Single Pixel Performance of a 32 × 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 × 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 $_{\mathrm{FWHM}}$ = 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 d), 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 $_{\mathrm{FWHM}}$ = 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).