L. Gottardi

Thermal fluctuation noise in a voltage biased superconducting transition edge thermometer

The current noise at the output of a microcalorimeter with a voltage biased superconducting transition edge thermometer is studied in detail. In addition to the two well-known noise sources: thermal fluctuation noise from the heat link to the bath and Johnson noise from the resistive thermometer, a third noise source strongly correlated with the steepness of the thermometer is required to fit the measured noise spectra. Thermal fluctuation noise, originating in the thermometer itself, fully explains the additional noise. A simple model provides quantitative agreement between the observed and calculated noise spectra for all bias points in the superconducting transition.

Transition-Edge Sensor Pixel Parameter Design of the Microcalorimeter Array for the X-Ray Integral Field Unit on Athena

The focal plane of the X-ray integral field unit (X-IFU) for ESA’s Athena X-ray observatory will consist of ~ 4000 transition edge sensor (TES) x-ray microcalorimeters optimized for the energy range of 0.2 to 12 keV. The instrument will provide unprecedented spectral resolution of ~ 2.5 eV at energies of up to 7 keV and will accommodate photon fluxes of 1 mCrab (90 cps) for point source observations. The baseline configuration is a uniform large pixel array (LPA) of 4.28” pixels that is read out using frequency domain multiplexing (FDM). However, an alternative configuration under study incorporates an 18 × 18 small pixel array (SPA) of 2” pixels in the central ~ 36” region. This hybrid array configuration could be designed to accommodate higher fluxes of up to 10 mCrab (900 cps) or alternately for improved spectral performance (< 1.5 eV) at low count-rates. In this paper we report on the TES pixel designs that are being optimized to meet these proposed LPA and SPA configurations. In particular we describe details of how important TES parameters are chosen to meet the specific mission criteria such as energy resolution, count-rate and quantum efficiency, and highlight performance trade-offs between designs. The basis of the pixel parameter selection is discussed in the context of existing TES arrays that are being developed for solar and x-ray astronomy applications. We describe the latest results on DC biased diagnostic arrays as well as large format kilo-pixel arrays and discuss the technical challenges associated with integrating different array types on to a single detector die.

EURECA - A European-Japanese micro-calorimeter array

The EURECA (EURopean-JapanEse Calorimeter Array) project aims to demonstrate the science performance and technological readiness of an imaging X-ray spectrometer based on a micro-calorimeter array for application in future X-ray astronomy missions, like Constellation-X and XEUS. The prototype instrument consists of a 5 × 5 pixel array of TES-based micro-calorimeters read out by by two SQUID-amplifier channels using frequency-domain-multiplexing (FDM). The SQUID-amplifiers are linearized by digital base-band feedback. The detector array is cooled in a cryogenfree cryostat consisting of a pulse tube cooler and a two stage ADR. A European-Japanese consortium designs, fabricates, and tests this prototype instrument. This paper describes the instrument concept, and shows the design and status of the various sub-units, like the TES detector array, LC-filters, SQUID-amplifiers, AC-bias sources, digital electronics, etc. Initial tests of the system at the PTB beam line of the BESSY synchrotron showed stable performance and an X-ray energy resolution of 1.58 eV at 250 eV and 2.5 eV @ 5.9 keV for the read-out of one TES-pixel only. Next step is deployment of FDM to read-out the full array. Full performance demonstration is expected mid 2009.

MiniGRAIL, the first spherical detector

An overview is given on the possibilities of building low cost omni-directional gravitational wave detectors using resonant spheres. Sensitivity curves of arrays and xylophones of spheres fabricated from different materials show that spherical detectors can be competitive with the large interferometers at frequencies above 1 kHz, with the additional advantage of being omni-directional and being able to determine the direction and polarization of the gravitational wave. MiniGRAIL is the first spherical resonant detector, being built at the Kamerlingh Onnes Laboratory of Leiden University in the Netherlands. The detector is planned to operate at a temperature of about 20 mK and will have a quantum limited strain sensitivity for a burst signal of the order of 4 × 10−21. We present the progress concerning cryogenics and transducer development.

Josephson effects in an alternating current biased transition edge sensor

We report the experimental evidence of the ac Josephson effect in a transition edge sensor (TES) operating in a frequency domain multiplexer and biased by ac voltage at MHz frequencies. The effect is observed by measuring the non-linear impedance of the sensor. The TES is treated as a weakly linked superconducting system and within the resistively shunted junction model framework. We provide a full theoretical explanation of the results by finding the analytic solution of the non-inertial Langevian equation of the system and calculating the non-linear response of the detector to a large ac bias current in the presence of noise.

