Preparation of an Actinium-228 Generator

Advances in targeted a-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 (Ac-225 and Ac-227) are equally "unfriendly" because the actinium alpha-, beta-, and gamma-emissions are difficult to resolve from the actinium daughters. To address these issues, we developed a method for isolating an actinium isotope (Ac-228) whose nuclear properties are well-suited for gamma-spectroscopy. This four-step procedure isolates Ra-228 from naturally occurring Th-232. The relatively long-lived Ra-228 (t(1/2) = 5.75(3) years) radioisotope subsequently decays to Ac-228. Because the Ac-228 decay rate [t(1/2) = 6.15(2) h] is fast, Ac-228 rapidly regenerates after being harvested from the Ra-228 parent. The resulting (22)(8)A cgenerator provides frequent and long-term access (of many years) to the spectroscopically "friendly" Ac-228 radionuclide. We have demonstrated that the Ac-228 product can be routinely "milked" from this generator on a daily basis, in chemically pure form, with high specific activity and in excellent yield (similar to 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 Ac-228 generator has the potential to broaden actinium access for the inorganic community, facilitating the characterization of actinium chemical behavior.