[en] We report recent progress at NIST on Mo/Cu Transition-Edge Sensors (TESs). While the signal-band noise of our sensors agrees with theory, we observe excess high-frequency noise. We describe this noise and demonstrate that it can be strongly suppressed by a magnetic field perpendicular to the plane of the sensor. Both the excess noise and α=(T/R)(dR/dT) depend strongly on field so our results show that accurate comparisons between devices are only possible when the field is well known or constant. We also present results showing the noise performance of TES designs incorporating parallel and perpendicular normal metal bars, an array of normal metal islands, and in wedge-shaped devices. We demonstrate significant reduction of high-frequency noise with the perpendicular bar devices at the cost of reduced α. Both the bars and the magnetic field are useful noise reduction techniques for bolometers

[en] We have measured the transport critical current density (J_c) in epitaxial quality films of YBa₂Cu₃O_7-δ some of which were covered by thin (10 nm) Ag films. The films, both with and without Ag, had J_c values greater than 10⁶ A/cm² in liquid nitrogen. The effect of the Ag was to greatly reduce the dependence of J_c on external magnetic fields in the case where the field was oriented in the plane of the film, that is, perpendicular to the c axis. It is unlikely that the effect is simply due to altered surface pinning, although qualitative agreement with critical state models is observed

[en] We report on the implementation of a multiplexer that uses superconducting quantum interference devices (SQUIDs) to read out low-impedance cryogenic detectors. Using prototype chips, a circuit was built which interfaces eight input SQUID channels with a close-packed array of eight transition-edge sensor (TES) infrared bolometers. Circuit elements were measured and crosstalk specifications are reported. Digital feedback is employed to flux-lock a single element in the array of SQUIDs

[en] We are developing arrays of Mo/Cu transition edge sensor-based detectors for use as X-ray microcalorimeters and sub-millimeter bolometers. We have fabricated 8x8 pixel X-ray microcalorimeter arrays using surface micromachining. Surface-micromachining techniques hold the promise of scalability to much larger arrays and may allow for the integration of in-plane multiplexer elements. In this paper we describe the surface micromachining process and recent improvements in the device geometry that provide for increased mechanical strength. We also present X-ray and heat pulse spectra collected using these detectors

[en] One challenge to using cryogenic detectors for X-ray spectroscopy on a scanning electron microscope is the implementation of infrared blocking filters. In order to achieve high X-ray transmission, these filters can be as thin as 250 nm and consequently are extremely fragile. To avoid breaking the filters, the cryostat must be evacuated slowly and by a trained operator. In this presentation, we describe the filter system currently used at NIST. In addition, we describe recent efforts to build a more robust and easy-to-use filter system. We present initial efforts to strengthen conventional aluminum-parylene filters with micromachined silicon grids that only reduce X-ray transmission by 2%. We also describe an automated pump-out system based on a commercial mass-flow controller

[en] Large-format (>1000 pixel) arrays of transition-edge sensors are being developed for applications ranging from X-ray materials analysis to submillimeter (SCUBA-2, ACT, SAFIRE) and X-ray (Constellation-X) astronomy. Cryogenic multiplexing allows instrumentation of these large detector arrays with a manageable number of wires and readout channels. We describe a SQUID-based, time-division multiplexer for X-ray microcalorimeter arrays, including a three-stage SQUID amplifier chain and room-temperature digital-feedback electronics. The prototype system has passed two milestone tests. First, energy resolution of 6.6 eV obtained from measuring 5.2 keV joule-heat pulses was not degraded by multiplexing 2, 4, or 8 biased detectors. Second, ⁵⁵Fe spectra obtained with four multiplexed detectors showed essentially the same resolution as those obtained from the same detectors while not multiplexed

[en] This paper reports on the behavior of high-quality YBa₂Cu₃O_7-x (YBC) thin films with Ag over-layers. The authors chose to study Ag in detail because of its widespread use as contact metallization and the author's earlier studies of proximity effects in YBC. The details of transport critical current measurements are presented. The Ag coatings can reduce normal state resistance while not degrading the critical current density, J_c

[en] The development of arrays of low-temperature detectors, including the superconducting transition-edge sensor (TES), has provided new capabilities for applications including astronomy, particle physics, and nuclear materials analysis. These applications require the implementation of large arrays of sensors. Due to constraints on cryogenic wiring heat load and complexity, it is impractical to route wires from room temperature to every pixel in a large low-temperature detector array. We have developed multiplexed readout circuits based on Superconducting Quantum Interference Devices (SQUIDs) to meet this need. We are implementing two SQUID multiplexer architectures: time-division multiplexed dc SQUID arrays operated at MHz frequencies, and frequency-division multiplexed rf SQUID arrays operated at microwave frequencies. Kilopixel TES arrays based on time-division multiplexed dc SQUIDs have reached maturity, and are being deployed in multiple instruments. Microwave frequency-division-multiplexed arrays of dissipationless rf SQUIDs coupled to superconducting microresonators have the promise of scaling to much larger array implementations in the future. The current status of these technologies, and future directions are described

[en] The development of thin-film dielectrics compatible with the epitaxial growth of YBa₂Cu₃O_7-δ (YBCO) is crucial to the fabrication of multilayer device and circuit structures. The authors of this paper investigate the YBCO/SrTiO₃ (STO) system by fabricating YBCO/STO bilayers and simple YBCO/STO/YBCO crossover structures. The thin films were deposited in situ by pulsed laser deposition and analyzed using x-ray diffraction and scanning electron microscopy. The film interfaces were characterized by secondary ion mass spectrometry (SIMS) depth profiling. The authors have developed photolithographic and wet-etching processes for patterning the crossovers which are compatible with these materials

[en] The application of cryogenic microcalorimeter detectors to γ-ray spectroscopy allows for measurements with unprecedented energy resolution. These detectors are ideally suited for γ-ray spectroscopy applications for which the measurement quality is limited by the spectral overlap of many closely spaced transitions using conventional detector technologies. The non-destructive analysis of mixed-isotope Pu materials is one such application where the precision can be potentially improved utilizing microcalorimeter detectors compared to current state-of-the-art high-purity Ge detectors (HPGe). The LANL-NIST γ-ray spectrometer, a 256-pixel microcalorimeter array based on transition-edge sensors (TESs), was recently commissioned and used to collect data on a variety of Pu isotopic standards to characterize the instrument performance. These measurements represent the first time the simultaneous readout of all 256 pixels for measurements of mixed-isotope Pu materials has been achieved. The LANL-NIST γ-ray spectrometer has demonstrated an average pixel resolution of 55 eV full-width-at-half-maximum at 100 keV, nearly an order of magnitude better than HPGe detectors. Some challenges of the analysis of many-channel ultra-high resolution data and the techniques used to produce quality spectra for isotopic analysis will be presented. The LANL-NIST γ-ray spectrometer has also demonstrated stable operation and obtained high resolution measurements at total array event rates beyond 1 kHz. For a total event rate of 1.25 kHz, approximately 5.6 cps/pixel, a 72.2 eV average FWHM for the 103 keV photopeak of ¹⁵³Gd was achieved