Thermal crosstalk of X-ray transition-edge sensor micro-calorimeters under frequency domain multiplexing readout

TL;DR

This study measures and analyzes thermal crosstalk in TES X-ray micro-calorimeter arrays with FDM readout, demonstrating crosstalk levels suitable for future space missions and estimating minimal impact on spectral resolution.

Contribution

It provides the first detailed characterization of thermal crosstalk in large TES arrays with FDM readout and suggests optimization strategies to further reduce crosstalk.

Findings

Crosstalk ratios are below 10^{-3}, meeting mission requirements.

Measured crosstalk levels are around 10^{-4} to 4×10^{-4}.

Spectral resolution degradation is less than 40 meV, within mission specifications.

Abstract

We have measured and characterized the thermal crosstalk in two different arrays of transition-edge sensor (TES) X-ray micro-calorimeters with frequency-domain multiplexing (FDM) readout. The TES arrays are fabricated at SRON and are a 8$\times$8 and a 32$\times$32 "kilo-pixel" uniform array. The amount of crosstalk is evaluated as the ratio between the averaged crosstalk signal and the X-ray pulse amplitudes. The crosstalk ratios (CR) for our detectors are compliant with the requirements for future X-ray space missions, such as Athena X-IFU (CR$< 10^{-3}$ for first-neighbour pixels): we measured a nearest-neighbour thermal crosstalk ratio at a level of $10^{-4}$, with a highest crosstalk ratio of $4\times 10^{-4}$ for the kilo-pixel array (worst case, center of array) and $1\times 10^{-4}$ for the 8$\times$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 crosstalk ratios, we have estimated the impact on the spectral resolution by means of noise equivalent power (NEP) 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.