Background <p>²²⁵Ac is one of the most promising short-lived radionuclides for application in targeted alpha therapy (TαT). However, quantitative detection of ²²⁵Ac and its daughter radionuclides by γ-spectroscopy has so far been unattainable. Achieving this would enable precise organ dosimetry and better targeting of the therapeutic potential of the ²²⁵Ac decay series.</p> Methods <p>In this study, an X-ray and γ-spectrum of an ²²⁵Ac sample (89 kBq) was recorded with a Metallic Magnetic Calorimeter (MMC) detector over a wide energy range of 5 to 125 keV.</p> Results <p>Here, we show the feasibility of detecting γ-lines of ²²⁵Ac, ²²¹Fr, ²¹³Bi, and ²⁰⁹Tl with ultra-high-resolution spectroscopy (FWHM of 23 eV @ 5.9 keV). By analyzing characteristic X-ray fluorescence lines, it is also possible to distinguish ²²¹Fr, ²¹⁷At, ²¹³Bi, ²¹³Po, and ²⁰⁹Pb&#xa0;in this sample. Additionally, particle induced X-ray emission from ²²⁵Ac is found.</p> Conclusions <p>To the best of our knowledge, this first MMC based X-ray and γ-spectrum of ²²⁵Ac demonstrates the detector’s potential, as it enables the separate detection of most nuclides in the ²²⁵Ac decay chain (all except ²²¹Ra and ²¹⁷Rn) with high sensitivity, excellent energy resolution, and precise energy calibration, paving the way for key technological advancements in research and clinical applications in nuclear medicine.</p>