S. J. Smith

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.

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...

The focal plane assembly for the Athena X-ray Integral Field Unit instrument

This paper summarizes a preliminary design concept for the focal plane assembly of the X-ray Integral Field Unit on the Athena spacecraft, an imaging microcalorimeter that will enable high spectral resolution imaging and point-source spectroscopy. The instruments sensor array will be a ~ 3840-pixel transition edge sensor (TES) microcalorimeter array, with a frequency domain multiplexed SQUID readout system allowing this large-format sensor array to be operated within the thermal constraints of the instruments cryogenic system. A second TES detector will be operated in close proximity to the sensor array to detect cosmic rays and secondary particles passing through the sensor array for off-line coincidence detection to identify and reject events caused by the in-orbit high-energy particle background. The detectors, operating at 55 mK, or less, will be thermally isolated from the instrument cryostats 2 K stage, while shielding and filtering within the FPA will allow the instruments sensitive sensor array to be operated in the expected environment during both on-ground testing and in-flight operation, including straylight from the cryostat environment, low-energy photons entering through the X-ray aperture, low-frequency magnetic fields, and high-frequency electric fields.

Heat Sinking, Crosstalk, and Temperature Uniformity for Large Close-Packed Microcalorimeter Arrays

In a large close-packed array of x-ray microcalorimeters, sufficient heat sinking is important to minimize thermal crosstalk between pixels and to make the bath temperature of all the pixels uniform. We have measured crosstalk in our 8 times 8 pixel arrays. The shapes of the thermal crosstalk pulses are reproduced well as a convolution of heat input from the source pixel and the thermal decay in the receiver pixel. The amount of the thermal crosstalk is clearly dependent on the degree of electrothermal feedback. We have compared the magnitude of thermal crosstalk with and without a heat-sinking copper layer on the backside of the silicon frame as a function of distance between the source and receiver pixels. Using the results obtained, we have estimated the degradation of energy resolution that is expected as a function of count rate. We have also studied the temperature distribution within an array due to continuous heating from the TES bias to estimate impacts on the uniformity of the pixel performance.

Bright source capabilities of the WFI and X-IFU

Trabajo presentado a la Conferencia: Exploring the Hot and Energetic Universe: The first scientific conference dedicated to the Athena X-ray observatory; celebrada en Madrid (Espana) del 8 a 10 de septiembre de 2015.-- et al.