The x-ray microcalorimeter spectrometer onboard of IXO

One of the instruments on the International X-ray Observatory (IXO), under study with NASA, ESA and JAXA, is the X-ray Microcalorimeter Spectrometer (XMS). This instrument, which will provide high spectral resolution images, is based on X-ray micro-calorimeters with Transition Edge Sensor thermometers. The pixels have metallic X-ray absorbers and are read-out by multiplexed SQUID electronics. The requirements for this instrument are demanding. In the central array (40 x 40 pixels) an energy resolution of < 2.5 eV is required, whereas the energy resolution of the outer array is more relaxed (≈ 10 eV) but the detection elements have to be a factor 16 larger in order to keep the number of read-out channels acceptable for a cryogenic instrument. Due to the large collection area of the IXO optics, the XMS instrument must be capable of processing high counting rates, while maintaining the spectral resolution and a low deadtime. In addition, an anti-coincidence detector is required to suppress the particle-induced background. In this paper we will summarize the instrument status and performance. We will describe the results of design studies for the focal plane assembly and the cooling systems. Also the system and its required spacecraft resources will be given.

Baseband Feedback for Frequency‐Domain‐Multiplexed Readout of TES X‐ray Detectors

The linear dynamic range available for Frequency‐Domain‐Multiplexed (FDM) read‐out of TES X‐ray detectors is seriously limited by the SQUID current amplifiers used for the read‐out of TES‐detectors. Baseband feedback is one of the ways to increase the linearity and dynamic range of SQUIDs for the TES signals. Baseband feedback is realized by demodulation, and low‐pass filtering of the AM‐signals at the amplified summing point, thereby retrieving the signals for each detector, and subsequent remodulation and summing of the individual detector signals with phase compensation for the delay and phase rotation at each carrier frequency. This algorithm creates sufficient gain‐bandwidth at and around each carrier frequency (1–10 MHz) to reduce the error signal at the input of the SQUID amplifier for both the AC‐carriers and the signals modulated onto them. The paper presents the principle, modeling, and initial results.

Progress on Frequency-Domain Multiplexing Development for High Count rate X-ray Microcalorimeters

Frequency‐domain multiplexing is one of the candidates for the readout of TES‐based imaging microcalorimeter arrays for applications such as IXO. The readout system uses the TES as the modulating element, a bandpass LC filter to separate the signals in frequency space, and a customized SQUID with a low input inductance and high dynamic range as cryogenic amplifier.We will show the latest experimental progress on the development of this system. The paper concentrates on our first results on multiplexing 2–7 channels at bias frequencies below 5 MHz using discrete LC filters and baseband feedback electronics. In addition to that, the scalability of the system will be addressed.

The x-ray microcalorimeter spectrometer onboard Athena

One of the instruments on the Advanced Telescope for High-Energy Astrophysics (Athena) which was one of the three missions under study as one of the L-class missions of ESA, is the X-ray Microcalorimeter Spectrometer (XMS). This instrument, which will provide high-spectral resolution images, is based on X-ray micro-calorimeters with Transition Edge Sensor (TES) and absorbers that consist of metal and semi-metal layers and a multiplexed SQUID readout. The array (32 x 32 pixels) provides an energy resolution of < 3 eV. Due to the large collection area of the Athena optics, the XMS instrument must be capable of processing high counting rates, while maintaining the spectral resolution and a low deadtime. In addition, an anti-coincidence detector is required to suppress the particle-induced background. Compared to the requirements for the same instrument on IXO, the performance requirements have been relaxed to fit into the much more restricted boundary conditions of Athena. In this paper we illustrate some of the science achievable with the instrument. We describe the results of design studies for the focal plane assembly and the cooling systems. Also, the system and its required spacecraft resources will be given.

The X‐Ray Microcalorimeter Spectrometer for the International X‐Ray Observatory

The International X‐Ray Observatory (IXO) is under formulation by NASA, ESA and JAXA for deployment in 2022. IXO emerged over the last 18 months as the NASA Constellation‐X and ESA/JAXA X‐Ray Evolving Universe Spectrometer (XEUS) missions were combined. The driving performance requirements for the X‐Ray Microcalorimeter Spectrometer (XMS) are a spectral resolution of 2.5 eV over the central 2’×2’ in the 0.3–7.0 keV band, and 10 eV to the edge of the 5’×5’ field of view (FOV). The XMS is now based on a microcalorimeter array of Transition‐Edge Sensor (TES) thermometers with Au/Bi absorbers and a SQUID MUX readout. One of the concepts studied as part of the mission formulation has a core 40×40 array corresponding to a 2’×2’ FOV with 3” pixels surrounded by an outer, annular 52×52 array of 6” pixels that extends the field of view to 5.4’×5.4’ with better than 10 eV resolution. There are several options for implementing the readout and cooling system of the XMS under study in the US, Europe and Japan. The ADR...