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We present that actinium-225 (t_1/2 = 9.92 d) is an α-emitting radionuclide with nuclear properties well-suited for use in targeted alpha therapy (TAT), a powerful treatment method for malignant tumors. Actinium-225 can also be utilized as a generator for ²¹³Bi (t_1/2 45.6 min), which is another valuable candidate for TAT. Actinium-225 can be produced via proton irradiation of thorium metal; however, long-lived ²²⁷Ac (t_1/2 = 21.8 a, 99% β^-, 1% α) is co-produced during this process and will impact the quality of the final product. Thus, accurate assays are needed to determine the ²²⁵Ac/²²⁷Ac ratio, which is dependent on beammore » energy, irradiation time and target design. Accurate actinium assays, in turn, require efficient separation of actinium isotopes from both the Th matrix and highly radioactive activation by-products, especially radiolanthanides formed from proton-induced fission. In this study, we introduce a novel, selective chromatographic technique for the recovery and purification of actinium isotopes from irradiated Th matrices. A two-step sequence of cation exchange and extraction chromatography was implemented. Radiolanthanides were quantitatively removed from Ac, and no non-Ac radionuclidic impurities were detected in the final Ac fraction. An ²²⁵Ac spike added prior to separation was recovered at ≥98%, and Ac decontamination from Th was found to be ≥10⁶. Finally, the purified actinium fraction allowed for highly accurate ²²⁷Ac determination at analytical scales, i.e., at ²²⁷Ac activities of 1–100 kBq (27 nCi to 2.7 μCi).« less