Optimized TES Microcalorimeters with 14 eV Energy Resolution at 30 keV for γ-Ray Measurements of the 229Th Isomer

We have developed a four-pixel array of superconducting transition-edge sensors with gold absorbers for the detection of a 29.2 keV $$\gamma$$-ray doublet decay from $${}^{229}$$Th. To identify the decay, an energy resolution better than 20 eV full width at half maximum (FWHM) is needed. We measured an energy resolution of 14 eV FWHM for 26 keV $$\gamma$$-ray decay from an $${}^{241}$$Am isotope in combined data of three pixels. We describe the design and the performance of the devices and discuss the baseline correction method to compensate the variation in the baseline, which was observed during the evaluation of the performance using the $${}^{241}$$Am isotope.