Thick Target Yield of Th-229 via Low Energy Proton Bombardment of Th-232

Actinium-225 is one of the more effective radioisotopes used in alpha radioimmunotherapy. Due to its ten-day half-life, it is more efficient to create its precursor, ²²⁹Th [Thorium-229] (t_1/2[half-life] = 7932 ± 55 years). In this work, ²²⁹Th was produced via 40 MeV [Mega electron Volts] proton bombardment of a thick ²³²Th [Thorium-232] target. The irradiation took place at the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Lab (ORNL). The target, consisting of 23 stacked natural thorium foils (137 mg/cm² [milligrams per square centimeter] each), was irradiated with 50 nA [nanoamps] of protons from HRIBF’s 25 MV [Mega Volt] tandem electrostatic accelerator for approximately 143 discontinuous hours. After 215 days post bombardment, allowing for the decay of short-lived protactinium and actinium isotopes and fission products, the target was chemically purified by a series of ion chromatography techniques. Thorium-229 was measured directly by γ-ray [gamma-ray] spectroscopy immediately after separation of the thorium fraction from the decay daughters of ²²⁸Th [Thorium-228] (t_1/2 = 1.9 years) and long long-lived fission products. The effective thick target cross section of ²²⁹Th is 205 ± 18 mb [millibarns] at a proton energy of 26.1 MeV. Variation of the effective cross-section as a function of proton energy is also reported.