[en] We describe a comparative investigation of the parameters of MoAu-bilayer TES bolometers designed for infrared detectors. A set of devices with variations in geometry were fabricated at the NASA/GSFC detector development facility. These detectors have different bilayer aspect ratios (providing differing normal state resistances and current densities), and have varieties of normal metal regions to study the effects of geometry on noise. These normal metal regions are oriented either parallel to or transverse to the direction of current flow, or both. The lowest noise detectors are found to have normal metal regions oriented transversely. For about a dozen different devices, we have measured a large set of parameters by means of a suite of tests. These include complex impedance measurements to derive time constants; IV curves to determine resistance and power; thermal conductance measurements; noise measurements as a function of device resistance; and direct resistance vs. temperature measurements

[en] Recent efforts in the Transition Edge Sensor (TES) bolometer/calorimeter community have focused on developing detectors whose noise properties are near the fundamental limits. These include the in-band phonon noise, the out-of-band Johnson noise, and the 1/f noise. We have investigated the noise performance of Mo/Au-bilayer TES bolometers designed for infrared detectors. These detectors use normal metal regions for the suppression of excess noise, which are oriented either parallel to ('bars') or transverse to ('stripes') the direction of current flow. Two nearly identical detectors, one with stripes and one with bars, were fabricated at the NASA/GSFC detector development facility. Significantly lower noise is found with the normal metal regions oriented transversely. We compare the detailed noise measurement and quantitative analysis of the noise level in each device as a function of the detector resistance. Our preliminary result is that the best detector features only moderate excess noise in both the in-band region and in the out-of-band region. This noise performance is suitable for instruments with multiplexed TES arrays, such as GSFC's FIBRE and SAFIRE

[en] We measured noise in a variety of Mo/Au transition-edge sensor (TES) X-ray calorimeters. We investigated the relationship between the noise, bias, and the superconducting phase transition in the TESs. Our square TES calorimeters have achieved very good energy resolutions (2.4 eV at 1.5 keV) but their resolutions have been limited by broadband white excess noise generated by the TES when it is biased in the phase transition. We have recently fabricated Mo/Cu TESs with interdigitated normal metal bars deposited on top of the bilayer. The new TES calorimeters have demonstrated little or no excess noise in the phase transition. These results point the way to development of TES calorimeters with higher energy resolution

[en] We have recently produced and tested two-dimensional arrays of Mo/Au transition-edge-sensor (TES) calorimeters with Bi/Cu absorbers. The arrays represent a significant step towards meeting the specifications of NASA's Constellation-X mission. The calorimeters are compactly spaced within 5x5 arrays of 250 μm square pixels necessary for an angular resolution of 5 arcsec. Lithographically produced absorbers hang over the substrate and wiring between the TESs for high filling fraction and high quantum efficiency. We designed the calorimeters with heat capacities and thermal couplings such that X-rays produce pulses with fall times of approximately 300 μs to allow relatively high count rates with low dead time. We read out up to four of the pixels simultaneously. The arrays demonstrated very good energy resolution (5 eV at 1.5 keV and 7 eV at 6 keV) and little crosstalk between neighboring pixels

[en] Transition Edge Sensors (TES) have found applications as astronomical detectors ranging from the microwave to the gamma ray energy bands. Each energy band, however, imposes a different set of requirements on the TES such as energy and timing resolution, focal plane coverage, and the mechanisms by which the signal is coupled to the detector. This paper focuses on the development of TESs optimized for the 0.1-10 keV energy range at the NASA Goddard Space Flight Center. Such detectors are suitable candidates for some of the upcoming X-ray observatories such as NeXT and Constellation-X. Ongoing efforts at producing, characterizing, and modeling such devices, as well as the latest results, are discussed

[en] We have developed transition-edge sensor (TES) microcalorimeter arrays with high count-rate capability and high energy resolution to carry out x-ray imaging spectroscopy observations of various astronomical sources and the Sun. We have studied the dependence of the energy resolution and throughput (fraction of processed pulses) on the count rate for such microcalorimeters with two different transition temperatures ( T _c). Devices with both transition temperatures were fabricated within a single microcalorimeter array directly on top of a solid substrate where the thermal conductance of the microcalorimeter is dependent upon the thermal boundary resistance between the TES sensor and the dielectric substrate beneath. Because the thermal boundary resistance is highly temperature dependent, the two types of device with different T _cs had very different thermal decay times, approximately one order of magnitude different. In our earlier report, we achieved energy resolutions of 1.6 and 2.3 eV at 6 keV from lower and higher T _c devices, respectively, using a standard analysis method based on optimal filtering in the low flux limit. We have now measured the same devices at elevated x-ray fluxes ranging from 50 Hz to 1000 Hz per pixel. In the high flux limit, however, the standard optimal filtering scheme nearly breaks down because of x-ray pile-up. To achieve the highest possible energy resolution for a fixed throughput, we have developed an analysis scheme based on the so-called event grade method. Using the new analysis scheme, we achieved 5.0 eV FWHM with 96% throughput for 6 keV x-rays of 1025 Hz per pixel with the higher T _c (faster) device, and 5.8 eV FWHM with 97% throughput with the lower T _c (slower) device at 722 Hz. (paper)

