 pA in the TES-coupled channels biased at 65% of the sensor normal resistance. This noise level is consistent with that predicted from bolometer thermal fluctuation (i.e., phonon) noise. Furthermore, the power spectral density is white over a range of frequencies down to ~100mHz, which enables CMB mapping on large angular scales that constrain the physics of inflation. Additionally, we report cross-talk measurements that indicate a level below 0.3%, which is less than the level of cross-talk from multiplexed readout systems in deployed CMB imagers. These measurements demonstrate the μmux as a viable readout technique for future CMB imaging instruments. [ABSTRACT FROM AUTHOR]

The number of elements in most cryogenic sensor arrays is limited by the technology available to multiplex signals from the arrays into a smaller number of wires and readout amplifiers. The largest demonstrated arrays of transition-edge sensor (TES) microcalorimeters contain roughly 250 detectors and use time-division multiplexing with Superconducting Quantum Interference Devices (SQUIDs). The bandwidth limits of this technology constrain the number of sensors per amplifier chain, a quantity known as the multiplexing factor, to several 10s. With microwave SQUID multiplexing, we can expand the readout bandwidth and enable much larger multiplexing factors. While microwave SQUID multiplexing of TES microcalorimeters has been previously demonstrated with small numbers of detectors, we now present a fully scalable demonstration in which 128 TES detectors are read out on a single pair of coaxial cables. [ABSTRACT FROM AUTHOR]

The so-called “optimal filter” analysis of a microcalorimeter's x-ray pulses is statistically optimal only if all pulses have the same shape, regardless of energy. However, the shapes of pulses from a nonlinear detector can and do depend on the pulse energy. A pulse-fitting procedure that we call “tangent filtering” accounts for the energy dependence of the shape and should, therefore, achieve superior energy resolution. We take a geometric view of the pulse-fitting problem and give expressions to predict how much the energy resolution stands to benefit from such a procedure. We also demonstrate the method with a case study of K-line fluorescence from several 3d transition metals. The method improves the resolution from 4.9 to 4.2 eV at the Cu K $\alpha$ line (8.0 keV). [ABSTRACT FROM PUBLISHER]

In superconducting transition-edge sensor X-ray detectors, we observe that as the thermal conductance (G) to the heat bath increases, the resistive transition broadens. Consequently, the sensitivity of films to deposited energy worsens. Using a two-fluid model for the superconducting-to-normal transition in a thin film, we show that this broadening can be attributed to the larger current (I0) necessary for biasing the film at a given point in the transition for higher-G devices, resulting in a higher Ic0/I0 ratio (Ic0 is the film's critical current at zero temperature). To recover a sharper transition, we fabricated rectangular films with varying numbers of internal normal-metal structures while keeping G constant, allowing the independent variation of both I0 and Ic0. We show that it is possible to manipulate the transition width and G independently, thus enabling fast thermal sensors with an excellent energy resolution. [ABSTRACT FROM AUTHOR]

We describe a series of microcalorimeter X-ray spectrometers designed for a broad suite of measurement applications. The chief advantage of this type of spectrometer is that it can be orders of magnitude more efficient at collecting X-rays than more traditional high-resolution spectrometers that rely on wavelength-dispersive techniques. This advantage is most useful in applications that are traditionally photon-starved and/or involve radiation-sensitive samples. Each energy-dispersive spectrometer is built around an array of several hundred transition-edge sensors (TESs). TESs are superconducting thin films that are biased into their superconducting-to-normal-metal transitions. The spectrometers share a common readout architecture and many design elements, such as a compact, 65 mK detector package, 8-column time-division-multiplexed superconducting quantum-interference device readout, and a liquid-cryogen-free cryogenic system that is a two-stage adiabatic-demagnetization refrigerator backed by a pulse-tube cryocooler. We have adapted this flexible architecture to mate to a variety of sample chambers and measurement systems that encompass a range of observing geometries. There are two different types of TES pixels employed. The first, designed for X-ray energies below 10 keV, has a best demonstrated energy resolution of 2.1 eV (full-width-at-half-maximum or FWHM) at 5.9 keV. The second, designed for X-ray energies below 2 keV, has a best demonstrated resolution of 1.0 eV (FWHM) at 500 eV. Our team has now deployed seven of these X-ray spectrometers to a variety of light sources, accelerator facilities, and laboratory-scale experiments; these seven spectrometers have already performed measurements related to their applications. Another five of these spectrometers will come online in the near future. We have applied our TES spectrometers to the following measurement applications: synchrotron-based absorption and emission spectroscopy and energy-resolved scattering; accelerator-based spectroscopy of hadronic atoms and particle-induced-emission spectroscopy; laboratory-based time-resolved absorption and emission spectroscopy with a tabletop, broadband source; and laboratory-based metrology of X-ray-emission lines. Here, we discuss the design, construction, and operation of our TES spectrometers and show first-light measurements from the various systems. Finally, because X-ray-TES technology continues to mature, we discuss improvements to array size, energy resolution, and counting speed that we anticipate in our next generation of TES-X-ray spectrometers and beyond. [ABSTRACT FROM AUTHOR]

