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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]

We describe an indium bump process applicable to our low-temperature detectors and associated readout. A titanium nitride under bump metallization layer is reactively sputtered onto wiring pads as a diffusion barrier and adhesion layer. Indium is thermally evaporated onto this layer at the desired thickness for bump bonding. Patterning for both titanium nitride and indium layers is accomplished by liftoff techniques. The process flow is compatible with many processes utilized for the fabrication of low-temperature detectors and other superconductive devices. [ABSTRACT FROM AUTHOR]

Feature sizes in integrated circuits have decreased substantially over time, and it has become increasingly difficult to three-dimensionally image these complex circuits after fabrication. This can be important for process development, defect analysis, and detection of unexpected structures in externally sourced chips, among other applications. Here, we report on a non-destructive, tabletop approach that addresses this imaging problem through x-ray tomography, which we uniquely realize with an instrument that combines a scanning electron microscope (SEM) with a transition-edge sensor (TES) x-ray spectrometer. Our approach uses the highly focused SEM electron beam to generate a small x-ray generation region in a carefully designed target layer that is placed over the sample being tested. With the high collection efficiency and resolving power of a TES spectrometer, we can isolate x-rays generated in the target from background and trace their paths through regions of interest in the sample layers, providing information about the various materials along the x-ray paths through their attenuation functions. We have recently demonstrated our approach using a 240 Mo/Cu bilayer TES prototype instrument on a simplified test sample containing features with sizes of ~ 1 μm. Currently, we are designing and building a 3000 Mo/Au bilayer TES spectrometer upgrade, which is expected to improve the imaging speed by factor of up to 60 through a combination of increased detector number and detector speed. [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]

The development of a superconducting analog to the transistor with extremely low power dissipation will accelerate the proliferation of low-temperature circuitry operating in the milliKelvin regime. The thin-film, magnetically actuated cryotron switch is a candidate building block for more complicated and flexible milliKelvin circuitry. We demonstrate its utility for implementing reconfigurable circuitry by integrating a cryotron switch into flux-summed code-division SQUID multiplexed readout for large arrays of transition-edge-sensor (TES) microcalorimeters. Code-division multiplexing eliminates the noise penalty of time-division multiplexing while being drop-in compatible with the latter's control electronics. However, code-division multiplexing is susceptible to single-point failure mechanisms which can result in an unconstrained demodulation matrix and the loss of information from many sensing elements. In the event of a failure, the integrated cryotron switch provides a zero-signal output from a single TES, enabling the demodulation matrix used to compute TES signals from SQUID signals to be constrained and data recovered from the remaining sensors. This demonstration of configurable error correction provides both a real-world application of the cryotron switch and a foundation for more complex circuitry at milliKelvin temperatures. [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]

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]

We are developing code-division multiplexing (CDM) systems for TES arrays with the goal of reaching multiplexing factors in the hundreds. We report on x-ray measurements made with a four-channel prototype CDM system that employs a flux-summing architecture, emphasizing data-analysis issues. We describe an empirical method to determine the demodulation matrix that minimizes cross-talk. This CDM system achieves energy resolutions of between 2.3 eV and 3.0 eV FWHM at 5.9 keV. [ABSTRACT FROM AUTHOR]

We report here a real-time pulse processing algorithm for superconducting transition-edge sensor (TES) based x-ray microcalorimeters. TES-based microcalorimeters offer ultra-high energy resolutions, but the small volume of each pixel requires that large arrays of identical microcalorimeter pixels be built to achieve sufficient detection efficiency. That in turn requires as much pulse processing as possible must be performed at the front end of readout electronics to avoid transferring large amounts of data to a host computer for post-processing. Therefore, a real-time pulse processing algorithm that not only can be implemented in the readout electronics but also achieve satisfactory energy resolutions is desired. We have developed an algorithm that can be easily implemented in hardware. We then tested the algorithm offline using several data sets acquired with an 8\times8 Goddard TES x-ray calorimeter array and 2\times16 NIST time-division SQUID multiplexer. We obtained an average energy resolution of close to 3.0 eV at 6 keV for the multiplexed pixels while preserving over 99% of the events in the data sets. [ABSTRACT FROM AUTHOR]

