[en] We have recently reported on a theoretical digital signal-processing algorithm for improved energy and position resolution in position-sensitive, transition-edge sensor (PoST) X-ray detectors [Smith et al., Nucl. Instr. and Meth. A 556 (2006) 237]. PoST's consists of one or more transition-edge sensors (TES's) on a large continuous or pixellated X-ray absorber and are under development as an alternative to arrays of single-pixel TES's. PoST's provide a means to increase the field-of-view for the fewest number of read-out channels. In this contribution we extend the theoretical correlated energy-position optimal filter (CEPOF) algorithm (originally developed for 2-TES continuous absorber PoST's) to investigate the practical implementation on multi-pixel single TES PoST's or Hydras. We use numerically simulated data for a nine-absorber device, which includes realistic detector noise, to demonstrate an iterative scheme that enables convergence on the correct photon absorption position and energy without any a priori assumptions. The position sensitivity of the CEPOF implemented on simulated data agrees very well with the theoretically predicted resolution. We discuss practical issues such as the impact of random arrival phase of the measured data on the performance of the CEPOF. The CEPOF algorithm demonstrates that full-width-at-half-maximum energy resolution of<8 eV coupled with position-sensitivity down to a few 100 eV should be achievable for a fully optimized device.

[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] 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] We describe optimal filtering algorithms for determining energy and position resolution in position-sensitive Transition Edge Sensor (TES) Distributed Read-Out Imaging Devices (DROIDs). Improved algorithms, developed using a small-signal finite-element model, are based on least-squares minimisation of the total noise power in the correlated dual TES DROID. Through numerical simulations we show that significant improvements in energy and position resolution are theoretically possible over existing methods

[en] We report on a new optimal filtering algorithm for improved energy and position resolution in position sensitive Transition Edge Sensor (TES) based X-ray detectors. This algorithm takes account of the noise correlation between the detector outputs to minimise the variance on the estimated photon energy and position. Using numerical methods we show that improved energy and position resolution can be obtained than from previously published methods. Our simulations also reveal the trade-offs resulting from changes in the thermal conductances and heat capacities of the detector elements

[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] We report on the experimental characterisation and modelling of Transition Edge Sensor (TES)-based Distributed Read-Out Imaging Devices (DROIDs), for use as position-sensitive detectors in X-ray astronomy. Latest experimental results from prototype DROIDs using Ir TESs with Au absorbers are reported. Through modelling and the development of signal processing algorithms we are able to design the DROID for optimum spectral and spatial resolution depending upon application

[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.

[en] We are developing arrays of position-sensitive magnetic calorimeter (PoSM) X-ray detectors for future astronomy missions. The PoSM consists of multiple absorbers thermally coupled to one magnetic sensor. Each absorber element has a different thermal coupling to the sensor. This results in a distribution of different pulse shapes and enables position discrimination between the absorber elements. PoSMs are motivated by the desire to achieve the largest possible focal plane area with the fewest number of readout channels without compromising on spatial sampling. Optimizing the performance of PoSMs requires careful design of key parameters such as the thermal conductances between the absorbers, magnetic sensor and the heat sink, as well as the absorber heat capacities. We report on the first experimental results from four-absorber PoSMs, each absorber consisting of a two layer composite of bismuth and gold. The measured energy resolution (FWHM) was less than 5 eV for 6 keV X-rays into all four absorbers. Straightforward position discrimination by means of rise-time is also demonstrated.

[en] We are developing transition-edge sensor (TES) X-ray detectors optimized for high count-rate applications. These devices are fabricated on thick (300 μm) Si substrates, resulting in a 20 times increase in thermal conductance to the heat sink compared to our conventional membrane isolated TES's. Operating a TES with higher heat sink conductance requires 4.5 times more bias current. This results in a 2.7 times increase in β, the logarithmic derivative of resistance with respect to current. Noise measurements show a lower limit on the TES excess noise scales as (2β)^1/2, consistent with the near-equilibrium, non-linear expansion of the Ohmic Johnson noise. This is consistent with our membrane devices though the increased β means the theoretical best attainable resolution is degraded by 25-35%. We have tested devices with different contact geometries between the absorber, and the TES and substrate. This allows us to investigate the loss of athermal phonons to the substrate, which can degrade the resolution. Results show a correlation between the stem contact area and a low-energy tail in the spectral response at 5.9 keV due to the athermal phonon loss. In several devices tested we demonstrate a resolution of 4.1-5.6 eV, coupled with detector time constants as fast as 44 μs, representing an increase in detector response by 7 times compared to the membrane devices.