Preparation of an Actinium-228 Generator

Advances in targeted α-therapies have increased the interest in actinium (Ac), whose chemistry is poorly defined due to scarcity and radiological hazards. Challenges associated with characterizing Ac 3+ chemistry are magnified by its 5f 0 6d 0 electronic configuration, which precludes the use of many spectroscopic methods amenable to small amounts of material and low concentrations (like EPR, UV-vis, fluorescence). In terms of nuclear spectroscopy, many actinium isotopes ( 225 Ac and 227 Ac) are equally "unfriendly" because the actinium α-, β-, and γ-emissions are difficult to resolve from the actinium daughters. To address these issues, we developed a method for isolating an actinium isotope ( 228 Ac) whose nuclear properties are well-suited for γ-spectroscopy. This four-step procedure isolates 228 Ra from naturally occurring 232 Th. The relatively long-lived 228 Ra ( 1/2 = 5.75(3) years) radioisotope subsequently decays to 228 Ac. Because the 228 Ac decay rate [ 1/2 = 6.15(2) h] is fast, 228 Ac rapidly regenerates after being harvested from the 228 Ra parent. The resulting 228 Ac generator provides frequent and long-term access (of many years) to the spectroscopically "friendly" 228 Ac radionuclide. We have demonstrated that the 228 Ac product can be routinely "milked" from this generator on a daily basis, in chemically pure form, with high specific activity and in excellent yield (∼95%). Hence, in the same way that developing synthesis routes to new starting materials has advanced coordination chemistry for many metals by broadening access, this 228 Ac generator has the potential to broaden actinium access for the inorganic community, facilitating the characterization of actinium chemical behavior.