Code-division multiplexing (CDM) offers a path to reading out large arrays of transition edge sensor (TES) X-ray microcalorimeters with excellent energy and timing resolution. We demonstrate the readout of X-ray TESs with a 32-channel flux-summed code-division multiplexing circuit based on superconducting quantum interference device (SQUID) amplifiers. The best detector has energy resolution of 2.28 ± 0.12 eV FWHM at 5.9 keV and the array has mean energy resolution of 2.77 ± 0.02 eV over 30 working sensors. The readout channels are sampled sequentially at 160 ns/row, for an effective sampling rate of 5.12 μ/channel. The SQUID amplifiers have a measured flux noise of 0:17 μΦ0/√Hz (non-multiplexed, referred to the first stage SQUID). The multiplexed noise level and signal slew rate are sufficient to allow readout of more than 40 pixels per column, making CDM compatible with requirements outlined for future space missions. Additionally, because the modulated data from the 32 SQUID readout channels provide information on each X-ray event at the row rate, our CDM architecture allows determination of the arrival time of an X-ray event to within 275 ns FWHM with potential benefits in experiments that require detection of near-coincident events. [ABSTRACT FROM AUTHOR]

Time-division multiplexing (TDM) is a mature scheme for the readout of arrays of transition-edge sensors (TESs). TDM is based on superconducting-quantum-interference-device (SQUID) current amplifiers. Multiple spectrometers based on gamma-ray and X-ray microcalorimeters have been operated with TDM readout, each at the scale of 200 sensors per spectrometer, as have several astronomical cameras with thousands of sub-mm or microwave bolometers. Here we present the details of two different versions of our TDM system designed to read out X-ray TESs. The first has been field-deployed in two 160-sensor (8 columns $$\times $$ 20 rows) spectrometers and four 240-sensor (8 columns $$\times $$ 30 rows) spectrometers. It has a three-SQUID-stage architecture, switches rows every 320 ns, and has total readout noise of 0.41 $$\mu \Phi _{\mathrm {0}} / \surd $$ Hz. The second, which is presently under development, has a two-SQUID-stage architecture, switches rows every 160 ns, and has total readout noise of 0.19 $$\mu \Phi _{\mathrm {0}} / \surd $$ Hz. Both quoted noise values are non-multiplexed and referred to the first-stage SQUID. In a demonstration of this new architecture, a multiplexed 1-column $$\times $$ 32-row array of NIST TESs achieved average energy resolution of $$2.55\pm 0.01$$ eV at 6 keV. [ABSTRACT FROM AUTHOR]

X-ray emission spectroscopy (XES) is a powerful element-selective tool to analyze the oxidation states of atoms in complex compounds, determine their electronic configuration, and identify unknown compounds in challenging environments. Until now the low efficiency of wavelength-dispersive X-ray spectrometer technology has limited the use of XES, especially in combination with weaker laboratory X-ray sources. More efficient energy-dispersive detectors have either insufficient energy resolution because of the statistical limits described by Fano or too low counting rates to be of practical use. This paper updates an approach to high-resolution X-ray emission spectroscopy that uses a microcalorimeter detector array of superconducting transition-edge sensors (TESs). TES arrays are discussed and compared with conventional methods, and shown under which circumstances they are superior. It is also shown that a TES array can be integrated into a table-top time-resolved X-ray source and a soft X-ray synchrotron beamline to perform emission spectroscopy with good chemical sensitivity over a very wide range of energies. [ABSTRACT FROM AUTHOR]

