Frequency domain multiplexing (FDM) is an attractive option for the readout of imaging arrays of microcalorimeters. The most straightforward implementation of FDM is based on alternating voltage (AC) biasing of the individual microcalorimeters. It is clear that for future applications the performance in terms of energy resolution and count rate capability is crucial. We discuss the consequences of AC bias for the performance of a microcalorimeter, and make a direct experimental comparison between the performance of a TES based microcalorimeter AC and DC bias. Modeling, as well as experiments, show that the performance of the device is is very similar in terms of energy resolution and pulse shapes (100 s effective time constant). The measured energy resolution at 5.89 keV photon energy is 6.3 eV for AC bias at 46 kHz and 5.5 eV for DC bias.
A micro-calorimeter array with superconducting transition-edge sensors read out by a SQUID-based frequency-domain multiplexer is under development for the X-ray imaging spectrometer on board ESA's X-ray Evolving Universe Spectroscopy (XEUS) mission. The XEUS requirements are 2 and 5 eV FWHM energy resolution for 2 and 7 keV X-rays, respectively. An array of 32 x 32 pixels with 250 micron square pixels is envisaged.SRON and MESA + have developed 5 x 5 imaging micro-calorimeter prototype arrays along a bulk micromachining and a surface micromachining route. The present state of array design and development with emphasis on pixel-to-pixel performance measurements of thermal and I-V characteristics, sensor noise and energy resolution are presented.SRON and VTT are developing frequency-domain multiplexing with SQUID current amplifiers to read out the 32 x 32 array. The concept for the frequency division multiplexing read-out will be presented and its performance characteristics discussed. Recent results of sensor operation under AC-bias (500 kHz) are presented.
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