We present finite element simulations of the transition-edge sensor (TES) microcalorimeter behavior using COMSOL Multiphysics. The simulated detector has a large absorber with a size of 990 × 990 μ m2. The simulation calculates the TES response after x-ray impact for a single pixel with a known, realistic, current-dependent resistive transition R ( T , I ) . The simulation includes the full electrothermal feedback effects and heat conduction through the multiple contact points of the absorber to the TES thermometer and supporting membrane. The presented model, especially the 2D model, has been optimized for high accuracy, making it suitable for simulating the detector response when incident photons hit different positions on the absorber. We study the effects of position dependence using Principal Component Analysis with the aim of extracting correct photon energies from the pulses. The simulations show that the degradation, i.e., the FWHM broadening, due to the pulse variations will be less than 0.7 eV at below 2 keV, while it can be significant ( > 1 eV) at a higher energy band. The simulation result for position dependence can guide the design of large-absorber detectors for future x-ray missions, such as the Hot Universe Baryon Surveyor.

We have measured and characterized the thermal crosstalk in two different arrays of transition-edge sensor (TES) X-ray microcalorimeters with frequency-domain multiplexing (FDM) readout. The TES arrays are fabricated at SRON and are a 8 × 8 and a 32 × 32 'kilo-pixel' uniform array. The amount of crosstalk is evaluated as the ratio between the averaged crosstalk signal and the X-ray pulse amplitudes. The crosstalk ratios (CRs) for our detectors are compliant with the requirements for future X-ray space missions, such as Athena X-IFU (CR < 10^{-3 for first-neighbor pixels): We measured a nearest neighbor thermal CR at a level of 10^{-4, with a highest CR of 4× 10^{-4 for the kilo-pixel array (worst case, center of array) and 1× 10^{-4 for the 8 × 8 array, with a margin of improvement achievable by optimizing the Cu metallization and the width of the Si supporting structures (muntins) in the backside of the TES array chip. Based on the measured CRs, we have estimated the impact on the spectral resolution by means of noise equivalent power considerations and a Monte Carlo simulation, finding an average degradation in quadrature of less than 40 meV, compliant with the <0.2 eV requirement for Athena X-IFU.