In this paper we present a new measurement setup, where a transition-edge sensor detector array is used to detect X-rays in particle induced X-ray emission (PIXE) measurements with a 2 MeV proton beam. Transition-edge sensors offer orders of magnitude improvement in energy resolution compared to conventional silicon or germanium detectors, making it possible to recognize spectral lines in materials analysis that have previously been impossible to resolve, and to get chemical information from the elements. Our sensors are cooled to the operation temperature ( $$\sim $$ 65 mK) with a cryogen-free adiabatic demagnetization refrigerator, which houses a specially designed X-ray snout that has a vacuum tight window to couple in the radiation. For the best pixel, the measured instrumental energy resolution was 3.06 eV full width at half maximum at 5.9 keV. We discuss the current status of the project, benefits of transition-edge sensors when used in PIXE spectroscopy, and the results from the first measurements. [ABSTRACT FROM AUTHOR]

This work presents an x-ray absorption measurement by use of ionizing radiation generated by a femtosecond pulsed laser source. The spectrometer was a microcalorimetric array whose pixels are capable of accurately measuring energies of individual radiation quanta. An isotropic continuum x-ray spectrum in the few-keV range was generated from a laser plasma source with a water-jet target. X rays were transmitted through a ferrocene powder sample to the detector, whose pixels have average photon energy resolution ΔE=3.14 eV full-width-at-half-maximum at 5.9 keV. The bond distance of ferrocene was retrieved from this first hard-x-ray absorption fine-structure spectrum collected with an energy-dispersive detector. This technique will be broadly enabling for time-resolved observations of structural dynamics in photoactive systems. [ABSTRACT FROM AUTHOR]

Multiplexers based on the modulation of superconducting quantum interference devices are now regularly used in multi-kilopixel arrays of superconducting detectors for astrophysics, cosmology, and materials analysis. Over the next decade, much larger arrays will be needed. These larger arrays require new modulation techniques and compact multiplexer elements that fit within each pixel. We present a new in-focal-plane code-division multiplexer that provides multiplexing elements with the required scalability. This code-division multiplexer uses compact lithographic modulation elements that simultaneously multiplex both signal outputs and superconducting transition-edge sensor (TES) detector bias voltages. It eliminates the shunt resistor used to voltage bias TES detectors, greatly reduces power dissipation, allows different dc bias voltages for each TES, and makes all elements sufficiently compact to fit inside the detector pixel area. These in-focal plane code-division multiplexers can be combined with multi-GHz readout based on superconducting microresonators to scale to even larger arrays. [ABSTRACT FROM AUTHOR]

A transition-edge-sensor (TES) microcalorimeter's shunt resistor ( R) and thermal conductance to the cryogenic bath ( G) are often considered to be interchangeable knobs with which to control detector speed. Indeed, for otherwise-identical TES-parameter models, there are many combinations of R and G that give the same decay time-constant ( τ). However, our previous work showed that with time- or code-division-multiplexed readout, the distribution of signal-to-noise ratio with frequency, which depends strongly on R and G, is just as important as τ. Here, we present a set of calculations to select the optimal values of R and G, given a linear TES model and count-rate and energy-resolution requirements. Lower G and lower R make multiplexing easier. Our calculations also determine the allowed combination of SQUID-readout noise ( S) and multiplexer row-period ( t) and row-count ( N). Recent improvements to S and t in the NIST time-division-multiplexing architecture have allowed a NIST eight-pixel TES array to be read out with 2.70 eV (full-width at half-maximum) average energy resolution at 6 keV. The improvements make the X-ray Microcalorimeter Spectrometer co-proposed by NASA and NIST for ESA's Athena X-ray observatory straightforwardly achievable, including engineering margin, with N=16. [ABSTRACT FROM AUTHOR]

