Single pixel performance of a 32\(\times\)32 Ti/Au TES array with broadband X-ray spectra

We are developing a kilo-pixels Ti/Au TES array as a backup option for Athena X-IFU. Here we report on single-pixel performance of a 32\(\times\)32 array operated in a Frequency Division Multiplexing (FDM) readout system, with bias frequencies in the range 1-5 MHz. We have tested the pixels response at several photon energies, by means of a \(^{55}\)Fe radioactive source (emitting Mn-K\(\alpha\) at 5.9 keV) and a Modulated X-ray Source (MXS, providing Cr-K\(\alpha\) at 5.4 keV and Cu-K\(\alpha\) at 8.0 keV). First, we report the procedure used to perform the detector energy scale calibration, usually achieving a calibration accuracy better than \(\sim\) 0.5 eV in the 5.4 - 8.9 keV energy range. Then, we present the measured energy resolution at the different energies (best single pixel performance: \(\Delta\)E\(_{FWHM}\) = 2.40 \(\pm\) 0.09 eV @ 5.4 keV; 2.53 \(\pm\) 0.10 eV @ 5.9 keV; 2.78 \(\pm\) 0.16 eV @ 8.0 keV), investigating also the performance dependency from the pixel bias frequency and the count rate. Thanks to long background measurements (\(\sim\) 1 day), we finally detected also the Al-K\(\alpha\) line at 1.5 keV, generated by fluorescence inside the experimental setup. We analyzed this line to obtain a first assessment of the single-pixel performance also at low energy (\(\Delta\)E\(_{FWHM}\) = 1.91 eV \(\pm\) 0.21 eV @ 1.5 keV), and to evaluate the linearity of the detector response in a large energy band (1.5 - 8.9 keV).