[en] Lynx is an x-ray telescope, one of four large satellite mission concepts currently being studied by NASA to be a flagship mission. One of Lynx’s three instruments is an imaging spectrometer called the Lynx x-ray microcalorimeter (LXM), an x-ray microcalorimeter behind an x-ray optic with an angular resolution of 0.5 arc sec and ~2 m² of area at 1 keV. The LXM will provide unparalleled diagnostics of distant extended structures and, in particular, will allow the detailed study of the role of cosmic feedback in the evolution of the Universe. We discuss the baseline design of LXM and some parallel approaches for some of the key technologies. The baseline sensor technology uses transition-edge sensors, but we also consider an alternative approach using metallic magnetic calorimeters. We discuss the requirements for the instrument, the pixel layout, and the baseline readout design, which uses microwave superconducting quantum interference devices and high-electron mobility transistor amplifiers and the cryogenic cooling requirements and strategy for meeting these requirements. For each of these technologies, we discuss the current technology readiness level and our strategy for advancing them to be ready for flight. We also describe the current system design, including the block diagram, and our estimate for the mass, power, and data rate of the instrument.

[en] Superconducting transition-edge sensor (TES) microcalorimeters are being developed for a variety of potential astrophysics missions, including Athena. The X-ray integral field unit instrument on this mission requires close-packed pixels on a 0.25 mm pitch, and high quantum efficiency between 0.2 and 12 keV. In this work, we describe a new approach with 50 μm square TESs consisting of a Mo/Au bilayer, deposited on silicon nitride membranes to provide a weak thermal conductance to a ~ 50 mK heat bath. Larger TESs usually have additional normal metal stripes on top of the bilayer to reduce the noise. However, we have found that excellent spectral performance can be achieved without the need for any normal metal stripes on top of the TES. A spectral performance of 1.58 ± 0.12 eV at 5.9 keV has been achieved, the best resolution seen in any of our devices with this pixel size.

[en] The performance of transition-edge sensors (TES) and their SQUID multiplexed readouts is very sensitive to ambient magnetic field and its fluctuations. In order to run ground experiments on thousands of X-ray TES microcalorimeters with a small uniform ambient magnetic field (< 1 μT, with a uniformity < 0.1 μT), we need a very low ambient field to be trapped into the superconducting magnetic shields. We have designed a sub-Kelvin test platform to reach these specifications. For this purpose, we modeled a new design for the shielding consisting of a series of different mu-metal and superconducting shields, including a niobium shield at 50 mK, a cryoperm (A4K) shield at 3 K, and a mu-metal shield at 300 K. A magnetic field coil is used to vary the local perpendicular magnetic field over the TES array. To optimize this field, we have studied a number of different field-coil designs and the impact of the different shield geometries, in order to reach the required field uniformity.

[en] Transition-edge-sensor (TES) X-ray microcalorimeters have mostly been targeted at mid-band energies from 0.05-10 keV and high energies to above 100 keV. However, many other optimizations are possible. Here we present results from devices optimized for soft X-ray applications. For spectroscopy below 1 keV, the X-ray stopping power and heat capacity (C) of the TES itself are high enough that we can omit a separate absorber. The resulting devices have low C and the best-achievable energy resolution should be under 1 eV. We are interested in pursuing such devices primarily for astrophysical applications and laboratory astrophysics at LLNL's Electron-Beam Ion Trap. To this end, we have studied arrays in which 'bare' TESs are interspersed with broad-band pixels that have absorbers. By extending the absorbers to cover the area where the leads contact the low-energy pixels, we have eliminated a significant source of non-Gaussian detector response. The bare devices are in a different regime from our typical devices in that C is ten times lower and the conductance to the bath is four times lower. We have explored this regime through simultaneous fitting of noise and impedance data. These data cannot be fit by the simple model we employ to describe our typical broad-band devices. In this contribution we present X-ray spectra and the results from modeling.