Time-division SQUID multiplexers are used in many applications that require exquisite control of systematic error. One potential source of systematic error is the pickup of external magnetic fields in the multiplexer. We present measurements of the field sensitivity figure of merit, effective area, for both the first stage and second stage SQUID amplifiers in three NIST SQUID multiplexer designs. These designs include a new variety with improved gradiometry that significantly reduces the effective area of both the first and second stage SQUID amplifiers. [ABSTRACT FROM AUTHOR]

The absolute neutrino mass scale is still an outstanding challenge in both particle physics and cosmology. The calorimetric measurement of the energy released in a nuclear beta decay is a powerful tool to determine the effective electron-neutrino mass. In the last years, the progress on low temperature detector technologies has allowed to design large scale experiments aiming at pushing down the sensitivity on the neutrino mass below 1 eV. Even with outstanding performances in both energy (~ eV on keV) and time resolution (~ 1 μs) on the single channel, a large number of detectors working in parallel is required to reach a sub-eV sensitivity. Microwave frequency domain readout is the best available technique to readout large array of low temperature detectors, such as Transition Edge Sensors (TESs) or Microwave Kinetic Inductance Detectors (MKIDs). In this way a multiplex factor of the order of thousands can be reached, limited only by the bandwidth of the available commercial fast digitizers. This microwave multiplexing system will be used to readout the HOLMES detectors, an array of 1000 microcalorimeters based on TES sensors in which the 163 Ho will be implanted. HOLMES is a new experiment for measuring the electron neutrino mass by means of the electron capture (EC) decay of 163 Ho. We present here the microwave frequency multiplex which will be used in the HOLMES experiment and the microwave frequency multiplex used to readout the MKID detectors developed in Milan as well. [ABSTRACT FROM AUTHOR]

Single-pixel transition-edge sensors (TES) are useful for a variety of applications requiring the detection of photons from sub-millimeter wavelengths to gamma rays. Arrays of TES's are required in the next-generation instruments to continue to be useful. We report on the development and performance of an electronics system used to readout and process signals from arrays of TES detectors. The system utilizes high-performance analog and digital electronics, and digital signal-processing. Because the system digitizes signals early in the signal processing chain, it is possible to implement a digital-feedback system to flux lock a SQUID amplifier and digitally process the detector (feedback) signal. [ABSTRACT FROM AUTHOR]

We demonstrate the code-division multiplexing (CDM) readout of eight transition-edge sensor microcalorimeters. The energy resolution is 3.0 eV (full width at half-maximum) or better at 5.9 keV, with a best resolution of 2.3 eV and a mean of 2.6 eV over the seven modulated detectors. The flux-summing CDM system is described and compared with similar time-division multiplexing (TDM) readout. We show that the [formula] multiplexing disadvantage associated with TDM is not present in CDM. This demonstration establishes CDM as both a simple route to higher performance in existing TDM microcalorimetric experiments and a long-term approach to reaching higher multiplexing factors. [ABSTRACT FROM AUTHOR]

Multiplexed superconducting quantum interference device (SQUID) readout systems are critical for measuring large arrays of superconducting transition-edge sensors (TES). We demonstrate a code-division SQUID multiplexing (CDM) architecture that is modulated by Walsh codes. Measurements and simulations of a prototype multiplexer show that this modulation scheme is not degraded by SQUID-noise aliasing, suppresses parasitic pickup, and has low levels of crosstalk. These properties enable this architecture to scale to large TES arrays. Furthermore, CDM modulation suppresses the 1/f knee in the noise to below 20 mHz, suggesting the use of this circuit for low-frequency-noise mitigation in more general SQUID applications. [ABSTRACT FROM AUTHOR]

The authors present a prototype for a high-energy-resolution, high-count-rate, gamma-ray spectrometer intended for nuclear forensics and international nuclear safeguards. The prototype spectrometer is an array of 14 transition-edge-sensor microcalorimeters with an average energy resolution of 47 eV (full width at half maximum) at 103 keV. The resolution of the best pixel is 25 eV. A cryogenic, time-division multiplexer reads out the array. Several important topics related to microcalorimeter arrays are discussed, including cross-talk, the uniformity of detector bias conditions, fabrication of the arrays, and the multiplexed readout. The measurements and calculations demonstrate that a kilopixel array of high-resolution microcalorimeters is feasible. [ABSTRACT FROM AUTHOR]

