Actinium-225 photonuclear production in nuclear reactors using a mixed radium-226 and gadolinium-157 target

Background: Actinium-225 is one of the most promising radionuclides for targeted alpha therapy. Its limited availability significantly restricts clinical trials and potential applications of ²²⁵Ac-based radiopharmaceuticals. Methods: In this work, we examine the possibility of ²²⁵Ac production from the thermal neutron flux of a nuclear reactor. For this purpose, a target consisting of 1.4 mg of ²²⁶Ra(NO₃)₂ (T_1/2 = 1600 years) and 115.5 mg of 90 % enriched, stable ¹⁵⁷Gd₂O₃ was irradiated for 48 h in the Breazeale Nuclear Reactor with an average neutron flux of 1.7·10¹³ cm⁻²·s⁻¹. Gadolinium-157 has one of the highest thermal neutron capture cross sections of 0.25 Mb, and its neutron capture results in emission of high-energy, prompt γ-photons. Emitted γ-photons interact with ²²⁶Ra to produce ²²⁵Ra according to the ²²⁶Ra(γ, n)²²⁵Ra reaction. Gadolinium debulking and separation of undesirable, co-produced ²²⁷Ac from ²²⁵Ra was achieved in one step by using 60 g of branched DGA resin. After ²²⁵Ac ingrowth from ²²⁵Ra (T_1/2 = 14.8 d), ²²⁵Ac was extracted from the ²²⁶Ra and ²²⁵Ra fraction using 5 g of bDGA resin and then eluted using 5 mM HNO₃. Results: Measured activity of ²²⁵Ac showed that 6(1) kBq or 0.16(3) μCi (1σ) of ²²⁵Ra was produced at the end of bombardment from 0.9 mg of ²²⁶Ra. Conclusion: The developed ²²⁵Ac separation is a waste-free process which can be used to obtain pure ²²⁵Ac in a nuclear reactor.