Microcalorimeter detectors provide superior energy resolution for the detection of X-rays and gamma-rays. The technology utilizes a cryogenic transition-edge sensor (TES) coupled to a tin bulk absorber. We are working on fabrication methods for the production of arrays with many sensors. In this paper, we present data collected with an array of microcalorimeters using as many as 26 sensor elements simultaneously. Advances in sensor design have extended the useful dynamic range to photon energies up to ∼200 keV, while providing resolution performance in the 80–90 eV FWHM range, significantly better than planar high-purity germanium. These sensor arrays have applications in the measurement of nuclear materials. We present data collected from 153Gd, a highly-enriched uranium sample, and a plutonium isotopic standard source. We also demonstrate clean separation of the 235U 185.715 keV peak from the ubiquitous 226Ra 186.211 keV background peak interference. [ABSTRACT FROM AUTHOR]

We discuss recent developments in using cryogenic microcalorimeter detectors for x- and gamma-ray spectroscopy. We are currently operating a detector array consisting of thirteen pixels with time-domain multiplexed readout. With a single pixel from this detector, we have measured 97.43-keV gamma rays from 153-Gd with 22-eV resolution (FWHM). We have also made the first multiplexed array measurements of plutonium x- and gamma-rays with 45-eV resolution. We are currently testing a 66-pixel next-generation detector chip. Preliminary measurements with the new detector indicate improved energy linearity and single-pixel energy resolution of 50-100 eV at 100 keV. We present preliminary calibration data from this chip, and a high-statistics multiplexed 21-pixel spectrum of the Pu x-ray region between 90 and 130 keV. [ABSTRACT FROM AUTHOR]

Micro-X is a sounding-rocket experiment that will combine a transition edge sensor (TES) microcalorimeter array with an imaging mirror to obtain high-spectral-resolution images of astronomical X-ray sources. The instrument's resolution across the 0.3-2.5 keV band will be 2 eV. The first flight will target the region of the Bright Eastern Knot of the Puppis A supernova remnant and is slated for January 2011. The obtained high-resolution X-ray spectra will be used to ascertain the temperature and ionization state of the X-ray-emitting gas and to determine its velocity structure. The TES array is read out bya time-division superconducting quantum interference device (SQUID) multiplexing system. The detector front end assembly and the SQUID multiplexing circuit are cooled to the operating temperature of 50 mK with an adiabatic demagnetization refrigerator (ADR). The design of this refrigerator is tailored to the requirements of rocket flight. Stable operation of the TES array close to the ADR magnet will be achieved with a magnetic shielding system, which will be based on a combination of a bucking coil and high-permeability and superconducting shield materials to cancel out residual fields. We describe our progress in developing the Micro-X instrument. [ABSTRACT FROM AUTHOR]

We present results from the largest array of gamma-ray microcalorimeters operated to date. The microcalorimeters consist of Mo/Cu transition-edge sensors with attached Sn absorbers. The detector array contains 66 pixels each with an active area 2.25 mm2. Out of 66 pixels, 55 are active, and 31 were used to acquire a high statistics Pu gamma-ray spectrum. The energy resolution of the best 21 pixels was found to be 120 eV FWHM. The array is read out using time domain SQUID multiplexing. Here, we describe the multiplexing and present a high statistics Pu spectra. Because of the large collecting area of our array, the statistical error in the 240Pu line intensity is approximately 0.7%, which is comparable to the systematic error in a measurement with a 500 eV germanium sensor. Hence, we have reached an important threshold for demonstrating improved isotopic measurements with microcalorimeter sensors. With straightforward changes, we plan to achieve a resolution of about 50 eV FWHM with 256 multiplexed detectors. Finally, we present early estimates of on-chip heating within our sensor array. [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 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]

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]