Ultra-high-resolution γ- and X-ray spectroscopy of 225Ac and its daughter radionuclides using MMC-based detectors

Aims: To date, only 221Fr and 213Bi are imageable in the decay chain of 225Ac due to their suitable gamma lines. However, quantitative detection of ideally all radionuclides is desirable to precisely determine the distribution in the organism and thus the organ dose. Detectors used in SPECT but also high purity germanium detectors lack energy resolution and sensitivity to distinguish between more than the above-mentioned radionuclides.1 The resolution of the 225Ac-spectrum presented in this work using the magnetic micro-calorimeter (MMC) detectors is the superior to available detectors,2 permitting to differentiate between the γ- lines of the individual daughters of 225Ac in small energy ranges in the low energy region (<99 keV).
Methods: A gamma and X-ray spectrum obtained using an 80 kBq source of 225Ac, which was in equilibrium with its daughters was measured with an MMC detectors while a 55Fe source for calibration was present. The resulting histogram was compared with simultaneously measured spectra from a silicon drift detector (SDD). In addition, a theoretical gamma spectrum of 225Ac (Nucleonica) was used to assign the various signals to the γs of actinium and its daughters. ... mehrOther sources of signal were discussed.
Results: An ultra-high resolution (Full Width at Half Maximum (FWHM) of 23 eV @ 5.9 keV) enabled the assignment of the γ-lines of 225Ac, 221Fr, 213Bi, and 209Tl. Additionally, the characteristic X-ray fluorescence lines, of 221Fr, 217At, 213Bi, 213Po, and 209Pb could be recorded.
Conclusion: An γ- and X-ray spectrum of 225Ac with superior energy resolution was obtained, it enables the separate detection of most nuclides in the ²²⁵Ac decay chain (all except ²²¹Ra and ²¹⁷Rn). This first MMC spectrum of ²²⁵Ac demonstrates the detector’s potential which could potentially be used to achieve better dose distribution in future applications.3
References and Acknowledgments: [1] Apostolidis, C et al. Appl. Radiat. Isot. 2005, 62 (3), 383–387. [2] Sikorsky, T et al. Phys. Rev. Lett. 2020, 125 (14), 142503. [3] Maurer, K.; Unger, Schacherl, B. et al. Towards Cancer Theragnostic ‒ Probing the 225Ac decay chain with Ultra-High-Resolution MMC-based detectors. Submitted.
We acknowledge funding by Federal Ministry of Education and Research (BMBF) and the Baden-Württemberg Ministry of Science as part of the Excellence Strategy of the German Federal and State Governments as KIT future fields stage II project – InnoAlpha and the joint strategic alliance between Karlsruhe Institute of Technology (KIT) and German Cancer Research Center (DKFZ).