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

Introduction: The radionuclide 225Ac is a very promising candidate for the use in targeted α-therapy (TαT). However, its decay series includes four subsequent alpha decays, which pose certain limitations. As a result, the daughter radionuclides may be released from the chelator after the first alpha decay, potentially exerting therapeutic or destructive effects as well. To date, only 221Fr and 213Bi are imageable in the decay chain 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 lack energy resolution and sensitivity to distinguish between more than the above mentioned radionuclides.1 Although high purity germanium detectors measure with higher energy resolution they suffer from spatial resolution and in a heightened background of transitions, due to merging gamma lines of daughters in low energy ranges (<99 keV).2 The resolution of the 225Ac-spectrum presented in this work using the MMC3 (magnetic micro-calorimeter) detectors is the superior to available detectors making it possible to differentiate between the gamma lines of the individual daughters of 225Ac in small energy ranges (ΔE<1 keV) in the low energy region (<99 keV).
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Methods: A gamma spectrum obtained using an 80 kBq source of 225Ac, which was in equilibrium with its daughters was measured with the 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. Other sources of signal were discussed.
Results: A resolving power higher than E/∆E = 6,000 has been demonstrated with MMC detectors for 60 keV gammas from a 241Am source before.3 Thanks to this excellent energy resolution, the gamma lines related to the expected daughter nuclides of 225Ac (221Fr, 213Bi,209Tl) could be clearly identified. For example, gamma lines of 221Fr and 213Bi in the energy range of <0.5 keV could be separately resolved. Furthermore, many separate energy peaks could also be detected in the low energy region.
Conclusion: A γ-spectrum of 225Ac with superior energy resolution was obtained, enabling precise identification of the individual gamma lines of 225Ac and its daughter radionuclides. This improved resolution could potentially be used to achieve better dose distribution in future applications. Additionally, it would facilitate the detection and quantification of possible impurities.
References: 1 Usmani, S et al. Clin. Nucl. Med. 2019, 44 (5), 401–403. 2 Apostolidis, C et al. Appl. Radiat. Isot. 2005, 62 (3), 383–387. 3 Sikorsky, T et al. Phys. Rev. Lett. 2020, 125 (14), 142503.
Acknowledgments: 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).