The authors describe a microcalorimeter γ-ray detector with measured energy resolution of 42 eV full width at half maximum for 103 keV photons. This detector consists of a thermally isolated superconducting transition-edge thermometer and a superconducting bulk tin photon absorber. The absorber is attached with a technique compatible with producing arrays of high-resolution γ-ray detectors. The results of a detailed characterization of the detector, which includes measurements of the complex impedance, detector noise, and time-domain pulse response, suggest that a deeper understanding and optimization of the thermal transport between the absorber and thermometer could significantly improve the energy resolution of future detectors. [ABSTRACT FROM AUTHOR]

We present measurements from a series of transition-edge x-ray microcalorimeters designed for optimal energy resolution. We used the geometry of the sensors to control their heat capacity and employed additional normal metal features and a perpendicular magnetic field to control the sharpness of the superconducting-to-normal transition. These degrees of control allow an optimal selection of sensor saturation energy and noise. Successive design changes improved the measured energy resolution of the sensors from 4.5 eV full width at half maximum at 5.9 keV to 2.4 eV at 5.9 keV. Sensors with this energy resolution are well matched to applications in x-ray astrophysics and terrestrial materials analysis. [ABSTRACT FROM AUTHOR]

We present experimental results from a four-pixel array of transition-edge-sensor, x-ray microcalorimeters read out through a single amplifier channel via a time-division superconducting quantum interference device multiplexer. We map the dependence of the x-ray energy resolution of the microcalorimeters on multiplexer timing parameters. We achieve multiplexed, four-pixel resolution of 6.94±0.05 eV full width at half maximum of the Mn Kα complex near 5.9 keV, which is a degradation of only 0.44 eV from nonmultiplexed operation. An analysis of straightforward upgrades to the multiplexer predicts that a linear array of 32 of these pixels could he multiplexed with a degradation in resolution of only 0.1 eV. These results, the first demonstration of a time-division multiplexer for x-ray detectors, establish a clear path to the instrumentation of a kilopixel microcalorimeter array. [ABSTRACT FROM AUTHOR]

The noise in superconducting transition-edge sensors (TESs) commonly exceeds simple theoretical predictions. The reason for this discrepancy is presently unexplained. We have measured the amplitude and frequency dependence of the noise in TES sensors with eight different geometries. In addition, we have measured the dependence of the noise on operating resistance, perpendicular magnetic field, and bath temperature. We find that the unexplained noise contribution is inversely correlated with the temperature width of the superconducting-to-normal transition and is reduced by a perpendicular field and in certain geometries. These results suggest paths to improved sensor performance. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present the first performance results obtained with microwave multiplexed Transition Edge Sensors prototypes specifically designed for the HOLMES experiment, a project aimed at directly measuring the electron neutrino mass through the calorimetric measurement of the 163 Ho electron capture spectrum. The detectors required for such an experiment feature a high energy resolution at the Q–value of the transition, around ∼ 2.8 keV, and a fast response time combined with the compatibility to be multiplexed in large arrays in order to collect a large statistics while keeping the pile-up contribution as small as possible. In addition, the design has to be suitable for future ion-implantation of 163 Ho. The results obtained in these tests allowed us to identify the optimal detector design among several prototypes. The chosen detector achieved an energy resolution of (4.5 ± 0.3) eV on the chlorine K α line, at ∼ 2.6 keV, obtained with an exponential rise time of 14 μ s. The achievements described in this paper pose a milestone for the HOLMES detectors, setting a baseline for the subsequent developments, aiming to the actual ion-implantation of the 163 Ho nuclei. In the first section the HOLMES experiment is outlined along with its physics goal, while in the second section the HOLMES detectors are described; the experimental set-up and the calibration source used for the measurements described in this paper are reported in Sects. 3 and 4, respectively; finally, the details of the data analysis and the results obtained are reported in Sect. 6. [ABSTRACT FROM AUTHOR]