Actinium-225 Production via Proton Irradiation of Thorium-232

High energy proton spallation reactions on natural thorium metal targets have been utilized to produce multi mCi [milliCurie] quantities of Actinium-225. Theoretical cross sections for actinium and thorium isotopes as well as for a select number of the fission products produced in these reactions were generated by the Monte Carlo radiation transport code PHITS to simulate the experimental data obtained from sixteen irradiations of thorium metal targets with 25-210 µA [microampere] proton beams ranging in energies from 77 to 192 MeV. Irradiations were conducted at Brookhaven National Laboratory (BNL) and Los Alamos National Laboratory (LANL), while target dissolution and processing was carried out at Oak Ridge National Laboratory (ORNL). At ORNL, a series of ion exchange columns was employed to isolate actinium from dissolved thorium, protactinium, radium, and numerous fission products. PHITS simulated cross sections for the production of Actinium-225 range from 7.8 to 28.2 mb [millibarn] in the incident proton energy range of 77 to 192 MeV [mega-electron volt]. The experimental cumulative cross sections for Actinium-225 are less than the simulated values by approximately a factor of two and vary from 3.6 to 16.7 mb over the same energy range. Based on these data, production of curie quantities of Actinium-225 is possible by irradiating ~80 grams of Thorium-232 target (5.0 g cm^-2 [grams per square centimeter]) for 10 days at either BNL or LANL proton irradiation facilities. Thorough analysis of two Actinium-225/Bismuth-213 generators produced by this process and used by nuclear medicine researchers was performed. It was demonstrated that the generators are free of any long-lived radiocontaminants with the exception of the known contaminant Actinium-227 and its decay products.